The big-endian v. little-endian controversy, how to avoid stupid mistakes in hardware and software, ON HOLY WARS AND A PLEA FOR PEACE.
Related local pages:
Resent-From: pci-sig-request at znyx.com Resent-Date: Wed, 24 Jan 1996 23:50:25 GMT Date: Wed, 24 Jan 1996 23:50:25 GMT From: Tim at tile.demon.co.uk (Tim Eccles) Subject: Re: Big Endian question To: Mailing List Recipients <pci-sig-request at znyx.com> Alan Deikman writes: > I wrote: > > >Is there an example of a 32-bit processor that stores bytes 0123 as 0132 > >for a 32-bit number? > > Oops, I hit the send key too soon. I meant "0123" as "1032", where > the 16-bit parts of a 32-bit number were swapped. "Endian" discussions > always make me bug-eyed. But I remember reading that byte order some- > where but forgot where. Long ago, in UNIX times, this was the NUXI byte order. Herewith Danny Cohen's original paper, as posted recently to comp.arch, etc. Still makes a good read on a wet afternoon. >gnu> From: gnu at hoptoad.uucp (John Gilmore) >gnu> Newsgroups: comp.sys.m68k,comp.arch,comp.sys.intel >gnu> Subject: Byte Order: On Holy Wars and a Plea for Peace >gnu> Date: 30 Nov 86 01:29:46 GMT >gnu> [Not a single person objected to my posting this, so here it is. >gnu> Mod.sources.doc seems to be dead, so I am posting this here. Factual >gnu> followups to comp.arch, please. Send flames to yourself via email. >gnu> Note that the date of the article is 1980, so there are a few things >gnu> that have changed since then; nevertheless, the spirit of the article >gnu> is still relevant. --gnu] IEN 137 Danny Cohen U S C/I S I 1 April 1980 ON HOLY WARS AND A PLEA FOR PEACE INTRODUCTION This is an attempt to stop a war. I hope it is not too late and that somehow, magically perhaps, peace will prevail again. The latecomers into the arena believe that the issue is: "What is the proper byte order in messages?". The root of the conflict lies much deeper than that. It is the question of which bit should travel first, the bit from the little end of the word, or the bit from the big end of the word? The followers of the former approach are called the Little-Endians, and the followers of the latter are called the Big-Endians. The details of the holy war between the Little-Endians and the Big-Endians are documented in  and described, in brief, in the Appendix. I recommend that you read it at this point. [I have inserted it -- gnu] 13 A P P E N D I X Some notes on Swift's Gulliver's Travels: Gulliver finds out that there is a law, proclaimed by the grandfather of the present ruler, requiring all citizens of Lilliput to break their eggs only at the little ends. Of course, all those citizens who broke their eggs at the big ends were angered by the proclamation. Civil war broke out between the Little-Endians and the Big-Endians, resulting in the Big-Endians taking refuge on a nearby island, the kingdom of Blefuscu. Using Gulliver's unquestioning point of view, Swift satirizes religious wars. For 11,000 Lilliputian rebels to die over a controversy as trivial as at which end eggs have to be broken seems not only cruel but also absurd, since Gulliver is sufficiently gullible to believe in the significance of the egg question. The controversy is important ethically and politically for the Lilliputians. The reader may think the issue is silly, but he should consider what Swift is making fun of the actual causes of religious- or holy-wars. In political terms, Lilliput represents England and Blefuscu France. The religious controversy over egg-breaking parallels the struggle between the Protestant Church of England and the Catholic Church of France, possibly referring to some differences about what the Sacraments really mean. More specifically, the quarrel about egg-breaking may allude to the different ways that the Anglican and Catholic Churches distribute communion, bread and wine for the Anglican, but bread alone for the Catholic. The French and English struggled over more mundane questions as well, but in this part of Gulliver's Travels, Swift points up the symbolic difference between the churches to ridicule any religious war. For ease of reference please note that Lilliput and Little-Endians both start with an "L", and that both Blefuscu and Big-Endians start with a "B". This is handy while reading this note.] [End of appendix -- gnu] The above question arises from the serialization process which is performed on messages in order to send them through communication media. If the communication unit is a message - these problems have no meaning. If the units are computer "words" then one may ask in which order these words are sent, what is their size, but not in which order the elements of these words are sent, since they are sent virtually "at-once". If the unit of transmission is an 8-bit byte, similar questions about bytes are meaningful, but not the order of the elementary particles which constitute these bytes. If the units of communication are bits, the "atoms" ("quarks"?) of computation, then the only meaningful question is the order in which bits are sent. Obviously, this is actually the case for serial transmission. Most modern communication is based on a single stream of information ("bit-stream"). Hence, bits, rather than bytes or words, are the units of information which are actually transmitted over the communication channels such as wires and satellite connections. Even though a great deal of effort, in both hardware and software, is dedicated to giving the appearance of byte or word communication, the basic fact remains: bits are communicated. Computer memory may be viewed as a linear sequence of bits, divided into bytes, words, pages and so on. Each unit is a subunit of the next level. This is, obviously, a hierarchical organization. 2 If the order is consistent, then such a sequence may be communicated successfully while both parties maintain their freedom to treat the bits as a set of groups of any arbitrary size. One party may treat a message as a "page", another as so many "words", or so many "bytes" or so many bits. If a consistent bit order is used, the "chunk-size" is of no consequence. If an inconsistent bit order is used, the chunk size must be understood and agreed upon by all parties. We will demonstrate some popular but inconsistent orders later. In a consistent order, the bit-order, the byte-order, the word-order, the page-order, and all the other higher level orders are all the same. Hence, when considering a serial bit-stream, along a communication line for example, the "chunk" size which the originator of that stream has in mind is not important. There are two possible consistent orders. One is starting with the narrow end of each word (aka "LSB") as the Little-Endians do, or starting with the wide end (aka "MSB") as their rivals, the Big-Endians, do. In this note we usually use the following sample numbers: a "word" is a 32-bit quantity and is designated by a "W", and a "byte" is an 8-bit quantity which is designated by a "C" (for "Character", not to be confused with "B" for "Bit)". MEMORY ORDER The first word in memory is designated as W0, by both regimes. Unfortunately, the harmony goes no further. The Little-Endians assign B0 to the LSB of the words and B31 is the MSB. The Big-Endians do just the opposite, B0 is the MSB and B31 is the LSB. By the way, if mathematicians had their way, every sequence would be numbered from ZERO up, not from ONE, as is traditionally done. If so, the first item would be called the "zeroth".... Since most computers are not built by mathematicians, it is no wonder that some computers designate bits from B1 to B32, in either the Little-Endians' or the Big-Endians' order. These people probably would like to number their words from W1 up, just to be consistent. Back to the main theme. We would like to illustrate the hierarchically consistent order graphically, but first we have to decide about the order in which computer words are written on paper. Do they go from left to right, or from right to left? 3 The English language, like most modern languages, suggests that we lay these computer words on paper from left to right, like this: |---word0---|---word1---|---word2---|.... In order to be consistent, B0 should be to the left of B31. If the bytes in a word are designated as C0 through C3 then C0 is also to the left of C3. Hence we get: |---word0---|---word1---|---word2---|.... |C0,C1,C2,C3|C0,C1,C2,C3|C0,C1,C2,C3|..... |B0......B31|B0......B31|B0......B31|...... If we also use the traditional convention, as introduced by our numbering system, the wide-end is on the left and the narrow-end is on the right. Hence, the above is a perfectly consistent view of the world as depicted by the Big-Endians. Significance [consistently] decreases as the item numbers (address) increases. Many computers share with the Big-Endians this view about order. In many of their diagrams the registers are connected such that when the word W(n) is shifted right, its LSB moves into the MSB of word W(n+1). English text strings are stored in the same order, with the first character in C0 of W0, the next in C1 of W0, and so on. This order is very consistent with itself and with the English language. On the other hand, the Little-Endians have their view, which is different but also self-consistent. They believe that one should start with the narrow end of every word, and that low addresses are of lower order than high addresses. Therefore they put their words on paper as if they were written in Hebrew, like this: ...|---word2---|---word1---|---word0---| When they add the bit order and the byte order they get: ...|---word2---|---word1---|---word0---| ....|C3,C2,C1,C0|C3,C2,C1,C0|C3,C2,C1,C0| .....|B31......B0|B31......B0|B31......B0| In this regime, when word W(n) is shifted right, its LSB moves into the MSB of word W(n-1). 4 English text strings are stored in the same order, with the first character in C0 of W0, the next in C1 of W0, and so on. This order is very consistent with itself, with the Hebrew language, and (more importantly) with mathematics, because significance increases with increasing item numbers (address). It has the disadvantage that English character streams appear to be written backwards; this is only an aesthetic problem but, admittedly, it looks funny, especially to speakers of English. In order to avoid receiving strange comments about this orders the Little-Endians pretend that they are Chinese, and write the bytes, not right-to-left but top-to-bottom, like: C0: "J" C1: "O" C2: "H" C3: "N" ..etc.. Note that there is absolutely no specific significance whatsoever to the notion of "left" and "right" in bit order in a computer memory. One could think about it as "up" and "down" for example, or mirror it by systematically interchanging all the "left"s and "right"s. However, this notion stems from the concept that computer words represent numbers, and from the old mathematical tradition that the wide-end of a number (aka the MSB) is called "left" and the narrow-end of a number is called "right". This mathematical convention is the point of reference for the notion of "left" and "right". It is easy to determine whether any given computer system was designed by Little-Endians or by Big-Endians. This is done by watching the way the registers are connected for the "COMBINED-SHIFT" operation and for multiple-precision arithmetic like integer products; also by watching how these quantities are stored in memory; and obviously also by the order in which bytes are stored within words. Don't let the B0-to-B31 direction fool you!! Most computers were designed by Big-Endians, who under the threat of criminal prosecution pretended to be Little-Endians, rather than seeking exile in Blefuscu. They did it by using the B0-to-B31 convention of the Little-Endians, while keeping the Big-Endians' conventions for bytes and words. The PDP10 and the 360, for example, were designed by Big-Endians: their bit order, byte-order, word-order and page-order are the same. The same order also applies to long (multi-word) character strings and to multiple precision numbers. 5 Next, let's consider the new M68000 microprocessor. Its way of storing a 32-bit number, xy, a 16-bit number, z, and the string "JOHN" in its 16-bit words is shown below (S = sign bit, M = MSB, L = LSB): SMxxxxxxx yyyyyyyyL SMzzzzzzL "J" "O" "H" "N" |--word0--|--word1--|--word2--|--word3--|--word4--|.... |-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|..... |B15....B0|B15....B0|B15....B0|B15....B0|B15....B0|...... The M68000 always has on the left (i.e., LOWER byte- or word-address) the wide-end of numbers in any of the various sizes which it may use: 4 (BCD), 8, 16 or 32 bits. Hence, the M68000 is a consistent Big-Endian, except for its bit designation, which is used to camouflage its true identity. Remember: the Big-Endians were the outlaws. Let's look next at the PDP11 order, since this is the first computer to claim to be a Little-Endian. Let's again look at the way data is stored in memory: "N" "H" "O" "J" SMzzzzzzL SMyyyyyyL SMxxxxxxL ....|--word4--|--word3--|--word2--|--word1--|--word0--| .....|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-| ......|B15....B0|B15....B0|B15....B0|B15....B0|B15....B0| The PDP11 does not have an instruction to move 32-bit numbers. Its multiplication products are 32-bit quantities created only in the registers, and may be stored in memory in any way. Therefore, the 32-bit quantity, xy, was not shown in the above diagram. Hence, the above order is a Little-Endians' consistent order. The PDP11 always stores on the left (i.e., HIGHER bit- or byte-address) the wide-end of numbers of any of the sizes which it may use: 8 or 16 bits. However, due to some infiltration from the other camp, the registers of this Little-Endian's marvel are treated in the Big-Endians' way: a double length operand (32-bit) is placed with its MSB in the lower address register and the LSB in the higher address register. Hence, when depicted on paper, the registers have to be put from left to right, with the wide end of numbers in the LOWER-address register. This affects the integer multiplication and division, the combined-shifts and more. Admittedly, Blefuscu scores on this one. Later, floating-point hardware was introduced for the PDP11/45. Floating-point numbers are represented by either 32- or 64-bit quantities, which are 2 or 4 PDP11 words. The wide end is the one with the sign bit(s), the exponent and the MSB of the fraction. The narrow end is the one with the LSB of the fraction. On paper these formats are clearly shown with the wide end on the left and the narrow on the right, according to the centuries old mathematical conventions. On page 12-3 6 of the PDP11/45 processor handbook, , there is a cute graphical demonstration of this order, with the word "FRACTION" split over all the 2 or the 4 words which are used to store it. However, due to some oversights in the security screening process, the Blefuscuians took over, again. They assigned, as they always do, the wide end to the LOWer addresses in memory, and the narrow to the HIGHer addresses. Let "xy" and "abcd" be 32- and 64-bit floating-point numbers, respectively. Let's look how these numbers are stored in memory: ddddddddL ccccccccc bbbbbbbbb SMaaaaaaa yyyyyyyyL SMxxxxxxx ....|--word5--|--word4--|--word3--|--word2--|--word1--|--word0--| .....|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-| ......|B15....B0|B15....B0|B15....B0|B15....B0|B15....B0|B15....B0| Well, Blefuscu scores many points for this. The above reference in  does not even try to camouflage it by any Chinese notation. Encouraged by this success, as minor as it is, the Blefuscuians tried to pull another fast one. This time it was on the VAX, the sacred machine which all the Little-Endians worship. Let's look at the VAX order. Again, we look at the way the above data (with xy being a 32-bit integer) is stored in memory: "N" "H" "O" "J" SMzzzzzzL SMxxxxxxx yyyyyyyyL ...ng2-------|-------long1-------|-------long0-------| ....|--word4--|--word3--|--word2--|--word1--|--word0--| .....|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-| ......|B15....B0|B15....B0|B15....B0|B15....B0|B15....B0| What a beautifully consistent Little-Endians' order this is !!! So, what about the infiltrators? Did they completely fail in carrying out their mission? Since the integer arithmetic was closely guarded they attacked the floating point and the double-floating which were already known to be easy prey. 7 Let's look, again, at the way the above data is stored, except that now the 32-bit quantity xy is a floating point number: now this data is organized in memory in the following Blefuscuian way: "N" "H" "O" "J" SMzzzzzzL yyyyyyyyL SMxxxxxxx ...ng2-------|-------long1-------|-------long0-------| ....|--word4--|--word3--|--word2--|--word1--|--word0--| .....|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-|-C1-|-C0-| ......|B15....B0|B15....B0|B15....B0|B15....B0|B15....B0| Blefuscu scores again. The VAX is found guilty, however with the explanation that it tries to be compatible with the PDP11. Having found themselves there, the VAXians found a way around this unaesthetic appearance: the VAX literature (e.g., p. 10 of ) describes this order by using the Chinese top-to-bottom notation, rather than an embarrassing left-to-right or right-to-left one. This page is a marvel. One has to admire the skillful way in which some quantities are shown in columns 8-bit wide, some in 16 and other in 32, all in order to avoid the egg-on-the-face problem..... By the way, some engineering-type people complain about the "Chinese" (vertical) notation because usually the top (aka "up") of the diagrams corresponds to "low"-memory (low addresses). However, anyone who was brought up by computer scientists, rather than by botanists, knows that trees grow downward, having their roots at the top of the page and their leaves down below. Computer scientists seldom remember which way "up" really is (see 2.3 of , pp. 305-309). Having scored so easily in the floating point department, the Blefuscuians moved to new territories: Packed-Decimal. The VAX is also capable of using 4-bit-chunk decimal arithmetic, which is similar to the well known BCD format. The Big-Endians struck again, and without any resistance got their way. The decimal number 12345678 is stored in the VAX memory in this order: 7 8 5 6 3 4 1 2 ...|-------long0-------| ....|--word1--|--word0--| .....|-C1-|-C0-|-C1-|-C0-| ......|B15....B0|B15....B0| This ugliness cannot be hidden even by the standard Chinese trick. 8 SUMMARY (of the Memory-Order section) To the best of my knowledge only the Big-Endians of Blefuscu have built systems with a consistent order which works across chunk-boundaries, registers, instructions and memories. I failed to find a Little-Endians' system which is totally consistent. TRANSMISSION ORDER In either of the consistent orders the first bit (B0) of the first byte (C0) of the first word (W0) is sent first, then the rest of the bits of this byte, then (in the same order) the rest of the bytes of this word, and so on. Such a sequence of 8 32-bit words, for example, may be viewed as either 4 long-words, 8 words, 32 bytes or 256 bits. For example, some people treat the ARPA-internet-datagrams as a sequence of 16-bit words whereas others treat them as either 8-bit byte streams or sequences of 32-bit words. This has never been a source of confusion, because the Big-Endians' consistent order has been assumed. There are many ways to devise inconsistent orders. The two most popular ones are the following and its mirror image. Under this order the first bit to be sent is the LEAST significant bit (B0) of the MOST significant byte (C0) of the first word, followed by the rest of the bits of this byte, then the same right-to-left bit order inside the left-to-right byte order. Figure 1 shows the transmission order for the 4 orders which were discussed above, the 2 consistent and the 2 inconsistent ones. Those who use such an inconsistent order (or any other), and only those, have to be concerned with the famous byte-order problem. If they can pretend that their communication medium is really a byte-oriented link then this inconsistency can be safely hidden under the rug. A few years ago 8-bit microprocessors appeared and changed drastically the way we do business. A few years later a wide variety of 8-bit communication hardware (e.g., Z80-SIO and 2652) followed, all of which operate in the Little-Endians' order. 9 Now a wave of 16-bit microprocessors has arrived. It is not inconceivable that 16-bit communication hardware will become a reality relatively soon. Since the 16-bit communication gear will be provided by the same folks who brought us the 8-bit communication gear, it is safe to expect these two modes to be compatible with each other. The only way to achieve this is by using the consistent Little-Endians order, since all the existing gear is already in Little-Endians order. We have already observed that the Little-Endians do not have consistent memory orders for intra-computer organization. IF the 16-bit communication link could be made to operate in any order, consistent or not, which would give it the appearance of being a byte- oriented link, THEN the Big-Endians could push (ask? hope? pray?) for an order which transmits the bytes in left-to-right (i.e., wide-end first) and use that as a basis for transmitting all quantities (except BCD) in the more convenient Big-Endians format, with the most significant portions leading the least significant, maintaining compatibility between 16- and 32-bit communication, and more. However, this is a big "IF". Wouldn't it be nice if we could encapsulate the byte-communication and forget all about the idiosyncrasies of the past, introduced by RS232 and TELEX, of sending the narrow-end first? I believe that it would be nice, but nice things do not necessarily occur, especially if there is so much silicon against them. Hence, our choice now is between (1) Big-Endians' computer-convenience and (2) future compatibility between communication gear of different chunk size. I believe that this is the question, and we should address it as such. Short term convenience considerations are in favor of the former, and the long term ones are in favor of the latter. Since the war between the Little-Endians and the Big-Endians is imminent, let's count who is in whose camp. The founders of the Little-Endians party are RS232 and TELEX, who stated that the narrow-end is sent first. So do the HDLC and the SDLC protocols, the Z80-SIO, Signetics-2652, Intel-8251, Motorola-6850 and all the rest of the existing communication devices. In addition to these protocols and chips the PDP11s and the VAXes have already pledged their allegiance to this camp, and deserve to be on this roster. 10 The HDLC protocol is a full fledged member of this camp because it sends all of its fields with the narrow end first, as is specifically defined in Table 1/X.25 (Frame formats) in section 2.2.1 of Recommendation X.25 (see ). A close examination of this table reveals that the bit order of transmission is always 1-to-8. Always, except the FCS (checksum) field, which is the only 16-bit quantity in the byte-oriented protocol. The FCS is sent in the 16-to-1 order. How did the Blefuscuians manage to pull off such a fiasco?! The answer is beyond me. Anyway, anyone who designates bits as 1-to-8 (instead of 0-to-7) must be gullible to such tricks. The Big-Endians have the PDP10's, 370's, ALTO's and Dorado's... An interesting creature is the ARPANet-IMP. The documentation of its standard host interface (aka "LH/DH") states that "The high order bit of each word is transmitted first" (p. 4-4 of ), hence, it is a Big-Endian. This is very convenient, and causes no confusion between diagrams which are either 32- (e.g., on p. 3-25) and 16-bit wide (e.g., on p. 5-14). However, the IMP's Very Distant Host (VDH) interface is a Little-Endian. The same document (, again, p. F-18), states that the data "must consist of an even number of 8-bit bytes. Further, considering each pair of bytes as a 16-bit word, the less significant (right) byte is sent first". In order to make this even more clear, p. F-23 states "All bytes (data bytes too) are transmitted least significant (rightmost) bit first". Hence, both camps may claim to have this schizophrenic double-agent in their camp. Note that the Lilliputians' camp includes all the who's-who of the communication world, unlike the Blefuscuians' camp which is very much oriented toward the computing world. Both camps have already adopted the slogan "We'd rather fight than switch!". I believe they mean it. 11 SUMMARY (of the Transmission-Order section) There are two camps each with its own language. These languages are as compatible with each other as any Semitic and Latin languages are. All Big-Endians can talk to each other with relative ease. So can all the Little-Endians, even though there are some differences among the dialects used by different tribes. There is no middle ground. Only one end can go first. CONCLUSION Each camp tries to convert the other. Like all the religious wars of the past, logic is not the decisive tool. Power is. This holy war is not the first one, and probably will not be the last one either. The "Be reasonable, do it my way" approach does not work. Neither does the Esperanto approach of "let's all switch to yet a new language". Our communication world may split according to the language used. A certain book (which is NOT mentioned in the references list) has an interesting story about a similar phenomenon, the Tower of Babel. Little-Endians are Little-Endians and Big-Endians are Big-Endians and never the twain shall meet. We would like to see some Gulliver standing up between the two islands, forcing a unified communication regime on all of us. I do hope that my way will be chosen, but I believe that, after all, which way is chosen does not make too much difference. It is more important to agree upon an order than which order is agreed upon. How about tossing a coin ??? 12 time time | | \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / \ | | / <-MSB---------------LSB- -MSB---------------LSB-> order (1) | | order (2) time time | | / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ / | | \ <-MSB---------------LSB- -MSB---------------LSB-> order (3) | | order (4) Figure 1: Possible orders, consistent: (1)+(2), inconsistent: (3)+(4). 14 R E F E R E N C E S  Bolt Beranek & Newman. Report No. 1822: Interface Message Processor. Technical Report, BB&N, May, 1978.  CCITT. Orange Book. Volume VIII.2: Public Data Networks. International Telecommunication Union, Geneva, 1977.  DEC. PDP11 04/05/10/35/40/45 processor handbook. Digital Equipment Corp., 1975.  DEC. VAX11 - Architecture Handbook. Digital Equipment Corp., 1979.  Knuth, D. E. The Art of Computer Programming. Volume I: Fundamental Algorithms. Addison-Wesley, 1968.  Swift, Jonathan. Gulliver's Travel. Unknown publisher, 1726. 15 OTHER SLIGHTLY RELATED TOPICS (IF AT ALL) not necessarily for inclusion in this note Who's on first? Zero or One ?? People start counting from the number ONE. The very word FIRST is abbreviated into the symbol "1st" which indicates ONE, but this is a very modern notation. The older notions do not necessarily support this relationship. In English and French - the word "first" is not derived from the word "one" but from an old word for "prince" (which means "foremost"). Similarly, the English word "second" is not derived from the number "two" but from an old word which means "to follow". Obviously there is an close relation between "third" and "three", "fourth" and "four" and so on. Similarly, in Hebrew, for example, the word "first" is derived from the word "head", meaning "the foremost", but not specifically No. 1. The Hebrew word for "second" is specifically derived from the word "two". The same for three, four and all the other numbers. However, people have,for a very long time, counted from the number One, not from Zero. As a matter of fact, the inclusion of Zero as a full-fledged member of the set of all numbers is a relatively modern concept. Zero is one of the most important numbers mathematically. It has many important properties, such as being a multiple of any integer. A nice mathematical theorem states that for any basis, b, the first b^N (b to the Nth power) positive integers are represented by exactly N digits (leading zeros included). This is true if and only if the count starts with Zero (hence, 0 through b^N-1), not with One (for 1 through b^N). This theorem is the basis of computer memory addressing. Typically, 2^N cells are addressed by an N-bit addressing scheme. Starting the count from One, rather than Zero, would cause either the loss of one memory cell, or an additional address line. Since either price is too expensive, computer engineers agree to use the mathematical notation of starting with Zero. Good for them! The designers of the 1401 were probably ashamed to have address-0 and hid it from the users, pretending that the memory started at address-1. 16 This is probably the reason that all memories start at address-0, even those of systems which count bits from B1 up. Communication engineers, like most "normal" people, start counting from the number One. They never suffer by having to lose a memory cell, for example. Therefore, they are happily counting 1-to-8, and not 0-to-7 as computer people learn to do. ORDER OF NUMBERS. In English, we write numbers in Big-Endians' left-to-right order. I believe that this is because we SAY numbers in the Big-Endians' order, and because we WRITE English in Left-to-right order. Mathematically there is a lot to be said for the Little-Endians' order. Serial comparators and dividers prefer the former. Serial adders and multipliers prefer the latter order. When was the common Big-Endians order adopted by most modern languages? In the Bible, numbers are described in words (like "seven") not by digits (like "7") which were "invented" nearly a thousand years after the Bible was written. In the old Hebrew Bible many numbers are expressed in the Little-Endians order (like "Seven and Twenty and Hundred") but many are in the Big-Endians order as well. Whenever the Bible is translated into English the contemporary English order is used. For example, the above number appears in that order in the Hebrew source of The Book of Esther (1:1). In the King James Version it is (in English) "Hundred and Seven and Twenty". In the modern Revised American Standard Version of the Bible this number is simply "One Hundred and Twenty-Seven". INTEGERS vs. FRACTIONS Computer designers treat fix-point multiplication in one of two ways, as an integer-multiplication or as a fractional-multiplication. The reason is that when two 16-bit numbers, for example, are multiplied, the result is a 31-bit number in a 32-bit field. Integers are right justified; fractions are left justified. The entire difference is only a single 1-bit shift. As small as it is, this is an important difference. Hence, computers are wired differently for these kinds of multiplications. The addition/subtraction operation is the same for either integer/fraction operation. 17 If the LSB is B0 then the value of a number is SIGMA<B(i)*[(2)^i]>, for i=0,15, in the above example. This is, obviously, an integer. If the MSB is B0 then the value of a number is SIGMA<B(i)*[(1/2)^i]>, for i=0,15. This is, obviously, a fraction. Hence, after multiplication the Integerites would typically keep B0-B15, the LSH (Least Significant Half), and discard the MSH, after verifying that there is no overflow into it. The Fractionites would also keep B0-B15, which is the MSH, and discard the LSH. One could expect Integerites to be Little-Endians, and Fractionites to be Big-Endians. I do not believe that the world is that consistent. SWIFT's POINT It may be interesting to notice that the point which Jonathan Swift tried to convey in Gulliver's Travels in exactly the opposite of the point of this note. Swift's point is that the difference between breaking the egg at the little-end and breaking it at the big-end is trivial. Therefore, he suggests, that everyone does it in his own preferred way. We agree that the difference between sending eggs with the little- or the big-end first is trivial, but we insist that everyone must do it in the same way, to avoid anarchy. Since the difference is trivial we may choose either way, but a decision must be made. ***** An editied version of this note appears in Computer Magazine (IEEE) of October 1981. ***** -- Regards Tim EcclesOther copies of this essay (variously known as "IEN 137" or "ON HOLY WARS AND A PLEA FOR PEACE") on the net:
(With so many copies, why did I feel I had to put this one online ? Because none of the others give it enough context.)
Resent-From: pci-sig-request at znyx.com Resent-Date: Thu, 25 Jan 1996 14:51:14 -0800 From: John R Pierce <pierce at diamondmm.com> Cc: "'pci-sig at znyx.com'" <pci-sig at znyx.com> Subject: RE: Big Endian question Date: Thu, 25 Jan 1996 14:51:14 -0800 Encoding: 41 TEXT Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> ROTFL! Ya know that article misses one 'cute' tweak that DEC was doing with the VAX to resolve the 'backwardness' of ascii printed in little-endian words... They did their HEX dumps with byte 0 on the right, but the *-A-x-xxx-A-* stuff on the right was left to right.... They put the address between the two. This way words and dwords read l-r, and you could read the ascii too... F E D C B A 9 8 7 6 5 4 3 2 1 0 -ADDR- 0123456789ABCDEF 504F4E4D 4C4B4A49 48474645 44434241 00230040 *ABCEDFGHIJKLMNOP* 706F6E6D 6C6B6A69 68676665 64636261 00230050 *abcdefghijklmnop* It took a little getting used to (about 5 minutes ;) -jrp ----------
Should bridges automatically convert for you ? No !
BLlib http://members.tripod.com/~szanella/bllibeng.htm is a C++ library that can be used to transparently exchange alphanumeric and binary information between machines that have different architectures (Big endian - Little endian), hence its name (Big-Little library).
Resent-From: pci-sig-request at znyx.com Resent-Date: Tue, 14 Apr 1998 15:05:55 -0700 Date: Tue, 14 Apr 1998 14:57:49 -0700 (PDT) From: Phil Ronzone <phil at netcom.com> Subject: Big and little endian issues Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> > Philip Ronzone wrote: > > -- snip -- > > > Having worked with more than one piece of hardware that tried to "help" > > by doing byte swapping, I can only say "DON'T DO THAT!". > > I disagree. > > At V3 Semiconductor we have a line of bridge chips that perform on-the-fly > endian conversion for data moving between the PCI bus and a local processor > bus, or vice versa (sorry for the plug; I just wanted to have substance for my > example). OK. I'm sure that you do. Great. Etc. Now, unless your bridge chips are transferring ONLY monotonic data, also useless. Just HOW does your bridge chip KNOW which fields in the data stream are little-endian 16 and should NOT be swapped, big-endian 16 and DO need to be swapped, and 8-bit ASCII text, should not be touched? Oh, and how about 32-bit big-endian? That's NOT a case of just doing adjacent 16-bit swaps. And 64-bit big endian integer? Does your bridge handle these cases? I doubt it. If it does, PLEASE let me know -- it would be a MARVEL! No hardware can ever handle endian issues, UNLESS IT KNOWS the context of the data -- what is big-endian, what is little endian, what is text data, what is 16/32/64 bits, and so on. Too often I've seen a hardware "designer" mumble something like "oh, PCI is little endian, this is a big-endian processor, so I'll swap those bytes". Wrong. There are some very nice chips out there that can be programmed to both accept big/little endian addresses AND big/little descriptors in memory. They work really nice. Except when a hardware guy throws in a gratuitous swap. > More importantly, we have many customers that are happy with the endian- > conversion features of our bridges. They would surely disagree with your > advice. OK. Good for you. Since I'm not buying your product, then you should ignore me. Right? However, are you talking to the hardware guys that selected the chip, or did you talk to the SOFTWARE guys programming the chip? > > Byte swapping is inherently contextual. No hardware mechanism can ever > > know if those "next 4 bytes" are a text string and should NOT be swapped, > > or if those 4 bytes are "wrong endian" and should be swapped. > > > > The net effect (of hardware-provided swapping) is sometimes horrible. > > I agree, endian conversion is inherently contextual. > > However, in many applications, it usually fairly simple for software to arrange > to transfer blocks of similar-kind data (bytes, words, etc), and then engage > our DMA engines (with endian-conversion hardware) to deliver it without any > loss in performance. Where it is not possible (or not practical) to ensure that a > significant block has a uniform data type, software may be required to massage > a *portion* of the data, and then let the bridge's swapping hardware do the rest. Yes, altho simple is misleading. Time consuming is more apprropriate. Since EITHER WAY, software is going to have to swap something, somewhere, why noy just drop the hardware swap. It just adds to confusion. > For example, when a device chooses to DMA a block of string data, the > device can be programmed to perform swapping on a byte boundary. When > transferring a block of 16-bit shorts, the device can be programmed to swap > on a 16-bit boundary, etc. Furthermore, by having endian-conversion for > DMA data programmable through the DMA descriptor, the bridge does not > have to be re-programmed before transferring each block of data. This coupled > with DMA-chaining permits the transfer of several blocks of data with like > intra-block data types, but unlike inter-block data types to be converted and > delivered efficiently. Well, monotonic data (like, all 32-bit x-edian values) is rare. But, just how would a low level device drive KNOW what data is what? A disk driver operates at the level of "read or write block N. Here's the buffer address". This *IS* the whole point. I perhaps should have expanded on it first. I'm a software guide. HARDWARE PEOPLE, MOST OF THE TIME, DO NOT UNDERSTAND WHAT IS GOOD FOR PROGRAMMING (and vice versa I suppose). HARDWARE BYTE SWAPPING DOES MORE HARM THAN GOOD. I personally think that homocide should be legal ( :-) ) if one more hardware guys tells a software guy to "make it correct in software! Kaboom - like, just what is DMA for in the first place? Sure, a program CAN do all that extra, data massaging stuff, and burn lots of CPU cycles and delay things, but, I rather think hardware should do it right. (As an allied example, consider DMA hardware that requires I/O to start on some power of two, and have a length a power of two -- IDE controllers for example. Do you know how LONG it takes to align a buffer when a 120K byte IDE drive read starts on an odd byte address? A lot!) > > > In a system that swaps to/from PCI, but not to/from memory, I found > > that bi-endian devices (i.e., the DEC2114x Ethernet chips) were made > > "slow". The DEC2114x can specify either endian for both data and > > descriptor lists, but since the hardware didn't consistently swap, > > either the data or the descriptors always had to be swapped in > > software. > > > > Ugh. > > > > Thus, I assert that hardware swapping is useless. > > I have to disagree with Phillip's assertion. It is unfortunate that he has had > such bad experiences with devices that perform hardware endian conversion. > I would agree that, for the contextual reasons mentioned above, endian > conversion hardware is not the holy-grail for endian conversion -- it is simply > an accelerator to perform simple endian conversion cases without losing any > performance to those cases. It's not. If you're JUST transferring monotonic data over your chips, like a series of 32-bit big-endian integers, then you're fine. But, from your description, you'd be seeing all kinds of data. Data that has all kinds of formats, should that can be swapped, and some that can't. So why not leave it alone? It just makes it confusing. > Certainly, not every application will be able to get away from doing some endian > conversion in software. Nevertheless, even if you could only use the endian- > conversion hardware on 50% of the data being moved across the bridge (you > should be able to do much better), then you only take the hit in performance > on the unaccelerated 50% (or less) of the data.Please, don't discount the > performance gains that are to be had simply because > the hardware cannot handle every possible case. > > Michael Tresidder > V3 Semiconductor > tresidd at vcubed.com Just HOW is this "50%" to be determined? The low-level driver has NO knowledge of the data structure, so it can't do it. The application can't always do it, and it shouldn't do it (for example, you embed say, an Adobe postscript engine, which comes in object only format), because the source code should be the same for a big and little endian machine, so who would do it? Also, if I read your post correctly, you seem to imply that your bridge chip does only (??) 16-bit endian swapping. Is this right? "Cause that would screw up 32-bit big endian numbers, right? SUMMARY ------- Hardware, such as bridge chips per above, should NOT attempt to be smart and endian swaps. It only scrambles the data more. Hardware is better when I/O can be done by descriptor chains. Those descriptors are best when any byte alignment and any lenght can be any value. Please, end the alignment & modulo length restrictions. REFOCUS ------- Of course, perhaps I'm all wrong. So tell me, if one is reading a little-endian data IDE disk drive, with a DOS/W95 FAT-16 partition, on a MIPS big-endian system, just HOW would a bridge chip with endian swapping be useful? 1 - 16-bit only swapping will screw up 32-bit values. 2 - 32-bit only swapping will screw up 16-bit values. 3 - Swapping at all screws up byte fields (strings etc.). You haven't lived until you've programmed code on a big-endian machine to "reaasemble" bytes strings that were 32-bit endian swapped BUT read/written as 16-bit numbers. Confusion and complexity that would have been avoided by NOT trying to be helpful.
Resent-From: pci-sig-request at znyx.com Resent-Date: Thu, 25 Jan 1996 23:29:08 -0800 Mime-Version: 1.0 Date: Thu, 25 Jan 1996 23:29:08 -0800 From: David B Gustavson <dbg at sunrise.scu.edu> Subject: Resolving the Big endian question Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> I don't want to repeat my earlier posting on this, but just to alert the newcomers to the discussion: After many years of discussing these issues in the IEEE Bus Standards, it finally became clear (once one looks from a large-system viewpoint) that the solution has to be: The bridge between any two buses preserves the relative byte addresses of all data flowing through. (This is sometimes called the Address Invariance principle). That's what it takes to preserve character strings as they flow through a complex system. The point of this axiom is not to imply that character strings are particularly important--most users strongly want to preserve m-bit addresses or n-bit integers or whatever happens to be important for their application's efficiency. But the bridge never has enough information to be able to do this correctly in all situations. The best you can do is stop bridges from scrambling the data at each crossing in vain attempts to fixup certain kinds of data at the expense of others. If bridges obey the Address Invariance principle, then all you have to know is the meaning of the data where created, and the format such data needs on the destination machine, for software to correctly interpret all the data. Without that principle, you also have to know the state of all byte swappers in all bridges the data moved through at the time that data passed by, and also the path the data happened to take on that occasion. That is impossible in realistic complex systems. The missing piece was: how to tell the compiler on your destination machine what the original meaning of the data was. That problem was solved by ANSI/IEEE Std 1596.6-1993, which adds the syntax needed to handle the Endian and some other similar problems (like atomicity) that can break software drivers etc if not handled correctly. One of the beauties of this solution is that it is quite efficient: if data moves through an arbitrary world back into a processor like its originator, there is zero overhead. If it moves to a processor with different conventions, conversion costs are minimal, only affect the subset of the data that are used, and allow the data (plus possible computational enhancements) to flow onward to other destinations without further confusion. There's a sink hole under this well-plowed ground. Don't fall in. Don't pay the price of rediscovering all this again--it costs a LOT. It's very hard to have a broad enough perspective while designing the early chips in any new technology to see the downstream implications. We had to resolve these issues for SCI LAMP, because it is fundamentally based on a heterogeneous world with links and buses of many widths, processors of all types, growing like Topsy over time and technologies but interoperating and evolving. Fortunately, the IEEE groups had wrestled with the problems for many years, with S-100, Multibus, Multibus 2, VME, FutureBus, NuBus, SerialBus, etc etc etc., and the solution finally came together. (This is a very slippery problem because it is so confusing--many things that seem like Endian problems are really not, are merely relabeling of buses and data. Here and there labeling gets clarified, wherever addresses are multiplexed with data, but even that is not really fundamental. The fundamental thing is really how the externally addressed bytes end up in multibyte processor registers. The carry propagation direction in the register defines significance unambiguously.) --David B. Gustavson phone 415/961-0305 fax 415/961-3530 SCI (ANSI/IEEE Std 1596 Scalable Coherent Interface) chairman Exec. Director, SCIzzL: Assoc. SCI Local-Area MultiProcessor Users 1946 Fallen Leaf Lane, Los Altos, CA 94024-7206 <dbg at sunrise.scu.edu> For more info on SCI etc., see the Web: http://sunrise.scu.eduhttp://sunrise.scu.edu/
Resent-From: pci-sig-request at znyx.com Resent-Date: Tue, 23 Jan 1996 18:24:40 GMT From: ingvar.berg.swe3650 at oasis.icl.co.uk Date: Tue, 23 Jan 1996 18:24:40 GMT Reply-To: ingvar.berg.swe3650 at oasis.icl.co.uk Subject: RE: Endian strategy (was Re: PCI and bi-endian hosts) Priority: NORMAL Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> I saved the following explanation that appeared on this mail-list some time ago. Seems to me to be the FAQ needed. /Ingvar ---------- > We wrestled with these problems in various IEEE bus standards for over 10 years before we understood that there is only one solution that works in a general way, without being an ad hoc kluge that breaks when the system configuration gets changed. It becomes really obvious when you look beyond a single bridge and think of more general interconnected systems that use serial buses or cable/fiber buses of various widths to connect subsystems. The bridge never has enough information to do the conversions properly. If you cared enough, you could give it the information, by using fully tagged data as is done in some network protocols, but that will be slow and expensive, and requires a lot of software and processor architecture support to deliver that information to the hardware. That's not a practical solution. The solution is to never swap bytes in intermediate hardware, always delivering each byte in a multi-byte data type at its original relative address. I.e., byte 0 of a 64-bit integer on a little endian machine ends up in byte 0 on a big endian machine too. This clearly works fine for character strings, and scrambles everything else. The key is, it scrambles everything in a simple predictable way, which is not true if intermediate hardware reorders the bytes--then you have to know the behavior of every byte-swapper in the data path at the moment your data were transferred, a really impossible problem in complex systems. So, let the hardware preserve relative byte addresses. Then augment the data description information you feed your compiler so that it knows the original format of the data. For example, "integer" is uselessly vague--you need to know how wide the integer is, and whether it is big-endian or little-endian on the source machine. With that information, the compiler, knowing what kind of machine it is running on, can generate the right kind of loads/stores and swaps to make the computations work correctly. Without that information, or with hardware that swaps bytes, the job of untangling this always ends up in the hands of the programmer, and in my experience is a messy ad hoc job that is very hard to get right and keep right as the system evolves. It's a major time sink. So, we created a standard for the extra descriptive info you need to give the compiler. I think some experimental compilers have implemented these features, but I don't know their availability. Macros might be sufficient initially, even without compiler support. The standard is: ANSI/IEEE 1596.5-1993 IEEE Standard for Shared Data Formats Optimized for Scalable Coherent Interface (SCI) Processors Don't let the title fool you--the only thing SCI-specific in this is the name (limiting the scope made the project finite so it could actually be completed in time to be useful). The significance of SCI here is only that SCI supports distributed multiprocessing with heterogeneous processors, mixtures of big and little endian, with various bus widths, with arbitrary-width interconnects, with bridges, switches, adapters, etc., all evolving over time, with any mix of message-passing, shared memory, DMA, etc. SCI also places a very high premium on efficiency and low latency, every nanosecond counts in the memory path of a supercomputer. If there were a hardware solution, we would have used it. I see a lot of bridges that have swapping built in, and often swapping that can be turned on and off. If those ever get incorporated into larger systems that allow distributed I/O, data striping, etc they are going to create an incredible mess. Job security for driver writers... and added latency that will let cleaner-living competitors outrun you. Dave Gustavson p.s. Endianness isn't the only problem. In order to compute correctly, one has to convert data formats in many (context dependent) ways. E.g., ASCII-EBCDIC, integer widths (& signed/unsigned), floating point (IBM, DEC, IEEE, etc). Also, software needs to understand which transfers are "atomic"--it can break the OS if it writes an 8-byte pointer without realizing that the hardware breaks that into two 4-byte stores that might be separated by a read from a DMA controller or another processor. 1596.5 deals with all these issues too. --David B. Gustavson phone 415/961-0305 fax 415/961-3530 SCI (ANSI/IEEE Std 1596 Scalable Coherent Interface) chairman Exec. Director, SCIzzL: Assoc. SCI Local-Area MultiProcessor Users 1946 Fallen Leaf Lane, Los Altos, CA 94024-7206 <dbg at sunrise.scu.edu> For more info on SCI etc., see the Web: http://sunrise.scu.eduhttp://sunrise.scu.edu/
( see vlsi.html#pci for more PCI information.)
Is PCI inherently big-endian or little-endian ? While PCI is inherently little-endian (the first byte of memory is multiplexed with the least-significant 8 bits of the address), there are standard techniques for connecting big-endian and bi-endian hosts to the PCI bus. ("Address invariance" is the best technique).
Resent-From: pci-sig-request at znyx.com Resent-Date: Fri, 19 Jan 1996 18:22:00 -0700 From: "Monish Shah" <monish at mcsy2.fc.hp.com> Date: Fri, 19 Jan 1996 18:22:00 -0700 Subject: Re: Big Endian question Mime-Version: 1.0 Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> On Jan 19, 2:47pm, Jim Holeman wrote: > Subject: Big Endian question > > How are various designers and systems handling PCI designs interfacing to > systems with Big Endian processors (especially where the Big Endian > processor is using a 64 bit bus)? This is actually a very complicated problem with no easy answers. In fact, exactly what you have to do is very dependent on the specific function you are trying to implement and how your driver level software works. However, one trick that is likely to go a long way is the following: Request twice as much address space as you would have. Map in all your registers (or memory or whatever) twice - once with little endian byte ordering, another time with big endian byte ordering. Software can then choose which view it prefers. Note that this is much better than doing the byte swapping with a mode bit. You will inevitably run into cases where software will want some registers to be byte swapped but not others. If both versions are accessible simultaneously, software can pick which copy it will talk to for each register and choose the address accordingly. That is much easier than having to switch the mode bit. The next question is, do you swap bytes only within a 4 byte field or do you swap all 8 adjacent bytes as a single field, or what. The general answer is, swap to the natural size of the register. Example of 4 byte swapping: Bytes numbered 0123 4567 appear as 3210 7654. Example of 8 byte swapping: Bytes numbered 0123 4567 appear as 7654 3210. If you have a pair of 32 bit registers, follow example #1. If you have one 64 bit register, follow example #2. Other sizes are left as an exercise for the reader. My answer above was aimed at solving the problem for CPU accesses to a card. DMA is a different story. For DMA, the answer depends on the function. For a SCSI card, it is probably reasonable to not do any byte swapping on DMA because the data that you are DMAing is probably already in the correct byte order. I.e., if the system is big endian, it probably stored the data on the disk in big endian format to begin with, so it matches. Same goes for little endian. > holeman > > ____________________________________________________________________________ > > Jim Holeman Tandem Computers, Inc. > (512) 432-8755 (fax 8247) 14231 Tandem Boulevard > holeman at isd.tandem.com Austin, Tx 78728-6699 > > "A gentle answer turns away wrath" Monish Shah Hewlett Packard
Resent-From: pci-sig-request at znyx.com Resent-Date: Thu, 30 May 1996 10:59:18 +1000 Date: Thu, 30 May 1996 10:59:18 +1000 From: bull at highland.com.au (Geoff Bull) Subject: Re: Byte Lanes Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> Steve Belvin wrote: > I am developing a PCI expansion bridge and am dealing with byte ordering > issues. My expansion bus requires byte 0 to be on the lines 7:0 of the bus. > My experience says this is an unusual requirement for a bus. I am implementing > configuration registers internally that use both big endian and little endian > formats in the specifications which define them. > > I have also decided I need to implement byte swapping within my expansion > bridge in order to make big endian and little endian processor DWord formats > happy. I expect, however, for this capability to be used by few. The > expansion bridge will likely maintain byte 0 in lines 7:0 on both buses. > > My question is: > > Does PCI require byte 0 always be placed on lines 7:0 of the bus? > > I have looked the spec. over and do not see where this is expressed so directly > as it is in my expansion bus standard. An example where putting byte 0 on PCI > lines 31:24 may be desired is configuring the big-endian format registers. I > would prefer to configure those directly in software and not get involved in > byte swapping and byte lane issues - these can be one of the worst hazards to > be involved in. > > Steve Belvin Endian issues have been done to death here and you will find it all in the archive. I dredged out the following: >Resent-From: pci-sig-request at znyx.com >Resent-Date: Thu, 25 Jan 96 08:55:00 PST > >Gerhard Petrowitsc wrote: >>What I may have confused you with is perhaps that I said how the bytes are >>swapped. (0-7, 1-6 etc.) >>Sorry for that, I should have put it that way: If you connect a little and a >>big endian bus and want >>to keep the byte ordering ("first byte in a string is at address 0"), you >>must connect the busses >>as shown below (for 32bit busses) >> >> Little Endian bus Big Endian bus >> >> byte 0 Bit 7..0 <-> Bit 31..24 byte 0 >> byte 1 Bit 15..8 <-> Bit 23..16 byte 1 >> byte 2 Bit 23..16 <-> Bit 15..8 byte 2 >> byte 3 Bit 31..24 <-> Bit 7..0 byte 3 Wen-King Su wrote: >To confuse things even further, if A & D are multiplexed, during the >address phase the bus has to be reconnected like this: > > Little Endian bus Big Endian bus > byte 0 (LSB) Bit 7..0 <-> (LSB) Bit 7..0 byte 3 > byte 1 Bit 15..8 <-> Bit 15..8 byte 2 > byte 2 Bit 23..16 <-> Bit 23..16 byte 1 > byte 3 (MSB) Bit 31..24 <-> (MSB) Bit 31..24 byte 0 This is implementing what is called address invariance. Do anything else and your programmers will hate you forever! It prevents the address map of the little ended pci bus from being scrambled from your big-ended cpu's point of view. The programmer must do byte swaps when doing programmed IO. The above tranformation takes care of dma. If the cpu is bi-endian put any internal registers in the bridge on the little ended side so that their address map doesn't change when the cpu changes mode. You could take a look at some the PowerPC documents at: ftp://ftp.austin.ibm.com/pub/technology/spec It clearly specifies how to interface a bi-endian cpu to PCI. ============================== Geoff Bull, Hardware Engineer TPG Server Pty Ltd Suite 1, 348 Argyle Street, Moss Vale, NSW, 2577 AUSTRALIA Phone: +61 48 69 2066 Fax: +61 48 69 2080 Timezone: GMT+10 (Australia/NSW) Email: Geoff Bull <bull at highland.com.au>ftp://ftp.austin.ibm.com/pub/technology/spec
Controller Area Network (CAN) serialportdocs.html#can is Big Endian for this reason: by transmitting the most-significant bits first, the highest-priority messages will always gain control of the bus without "destructive" collisions. This relies on the fact that comparing 2 numbers gives the correct answer quickest by starting from the most-significant bit.
Most other protocols arbitrarily picked an endianness, for no particularly good reason.
The 68HC16 and 68HC11 controllers have 2 serial completely different serial ports:
The Intel x86 series of processors are little endian machines (therefore Linux, MS-DOS, MS-Windows, etc. that run on them are little-endian operating systems)
Most Unix machines (HP, Digital..) are big endian.
Most Motorola CPUs are big-endian.
/* ---------------- Start of Machine Configuration Section ----------------- */ /* The great thing about standards is that there are so many to choose from! */ #ifdef m68k #define mc68000 #endif /* Both Sun3 and SPARC machines turn on LITTLE_ENDIAN!!! Buy a DECstation, you * bozos. */ #ifdef BIG_ENDIAN #undef BIG_ENDIAN #endif #ifdef LITTLE_ENDIAN #undef LITTLE_ENDIAN #endif /* ------- Configuration: Selection of Byte Ordering ------- */ /* Big Endian: * MSB....................LSB * byte0 byte1 byte2 byte3 * * Little Endian: * MSB....................LSB * byte3 byte2 byte1 byte0 * * In big-endian, a pointer to a word points to the byte that * contains the most-significant-bit of the word. In little-endian, * it points to the byte containing the least-significant-bit. * */ #ifdef linux #define LITTLE_ENDIAN /* Intel x86 processors running Linux >=.99p7. */ #define sigvec sigaction #define sv_handler sa_handler #endif #ifdef vax #define LITTLE_ENDIAN /* The good 'ol VAX. */ #endif #ifdef MIPSEL #define LITTLE_ENDIAN /* MIPS processors in little-endian mode. */ #endif #ifdef wrltitan #define LITTLE_ENDIAN /* A DEC-WRL titan research machine (only 20 exist). */ /* NOT intended for the Ardent titan machine. */ #endif #ifdef MIPSEB #define BIG_ENDIAN /* MIPS processors in big-endian mode. */ #endif #ifdef mc68000 #define BIG_ENDIAN /* All 68xxx machines, such as Sun2's and Sun3's. */ #endif #ifdef macII #define BIG_ENDIAN /* Apple MacII (also a 68000, but being safe here.) */ #endif #ifdef sparc #define BIG_ENDIAN /* All SPARC-based machines. */ #endif #ifdef ibm032 #define BIG_ENDIAN /* IBM PC-RT and related machines. */ #endif #ifdef hp9000s300 #define BIG_ENDIAN /* HP 9000 machine. */ #endif #ifdef hp9000s800 #define BIG_ENDIAN /* HP 9000 machine. */ #endif #ifdef hp9000s820 #define BIG_ENDIAN /* HP 9000 machine. */ #endif
Resent-From: pci-sig-request at znyx.com Resent-Date: Tue, 23 Jan 1996 11:22:23 +1100 (EST) Date: Tue, 23 Jan 1996 11:22:23 +1100 (EST) From: Andrew Cagney <cagney at highland.com.au> Subject: Re: Proposal: Endian FAQ Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> Hello, If someone is going to build up an FAQ on the BE/LE issue. I strongly advise them to include references to documentation written by IBM/Apple/Mot (such as CHRP) along with a number of other papers. People wanting to learn more about the BE/LE issue are encouraged to read those documents. Key references on this would include: ftp://www.mot.com/pub/SPS/PowerPC/library/hw_spec/PPCPlatform.pdf The Appendix on Bi-endian support. Someone better net-connected could probably look around and find the URL pointing directly at that appendix. ftp://ftp.austin.ibm.com/pub/technology/spec/README That directory contains a number of papers some of which again discuss the issue of BE/LE support. Finally, if you're a hardware engineer, I strongly recommend that you discuss this issue with a software developer (that has had experience implementing network or machine independent IO code). If the two of you disagree as to how things should work, I suspect (based on experience) it is the hardware and not the software person that is confused. If the two of you are the same person, you've got serious problems ... :-) enjoy, Andrewftp://www.mot.com/pub/SPS/PowerPC/library/hw_spec/PPCPlatform.pdf ftp://ftp.austin.ibm.com/pub/technology/spec/README
Resent-From: pci-sig-request at znyx.com Resent-Date: Tue, 27 Feb 96 10:15:34 -0800 Date: Tue, 27 Feb 96 10:15:34 -0800 From: Alan Deikman <alan at znyx.com> Subject: Re: endian conversion - what's up ? Precedence: list Resent-Sender: pci-sig-request at znyx.com To: Mailing List Recipients <pci-sig-request at znyx.com> At 08:27 AM 2/27/96 PST, dave parker wrote: > This is a question that I have been trying to get an answer to for a > while. Please indulge me with your thoughts ! > > The PCI spec states that PCI is a little-endian bus. This suggests to I guess you missed it, but there was a long thread on this forum on this subject with several well-informed and insightful posts. A quick search of the most recent tar archive yields the following. Don't miss article #1967. >1912:Subject: Big Endian question >1915:Subject: Re: Big Endian question >1915:> Subject: Big Endian question >1919:Subject: Re: Big Endian question >1920:Subject: Re: Big Endian question >1923:Subject: Re: Big endian question >1924:Subject: Endianess (es) >1926:Subject: Re: Big endian question, don't swap! >1932:Subject: Proposal: Endian FAQ >1937:Subject: Re: Proposal: Endian FAQ >1941:Subject: Re: Proposal: Endian FAQ >1943:Subject: RE: Endian strategy (was Re: PCI and bi-endian hosts) >1966:Subject: Re(3): Big endian question >1967:Subject: Re: Big Endian question >1967:>gnu> Subject: Byte Order: On Holy Wars and a Plea for Peace >1970:Subject: Re: Re(3): Big endian question >1971:Subject: Re: Re(3): Big endian question >1972:Subject: RE: Big Endian question >1972:Subject: Re: Big Endian question >1977:Subject: Resolving the Big endian question >1978:Subject: >1979:Subject: Re(5): Big endian question The number at the beginning of each line is the article number. To retrieve the archive, send e-mail to email@example.com. In the Subject: line, put the word "archive" without the quotes. In the body of the message put get 9601tar for the "tar" version, OR get 9601zip for the "zip" version. BOTH files are UUencoded, which most mailers can deal with. If not, ask help from a UNIX drone near you. They are easy to spot by their social habits if you know what to look for. Regards, -------------------------------- Alan Deikman, ZNYX Corporation alan at znyx.com
(see above for some other floating-point comments).
Date: Fri, 12 Mar 1999 07:06:04 +0100 From: Mathias Brossard <f95mabr at dd.chalmers.se> X-Accept-Language: en, fr To: ogrimes at bellatlantic.net CC: "f-cpu at egroups.com" <f-cpu at egroups.com> Subject: [f-cpu] Re: Instruction Set debate
> > BTW, which adressing mode should we support ? > > I have been wondering about this. I am really unsure... The F21 does it by > saying "the following instruction is actually an immediate value to be added to > the number on the top of the stack." That could be a fairly good way of doing > it...I thought of this too. So I asked a friend who has done some RISC assembler, and he said RISC processors load constants in small parts. His answer to my idea, was (vaguely translated from swedish) "That's how it's done on Alphas and Mips, do you think they wouldn't have thought of it?". So I doubted a little... If you use this then it's not strictly a fixed length instruction set anymore. This means only that the decoder will have to tag the immediate value as "non-code". > Bloat must be eliminated! The eternal computer dilemma size versus speed. I always go for speed. > > Does that fit into a 32 bit instruction ? > > Operation (7 bits), A (6 bits) b(6 bits) c (6 bits) Unless we add more registers > to enhance the SIMD capabilitys of the design... Ok I'm currently thinking that we could do with 32 bits instructions, things I was trying to do with 64 bits. For that we would have to do a nice decoder that can decode and launch many instructions each cycle. Although a little more complicated it's possible: the advantages of my idea: - simpler CPU - puts the burden on the compiler. It's the compiler that finds the intrisic parallelism and not the CPU that has to do it on-the-fly. > > I'll admit these are grey areas in my knowledge... I don't know if the > >whole OS can run in interrupted: > > Linux does but any *GOOD* OS doesn't. If Linux does, so do we... Is that on all architecture ? > > a program should be able to call the OS in running mode and then the OS > > would disable the interrupt... just an idea... > > That is how intel processors handle it. Is that "How It Should Be Done(tm)" ? > > Do we have OS specialists ? We need some light... > > I have studied operating system design for years in hopes of developing my own. > I am among the keenest specialists in operating system design that you will > find. :) I said I would check on IEEE754, so I did: The IEEE 754 standard specifies: * Storage formats: IEEE754 C Type Bits Exponent Mantissa ----------------------------------------------------------------- - Single float 32 8 24 - Double double 64 11 53 - Double ext. long double >=80 >=15 >=64 ----------------------------------------------------------------- * Precise specifications of the results of operations Operations specified: - Addition - Substraction - Multiplication - Division - Square root - Remainder (modulo) - Conversion to/from integer - Conversion to/from printed base-10 * Special values - + or - 0 - Denormalized number - NaN (Not a Number) - + or - Infinity * Specified runtim behavior on illegal operations - Overflow to infinity - Underflow to zero - Division by zero - Invalid operation - Inexact operation There are much more details on http://www.loria.fr/serveurs/CCH/documentation/IEEE754 If you intend to make some FPU modules, this is a Must. Mathias -----------
From: "Kevin D. Kissell" <kevink at neu.sgi.com> Newsgroups: comp.arch Subject: Re: Little Endian vs. Big Endian Date: 19 Oct 1995 13:57:24 GMT Organization: The Institute for Impure Science Lines: 44 Message-ID: <firstname.lastname@example.org> References: <1995Oct19.email@example.com> NNTP-Posting-Host: jones.neu.sgi.com Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 1.1IS (X11; I; IRIX 5.3 IP22) X-URL: news:1995Oct19.firstname.lastname@example.org hcc at e0sun3 (Hsin-Chou Chi) wrote: >Hi there. > >Excuse me if this issue has been raised before, but does >anybody know what are the pros and cons of Little Endian >vs. Big Endian? I know what these two schemes mean. >However, it is not clear to me why some machines choose >one rather than the other. Oh Nooooooooo! The Holy Wars continue! ;-) The *bad* reason for doing little-endian machines is that one can take the address of an operand, and if one "knows" that the range of values is expressed within a small number of bits, one can reference the low-order part of the operand at the address without knowing its actual size. This kind of code really existed in UNIX 15 years ago, but was always agressively stupid, and has long since been cleaned up. I once had a discussion with an AI type who insisted that little-endian machines were better for implementing tagged data types in software, but when pressed for why, he was never able to explain. Maybe it's true, but I doubt it. The *lame* reason for doing big-endian machines is that, if one takes a byte-wise hex dump of memory, one can read the values of multi-byte data items without shuffling the bytes in your head. This is not agressively stupid, but it is less and less important as fewer people have to look at hex dumps. A slightly better reason for doing big-endian machines is that the standard byte-ordering for internet packet headers is big-endian, and little-endian machines have to burn a few extra cycles per packet to put them in canonical order. So, in the abstract, big endian is preferable, but for fairly silly second-order reasons. -- Opinions expressed may not be Kevin D. Kissell those of the author, let alone Silicon Graphics Core Technology Group those of Silicon Graphics. Cortaillod, Switzerland kevink at neu.sgi.com
"Two font files were necessary to allow for both big and little endian machines. DEC, Linux and DOS store binary files the same way (storing the low byte first followed by the high byte) whereas SUN and SGI store the high and low byte the other way around." -- Noel J. Rode <noel at rdt.monash.edu.au> May 1994 in file "genghis.txt".
Rather than choosing Big endian or little endian, there is a 3rd option. Some machines *only* access memory in word-aligned units of one full word. This makes the memory interface a lot simpler. (I hear the Alpha does this).
It seems likely to me that the Freedom CPU will have a multiplexed address/data bus (rather than a 64 bit address bus and a independent 64 bit data bus). Will the lsb address wire be the same as the lsb data wire or will the lsb address wire be the same as the MSb data wire ? Is this difference visible to software ?
Definitions of lsb and MSb: The least significant bit (lsb) of the data bus is the bit that always changes when you increment a integer. Toggling the least significant bit (lsb) of a address generally makes it point to a different register of the same destination device. Toggling the Most Significant bit (MSb) and bits close to it invariably makes a address point to a different "page" on a RAM, and usually a completely different device. Reading anywhere (forwards or backwards) from the same (typically 1 Kword ?) "page" of RAM is fast; there is a fixed overhead for switching to an other page of RAM.
OK, here's my summary on the Big endian v. little endian debate so far:
Humans are confused by endian issues. Ekkehard Morgenstern correctly understands how x86 machines and 68K machines represent integers, but his definitions of the terms "big endian" and "small endian" are exactly the *opposite* of the way Danny Cohen defined them in his classic 1980 paper ( http://www.rdrop.com/~cary/html/endian_faq.html ).
Pros for MSb at lowest address:
address: 98 99 9a 9b 9c 9d 9e 9f data : 74 65 73 74 00 00 00 05is the string "test" followed by the integer 5. (The x86 sort of machine would interpret the last part as the integer 0x0500_0000).
Pros for lsb at lowest address:
Pros for Neither Endian:
Items that make endian somewhat irrelevant:
Let's say I have the string "abcd" at memory location 0x1234_5678. If I probe the physical voltages on these lines, I see
With lsb-lsb, I see on my address/data lines MSb of address ...... LSb of address address phase: 12 34 56 78 data phase: 64 63 62 61 With lsb address -- MSB data, I see on my address/data lines MSb of address ...... LSb of address address phase: 12 34 56 78 data phase: 86 46 c6 26 Is this right ? for some reason I was expecting to see one or the other have data phase: 61 62 63 64 .
Anyway, while things seem to be transparent to *strings*, it seems that if you store *addresses* in RAM and then read them back one byte at a time (or misaligned), ... the software might be able to tell the difference. I think. (?)
What does Dr. Philip Emeagwali mean by "left-handed algorithms" when he says http://inventors.miningco.com/library/weekly/aa111097.htm ... http://inventors.about.com/library/weekly/aa111097a.htm
Computers that are commercially available are symmetric or non-handed but it is possible that some existing software and algorithms are left- or right-handed. I have demonstrated that you can apply a righted-handed algorithm and software to a right-handed computer. ... a left-handed computer must have left-handed software and algorithms that also complements it. Therefore, efforts to implement a left-handed software on a right-handed computer may be as awkward as putting your left shoe on your right leg.
Date: Thu, 12 Nov 1998 08:56:58 +0100 From: Walter Scherer Add to Address Book Subject: Re: uProcessor Busses, Current, Past and Future Organization: Walter Scherer To: d_cary at my-dejanews.com Hello! d_cary at my-dejanews.com wrote: > I collect endian-related information at > http://www.rdrop.com/~cary/html/endian_faq.html Nice FAQ. > If you find other related information, please tell me about it. I was building a PC clone mainboard with MC68040s once - a strange beast called the Freak40. It was a hobby project. A big endian MC68340 was handling the little endian ISA bus. Turns out I was doing the right thing - swapped the two byte lanes to get the addresses right. It was indeed possible to read the copyright string out of the onboard ROM of a VGA and it was readable without NUXI effects. Tschau ------ Walter
From: "Paul Lutus" <email@example.com> Subject: Re: HELP: struct data members alignment??? Date: 07 Jun 1999 00:00:00 GMT About this subject, and in seemingly endless threads, we get two kinds of posts: 1. Why can't I just write raw structure data to a file or network socket? Writing it as text or packaging it in a portable, binary form is a lot of work, and I probably won't be moving the program to a different platform anyway. 2. How come my program on platform B cannot read the raw structure data from platform A? The answer to (1) is (2). The answer to (2) is (1). -- Paul Lutus www.arachnoid.com
Mailing-List: contact f-cpu-owner at egroups.com Precedence: list X-URL: http://www.egroups.com/list/f-cpu/ X-Mailing-List: f-cpu at egroups.com Delivered-To: listsaver-egroups-f-cpu at egroups.com X-Lotus-FromDomain: PRS GMBH From: "Ekkehard Morgenstern" <emorgens at prs-gmbh.de> To: f-cpu at egroups.com Date: Tue, 17 Nov 1998 14:29:02 +0100 Mime-Version: 1.0 Subject: [f-cpu] Re: Big endian v. little endian Hi Dave, you wrote: > I collect on Big endian vs. little endian information at > http://www.rdrop.com/~cary/html/endian_faq.html. I have not fully read it yet, but my first (subjective) impression was that the illustrations/figures on your page are quite confusing. :) > So far as I can tell, neither one is significantly better than the other. Both have advantages and disadvantages. >From the programmers point of view, MSB..LSB is better, because the data is readable in a hex dump. Basically, the memory layout for both architectures is as follows: MSB = Most Significant Byte (carrying the 'higher' value of a number) LSB = Least Significant Byte (carrying the 'lower' value of a number) so, in a hexadecimal number 0x12345678, 0x12 would be the MSB, while 0x78 would be the LSB. There are two common models for byte-addressed processors, where in one model, numbers are stored in LSB..MSB order (Intel x86 etc), and another model, where numbers are stored from MSB..LSB (Motorola 68K,PPC etc.); So, in memory it looks like this (for our example 0x12345678): data a: 78 56 34 12 (Intel x86 etc.) data b: 12 34 56 78 (Motorola 68K, PPC etc.) address: 00003000 00003001 00003002 00003003 Since, in model 'a', the number 'ends' on its most significant byte (=0x12), the model is called 'big endian'. Model 'b' is called 'little endian' because the number ends on its least significant byte (=0x78). I don't like to use the terms 'little endian' and 'big endian' because they are likely to be confused. I prefer the notation MSB..LSB and LSB..MSB, resp. The LSB..MSB order generally can make debugging very difficult, because looking at the memory can't give you much hints about the use of the data. MSB..LSB however, stores the data readable in memory, so you can easily see where 32 bit data or 16 bit data is being used. LSB..MSB order is an artefact from the 8 bit era, where 16 bit was an exception. The MSB was then fetched during the next cycle. For compatibility reasons, Intel continued with this model in its x86 series till today. When dealing with different operand sizes it can be actually an advantage, because you don't have to read the whole word of data. However, on most modern processors, it does not make a difference, whether you read a byte or a whole 32 bit word, because the memory load/store interfaces already read whole words, whether you use the remaining bits or not. > > Douglas Seay @capgemini.fr mentioned > > Well, not always. I had the habbit of doubling integers with > > var <<= 1; > > > > which went south on a little endian machine. This is wrong; it depends on the internal architecture of a shift register in which direction the bits are shifted, not on the 'endianness' of the processor. Besides, the C programming language clearly defines that a left shift by 1 is the same as a multiplication by 2. In BCPL this was not so. On mainframes, there's generally a greater variety of architectures; since in BCPL, the actual behaviour of a shift was not defined, it was more likely that code using shifts would break. > I (and others) > will need to have complete understanding of these low-level issues > in order to make a functional CPU. Sure. Processor design is always low-level, _very_ low level. BTW, are there any hardware designers out there? I suspect we'll never manage to build a working CPU if we have discussions over basic principles of processors. > Rather than choosing Big endian or little endian, there is a 3rd option. > Some machines *only* access memory in word-aligned units of one full word. > This makes the memory interface a lot simpler. Yes, word-addressed machines are a lot simpler to implement, both in hardware and software. Greetings, Ekkehard. --- Ekkehard Morgenstern homepage: http://flnca.homepage.nu The statements made in this mail represent my own personal opinion, not the one of my employer. ------------------------------------------------------------------------ Free Web-based e-mail groups -- http://www.eGroups.com
To: f-cpu at egroups.com From: David Cary <d.cary at ieee.org> Subject: Big endian v. little endian
I collect on Big endian vs. little endian information at http://www.rdrop.com/~cary/html/endian_faq.html .
So far as I can tell, neither one is significantly better than the other. I've seen a couple of messages in the f-cpu archive where someone seemed to think one or the other was far superior. I would appreciate being informed about any real benefits one may have over the other.
Douglas Seay @capgemini.fr mentioned
>Well, not always. I had the habbit of doubling integers with >var <<= 1; > >which went south on a little endian machine.
I don't understand. I thought this will double the integer on either big-endian or little-endian machines. Please explain exactly what went wrong -- I (and others) will need to have complete understanding of these low-level issues in order to make a functional CPU.
Finally, a chance to choose a endianness based on technical rather than political reasons :-).
Rather than choosing Big endian or little endian, there is a 3rd option. Some machines *only* access memory in word-aligned units of one full word. This makes the memory interface a lot simpler.
Mailing-List: contact f-cpu-owner at egroups.com X-URL: http://www.egroups.com/list/f-cpu/ X-Mailing-List: f-cpu at egroups.com Delivered-To: listsaver-egroups-f-cpu at egroups.com X-Sender: cary at agora.rdrop.com Date: Wed, 18 Nov 1998 03:39:46 -0500 To: f-cpu at egroups.com From: David Cary <d.cary at ieee.org> Subject: [f-cpu] Re: Big endian v. little endian It seems likely to me that the Freedom CPU will have a multiplexed address/data bus (rather than a 64 bit address bus and a independent 64 bit data bus). Will the lsb address wire be the same as the lsb data wire or will the lsb address wire be the same as the MSb data wire ? Is this difference visible to software ? Definitions of lsb and MSb: The least significant bit (lsb) of the data bus is the bit that always changes when you increment a integer. Toggling the least significant bit (lsb) of a address generally makes it point to a different register of the same destination device. Toggling the Most Significant bit (MSb) and bits close to it invariably makes a address point to a different "page" on a RAM, and usually a completely different device. Reading anywhere (forwards or backwards) from the same (typically 1 Kword ?) "page" of RAM is fast; there is a fixed overhead for switching to an other page of RAM. OK, here's my summary on the Big endian v. little endian debate so far: Humans are confused by endian issues. Ekkehard Morgenstern correctly understands how x86 machines and 68K machines represent integers, but his definitions of the terms "big endian" and "small endian" are exactly the *opposite* of the way Danny Cohen defined them in his classic 1980 paper ( http://www.rdrop.com/~cary/html/endian_faq.html ). Pros for MSb at lowest address: * For people used to reading English text, it's slightly easier to understand dump files. (Ekkehard Morgenstern pointed this out). For example, with a 68K sort of machine. address: 98 99 9a 9b 9c 9d 9e 9f data : 74 65 73 74 00 00 00 05 is the string "test" followed by the integer 5. (The x86 sort of machine would interpret the last part as the integer 0x0500_0000). Pros for lsb at lowest address: * For people used to reading Hebrew text, it's slightly easier to understand dump files. * We want this chip to interface with the PCI bus (Olaf Naumann reminded me of this), and the PCI bus is inherently lsB at lowest address (Is this true ?). The guy coding the Freedom CPU Simulator claims that multi-word integer math is easer this way. (How long before a pre-alpha version of this code is online ?) (But then, how often are we really going to need integers so big they won't fit in a single 64 bit word ? We *never* need *addresses* so big they won't fit into a single 64 bit word.) Pros for Neither Endian: * Reading and writing only whole, aligned words results in much simpler hardware and software. (Ekkehard Morgenstern pointed this out) Items that make endian somewhat irrelevant: * RAM doesn't care which endian you make it. * Other devices are so slow that we have more than 10 CPU clock cycles after writing one word to it before it's ready to accept the next word. While we're waiting, we have plenty of clock cycles available to shuffle around the next word to whatever endian is needed by that device. ( Standard PCI now has top speed of 33 MHz; most I/O devices are much slower. Many processors have over 300 MHz clock speeds ). -- + David Cary "http://www.rdrop.com/~cary/" | "icbmto:N36 08.830' W97 03.443'" | Future Tech, Unknowns, machine vision, <*> O- ------------------------------------------------------------------------ Free Web-based e-mail groups -- http://www.eGroups.com Mailing-List: contact f-cpu-owner at egroups.com Precedence: list X-URL: http://www.egroups.com/list/f-cpu/ X-Mailing-List: f-cpu at egroups.com Delivered-To: listsaver-egroups-f-cpu at egroups.com From: AlphaRISC at aol.com Date: Wed, 18 Nov 1998 10:21:34 EST To: f-cpu at egroups.com Mime-Version: 1.0 Subject: [f-cpu] Re: Big endian v. little endian Status: U In a message dated 11/18/98 4:22:19 AM Central Standard Time, d.cary at ieee.org writes: > Items that make endian somewhat irrelevant: > * RAM doesn't care which endian you make it. > * Other devices are so slow that we have more than 10 CPU clock cycles > after writing one word to it before it's ready to accept the next word. > While we're waiting, we have plenty of clock cycles available to shuffle > around the next word to whatever endian is needed by that device. ( > Standard PCI now has top speed of 33 MHz; most I/O devices are much slower. > Many processors have over 300 MHz clock speeds ). And I'll be using those extra clocks to execute other threads, not waiting on a stalled bus. In a message dated 11/18/98 7:58:52 AM Central Standard Time, Wolfgang.Jung at micromata.de writes: about selectable endians: > Cons: > - CPU design follows the laziness of the programmers. > - More transistors in the MMU interface. > - 1 bit more in the TLB (could also affect max. cacheable area) It's no big deal. When I release the code the first thing I expect is that soembody will hack this into it. ALPHA ------------------------------------------------------------------------ Free Web-based e-mail groups -- http://www.eGroups.com
Send comments, suggestions, bug reports to
Return to index