Whether double floats can silently have 80 bit accumulators is a controversial thing. Numerical analysis people like it. Computer science types seem not to because it's unpredictable. I lean towards, "we should have it, but it should be explicit", but this is not the most considered opinion. I think there's a legitimate reason why Intel included it in x87, and why DSPs include it.
Numerical analysis people do not like it. Having _explicitly controlled_ wider accumulation available is great. Having compilers deciding to do it for you or not in unpredictable ways is anathema.
During an internship in 1986 I wrote C code for a machine with 10-bit bytes, the BBN C/70. It was a horrible experience, and the existence of the machine in the first place was due to a cosmic accident of the negative kind.
I wrote code on a DECSYSTEM-20, the C compiler was not officially supported. It had a 36-bit word and a 7-bit byte. Yep, when you packed bytes into a word there were bits left over.
And I was tasked with reading a tape with binary data in 8-bit format. Hilarity ensued.
I think the pdp-10 could have 9 bit bytes, depending on decisions you made in the compiler. I notice it's hard to Google information about this though. People say lots of confusing, conflicting things. When I google pdp-10 byte size it says a c++ compiler chose to represent char as 36 bits.
5. Arithmetic is an explicit choice. '+' overflowing is illegal behavior (will crash in debug and releasesafe), '+%' is 2's compliment wrapping, and '+|' is saturating arithmetic.
6. f16, f32, f64, f80, and f128 are the respective but length IEEE floating point types.
The question of the length of a byte doesn't even matter. If someone wants to compile to machine whose bytes are 12 bits, just use u12 and i12.
How does 5 work in practice? Surely no one is actually checking if their arithmetic overflows, especially from user-supplied or otherwise external values. Is there any use for the normal +?
This doesn't feel like a serious question, but in case this is still a mystery to you… the name bit is a portmanteau of binary digit, and as indicated by the word "binary", there are only two possible digits that can be used as values for a bit: 0 and 1.
How philosophical do you want to get? Technically, voltage is a continuous signal, but we sample only at clock cycle intervals, and if the sample at some cycle is below a threshold, we call that 0. Above, we call it 1. Our ability to measure whether a signal is above or below a threshold is uncertain, though, so for values where the actual difference is less than our ability to measure, we have to conclude that a bit can actually take three values: 0, 1, and we can't tell but we have no choice but to pick one.
The latter value is clearly less common than 0 and 1, but how much less? I don't know, but we have to conclude that the true size of a bit is probably something more like 1.00000000000000001 bits rather than 1 bit.
I don't think the term word has any consistent meaning. Certainly x86 doesn't use the term word to mean smallest addressable unit of memory. The x86 documentation defines a word as 16 bits, but x86 is byte addressable.
ARM is similar, ARM processors define a word as 32-bits, even on 64-bit ARM processors, but they are also byte addressable.
As best as I can tell, it seems like a word is whatever the size of the arithmetic or general purpose register is at the time that the processor was introduced, and even if later a new processor is introduced with larger registers, for backwards compatibility the size of a word remains the same.
I like the Rust approach more: usize/isize are the native integer types, and with every other numeric type, you have to mention the size explicitly.
On the C++ side, I sometimes use an alias that contains the word "short" for 32-bit integers. When I use them, I'm explicitly assuming that the numbers are small enough to fit in a smaller than usual integer type, and that it's critical enough to performance that the assumption is worth making.
<cstdint> has int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, and uint64_t. I still go back and forth between uint64_t, size_t, and unsigned int, but am defaulting to uint64_t more and more, even if it doesn't matter.
Me? I just dabble with documenting an unimplemented "50% more bits per byte than the competition!" 12-bit fantasy console of my own invention - replete with inventions such as "UTF-12" - for shits and giggles.
Yes, I'm trying to figure out which are still relevant and whether they target a modern C++, or intend to. I've been asking for a few years and haven't gotten positive answers. The only one that been brought up is TI, I added info in the updated draft: https://isocpp.org/files/papers/D3477R1.html
They can just target C++23 or earlier, right? I have a small collection of SHARCs but I am not going to go crying to the committee if they make C++30 (or whatever) not support CHAR_BIT=32
TI DSP Assembler is pretty high level, it's "almost C" already.
Writing geophysical | military signal and image processing applications on custom DSP clusters is suprisingly straightforward and doesn't need C++.
It's a RISC architecture optimised for DSP | FFT | Array processing with the basic simplification that char text is for hosts, integers and floats are at least 32 bit and 32 bits (or 64) is the smallest addressable unit.
Fantastic architecture to work with for numerics, deep computational pipelines, once "primed" you push in raw aquisition samples in chunks every clock cycle and extract processed moving window data chunks every clock cycle.
A single ASM instruction in a cycle can accumulate totals from vector multiplication and modulo update indexes on three vectors (two inputs and and out).
Signed integers did not have to be 2’s complement, there were 3 valid representations: signed mag, 1s and 2s complement. Modern C and C++ dropped this and mandate 2s complement (“as if” but that distinction is moot here, you can do the same for CHAR_BIT). So there is certainly precedence for this sort of thing.
Don’t break perfection!! Just accumulate more perfection.
What we need is a new C++ symbol that reliably references eight bit bytes, without breaking compatibility, or wasting annnnnny opportunity to expand the kitchen sink once again.
I propose “unsigned byte8” and (2’s complement) “signed byte8”. And “byte8” with undefined sign behavior because we can always use some more spice.
“unsigned decimal byte8” and “signed decimal byte8”, would limit legal values to 0 to 10 and -10 to +10.
For the damn accountants.
“unsigned centimal byte8” and “signed centimal byte8”, would limit legal values to 0 to 100 and -100 to +100.
For the damn accountants who care about the cost of bytes.
Also for a statistically almost valid, good enough for your customer’s alpha, data type for “age” fields in databases.
Well then someone somewhere with some mainframe got so angry they decided to write a manifesto to condemn kids these days and announced a fork of Qt because Qt committed the cardinal sin of adopting C++20. So don’t say “a problem literally nobody has”, someone always has a use case; although at some point it’s okay to make a decision to ignore them.
> because Qt committed the carnal sin of adopting C++20
I do believe you meant to write "cardinal sin," good sir. Unless Qt has not only become sentient but also corporeal when I wasn't looking and gotten close and personal with the C++ standard...
"Many of us got our first exposure to Qt on OS/2 in or around 1987."
Uh huh.
> someone always has a use case;
No he doesn't. He's just unhinged. The machines this dude bitches about don't even have a modern C++ compiler nor do they support any kind of display system relevant to Qt. They're never going to be a target for Qt.
Further irony is this dude proudly proclaims this fork will support nothing but Wayland and Vulkan on Linux.
"the smaller processors like those in sensors, are 1's complement for a reason."
The "reason" is never explained.
"Why? Because nothing is faster when it comes to straight addition and subtraction of financial values in scaled integers. (Possibly packed decimal too, but uncertain on that.)"
Is this a justification for using Unisys mainframes, or is the implication that they are fastest (not that this is even close to being true - as any dinosaurs are decomissioned they're fucking replaced with x86 Xeon hardware running emulation, I don't think Unisys makes any non x86 hardware anymore) because of 1's complement? Anyway, may need to refresh that CS education.
There's some rambling about the justification being data conversion, but what serialization protocols mandate 1's complement anyway, and if those exist someone has already implemented 2's complement supporting libraries for the past 50 years since that has been the overwhelming status quo. We somehow manage to deal with endianness and decimal conversions as well.
"Passing 2's complement data to backend systems or front end sensors expecting 1's complement causes catastrophes."
99.999% of every system MIPS, ARM, x86, Power, etc for the last 40 years uses 2's complement, so this has been the normal state of the world since forever.
Also the enterpriseist of languages, Java somehow has survived mandating 2's complement.
This is all very unhinged.
I'm not holding my breath to see this ancient Qt fork fully converted to "modified" Barr spec but that will be a hoot.
> A byte is 8 bits, which is at least large enough to contain the ordinary literal encoding of any element of the basic character set literal character set and the eight-bit code units of the Unicode UTF-8 encoding form and is composed of a contiguous sequence of bits, the number of which is bits in a byte.
But instead of the "and is composed" ending, it feels like you'd change the intro to say that "A byte is 8 contiguous bits, which is".
We can also remove the "at least", since that was there to imply a requirement on the number of bits being large enough for UTF-8.
Personally, I'd make a "A byte is 8 contiguous bits." a standalone sentence. Then explain as follow up that "A byte is large enough to contain...".
I'm totally fine with enforcing that int8_t == char == 8-bits, however I'm not sure about spreading the misconception that a byte is 8-bits. A byte with 8-bits is called an octet.
At the same time, a `byte` is already an "alias" for `char` since C++17 anyway[1].
- CHAR_BIT cannot go away; reams of code references it.
- You still need the constant 8. It's better if it has a name.
- Neither the C nor C++ standard will be simplified if CHAR_BIT is declared to be 8. Only a few passages will change. Just, certain possible implementations will be rendered nonconforming.
- There are specialized platforms with C compilers, such as DSP chips, that are not byte addressable machines. They are in current use; they are not museum pieces.
They still exist. You can still run OS 2200 on a Clearpath Dorado.[1] Although it's actually Intel Xeon processors doing an emulation.
Yes, indexing strings of 6-bit FIELDATA characters was a huge headache. UNIVAC had the unfortunate problem of having to settle on a character code in the early 1960s, before ASCII was standardized. At the time, a military 6-bit character set looked like the next big thing. It was better than IBM's code, which mapped to punch card holes and the letters weren't all in one block.
This would be a great setup for a time travelling science fiction where there is some legacy UNIVAC software that needs to be debugged, and John Titor, instead of looking for an IBM 5100, came back to the year 2024 to find a pre-P3477R0 compiler.
idk. by today most software already assumes 8 bit == byte in subtle ways all over the place to a point you kinda have to use a fully custom or at least fully self reviewed and patched stack of C libraries
so delegating such by now very very edge cases to non standard C seems fine, i.e. seems to IMHO not change much at all in practice
and C/C++ compilers are anyway full of non standard extensions and it's not that CHAR_BIT go away or you as a non-standard extension assume it might not be 8
> most software already assumes 8 bit == byte in subtle ways all over the place
Which is the real reason why 8-bits should be adopted as the standard byte size.
I didn't even realize that the byte was defined as anything other than 8-bits until recently. I have known, for decades, that there were non-8-bit character encodings (including ASCII) and word sizes were all over the map (including some where word size % 8 != 0). Enough thought about that last point should have helped me realize that there were machines where the byte was not 8-bits, yet the rarity of encountering such systems left me with the incorrect notion that a byte was defined as 8-bits.
Now if someone with enough background to figure it out doesn't figure it out, how can someone without that background figure it out? Someone who has only experienced systems with 8-bit bytes. Someone who has only read books that make the explicit assumption of 8-bit bytes (which virtually every book does). Anything they write has the potential of breaking on systems with a different byte size. The idea of writing portable code because the compiler itself is "standards compliant" breaks down. You probably should modify the standard to ensure the code remains portable by either forcing the compiler for non-8-bit systems to handle the exceptions, or simply admitting that compiler does not portable code for non-8-bit systems.
The UNIVAC 1108 (and descendants) mainframe architecture was not discontinued in 1986. The company that owned it (Sperry) merged with Burroughs in that year to form Unisys. The platform still exists, but now runs as a software emulator under x86-64. The OS is still maintained and had a new release just last year. Around the time of the merger the old school name “UNIVAC” was retired in a rebranding, but the platform survived.
Its OS, OS 2200, does have a C compiler. Not sure if there ever was a C++ compiler, if there once was it is no longer around. But that C compiler is not being kept up to date with the latest standards, it only officially supports C89/C90 - this is a deeply legacy system, most application software is written in COBOL and the OS itself itself is mainly written in assembler and a proprietary Pascal-like language called “PLUS”. They might add some features from newer standards if particularly valuable, but formal compliance with C99/C11/C17/C23/etc is not a goal.
The OS does contain components written in C++, most notably the HotSpot JVM. However, from what I understand, the JVM actually runs in x86-64 Linux processes on the host system, outside of the emulated mainframe environment, but the mainframe emulator is integrated with those Linux processes so they can access mainframe files/data/apps.
So please do excuse my ignorance, but is there a "logic" related reason other than hardware cost limitations ala "8 was cheaper than 10 for the same number of memory addresses" that bytes are 8 bits instead of 10? Genuinely curious, as a high-level dev of twenty years, I don't know why 8 was selected.
To my naive eye, It seems like moving to 10 bits per byte would be both logical and make learning the trade just a little bit easier?
One example from the software side: A common thing to do in data processing is to obtain bit offsets (compression, video decoding etc.). If a byte would be 10 bits you would need mod%10 operations everywhere which is slow and/or complex. In contrast mod%(2^N) is one logic processor instruction.
One thought is that it's always a whole number of bits (3) to bit-address within a byte. It's 3.5 bits to bit address a 10 bit byte. Sorta just works out nicer in general to have powers of 2 when working on base 2.
Another part of it is the fact that it's a lot easier to represent stuff with hex if the bytes line up.
I can represent "255" with "0xFF" which fits nice and neat in 1 byte. However, now if a byte is 10bits that hex no longer really works. You have 1024 values to represent. The max value would be 0x3FF which just looks funky.
Coming up with an alphanumeric system to represent 2^10 cleanly just ends up weird and unintuitive.
We probably wouldn't have chosen hex in a theoretical world where bytes were 10 bits, right? It would probably be two groups of 5 like 02:21 == 85 or 5 groups of 2 0x21011 == 85
Many circuits have ceil(log_2(N_bits)) scaling wrt to propagation delay/other dimensions so you’re just leaving efficiency on the table if you aren’t using a power of 2 for your bit size.
Because modern computing has settled on the Boolean (binary) logic (0/1 or true/false) in the chip design, which has given us 8 bit bytes (a power of two). It is the easiest and most reliable to design and implement in the hardware.
On the other hand, if computing settled on a three-valued logic (e.g. 0/1/«something» where «something» has been proposed as -1, «undefined»/«unknown»/«undecided» or a «shade of grey»), we would have had 9 bit bytes (a power of three).
10 was tried numerous times at the dawn of computing and… it was found too unwieldy in the circuit design.
> On the other hand, if computing settled on a three-valued logic (e.g. 0/1/«something» where «something» has been proposed as -1, «undefined»/«unknown/undecided» or a «shade of grey»), we would have had 9 bit bytes (a power of three).
Is this true? 4 ternary bits give you really convenient base 12 which has a lot of desirable properties for things like multiplication and fixed point. Though I have no idea what ternary building blocks would look like so it’s hard to visualize potential hardware.
It is hard to say whether it would have been 9 or 12, now that people have stopped experimenting with alternative hardware designs. 9-bit byte designs certainly did exist (and maybe even the 12-bit designs), too, although they were still based on the Boolean logic.
I have certainly heard an argument that ternary logic would have been a better choice, if it won over, but it is history now, and we are left with the vestiges of the ternary logic in SQL (NULL values which are semantically «no value» / «undefined» values).
I'm fairly sure it's because the English character set fits nicely into a byte. 7 bits would have have worked as well, but 7 is a very odd width for something in a binary computer.
If you're ignoring what's efficient to use then just use a decimal data type and let the hardware figure out how to calculate that for you best. If what's efficient matters then address management, hardware operation implementations, and data packing are all simplest when the group size is a power of the base.
likely mostly as a concession to ASCII in the end. you used a typewriter to write into and receive terminal output from machines back in the day. terminals would use ASCII. there were machines with all sorts of smallest-addressable-sizes, but eight bit bytes align nicely with ASCII. makes strings easier. making strings easier makes programming easier. easier programming makes a machine more popular. once machines started standardizing on eight bit bytes, others followed. when they went to add more data, they kept the byte since code was written for bytes, and made their new registeres two bytes. then two of those. then two of those. so we're sitting at 64 bit registers on the backs of all that that came before.
Possible, but likely slow. There's nothing in the "C abstract machine" that mandates specific hardware. But, the bitshift is only a fast operation when you have bits. Similarly with bitwise boolean operations.
In the spirit of redefining the kilobyte, we should define byte as having a nice, metric 10 bits. An 8 bit thing is obviously a bibyte. Then power of 2 multiples of them can include kibibibytes, mebibibytes, gibibibytes, and so on for clarity.
Appeasing that attitude is what prevented Microsoft from migrating to LP64. Would have been an easier task if their 32-bit LONG type never existed, they stuck with DWORD, and told the RISC platforms to live with it.
On RISC machines, it can be very useful to have the concept of "words," because that indicates things about how the computer loads and stores data, as well as the native instruction size. In DSPs and custom hardware, it can indicate the only available datatype.
The land of x86 goes to great pains to eliminate the concept of a word at a silicon cost.
I'm saying the term "Word" abstracting the number of bytes a CPU can process in a single operation is an outdated concept. We don't really talk about word-sized values anymore. Instead we mostly explicit on the size of value in bits. Even the idea of a CPU having just one relevant word size is a bit outdated.
There were/are C++ compilers for PDP-10 (9 bit byte). Those haven't been maintained AFAICT, but there are C++ compilers for various DSP's where the smallest unit of access is 16 or 32 bits that are still being sold.
Here's a bit of 40 year old code I wrote which originally ran on 36-bit PDP-10 machines, but will work on non-36 bit machines.[1] It's a self-contained piece of code to check passwords for being obvious. This will detect any word in the UNIX dictionary, and most English words, using something that's vaguely like a Bloom filter.
This is so old it predates ANSI C; it's in K&R C. It used to show up on various academic sites. Now it's obsolete enough to have scrolled off Google.
I've seen copies of this on various academic sites over the years, but it seems to have finally scrolled off.
I think we can dispense with non 8-bit bytes at this point.
The tms320c28x DSPs have 16 bit char, so e.g. the Opus audio codec codebase works with 16-bit char (or at least it did at one point -- I wouldn't be shocked if it broke from time to time, since I don't think anyone runs regression tests on such a platform).
For some DSP-ish sort of processors I think it doesn't make sense to have addressability at char level, and the gates to support it would be better spent on better 16 and 32 bit multipliers. ::shrugs::
I feel kind of ambivalent about the standards proposal. We already have fixed size types. If you want/need an exact type, that already exists. The non-fixed size types set minimums and allow platforms to set larger sizes for performance reasons.
Having no fast 8-bit level access is a perfectly reasonable decision for a small DSP.
Might it be better instead to migrate many users of char to (u)int8_t?
The proposed alternative of CHAR_BIT congruent to 0 mod 8 also sounds pretty reasonable, in that it captures the existing non-8-bit char platforms and also the justification for non-8-bit char platforms (that if you're not doing much string processing but instead doing all math processing, the additional hardware for efficient 8 bit access is a total waste).
I thinks it's fine to relegate non 8 bit chars to non-standard C given that a lot of software anyway assumes 8bit bytes already implicitly. Non standard extensions for certain use-cases isn't anything new for C compilers. Also it's a C++ proposal I'm not sure if you program DSPs with C++ :think:
Why? Pls no. We've been told (in school!) that byte is byte. Its only sometimes 8bits long (ok, most of the time these days). Do not destroy the last bits of fun.
Is network order little endian too?
Why can’t it be 8?, the fact that it’s a trit doesn’t put any constraint on the byte (tryte ? size). You could actually make it 5 or 6 trits (~9.5 bits) for similar information density. The Setun used 6 trit addressable units.
Ignoring this C++ proposal, especially because C and C++ seem like a complete nightmare when it comes to this stuff, I've almost gotten into the habit of treating a "byte" as a conceptual concept. Many serial protocols will often define a "byte", and it might be 7, 8, 9, 11, 12, or whatever bits long.
One fun fact I found the other day: ASCII is 7 bits, but when it was used with punch cards there was an 8th bit to make sure you didn't punch the wrong number of holes. https://rabbit.eng.miami.edu/info/ascii.html
A 9-bit byte is found on 36-bit machines in quarter-word mode.
Parity is for paper tape, not punched cards. Paper tape parity was never standardized. Nor was parity for 8-bit ASCII communications. Which is why there were devices with settings for EVEN, ODD, ZERO, and ONE for the 8th bit.
Punched cards have their very own encodings, only of historical interest.
>A 9-bit byte is found on 36-bit machines in quarter-word mode.
I've only programmed in high level programming languages in 8-bit-byte machines. I can't understand what you mean by this sentence.
So in a 36-bit CPU a word is 36 bits. And a byte isn't a word. But what is a word and how does it differ from a byte?
If you asked me what 32-bit/64-bit means in a CPU, I'd say it's how large memory addresses can be. Is that true for 36-bit CPUs or does it mean something else? If it's something else, then that means 64-bit isn't the "word" of a 64-bit CPU, so what would the word be?
the fact that this isn't already done after all these years is one of the reasons why I no longer use C/C++. it takes years and years to get anything done, even the tiniest, most obvious drama free changes. contrast with Go, which has had this since version 1, in 2012:
fgetc(3) and its companions always return character-by-character input as an int, and the reason is that EOF is represented as -1. An unsigned char is unable to represent EOF. If you're using the wrong return value, you'll never detect this condition.
However, if you don't receive an EOF, then it should be perfectly fine to cast the value to unsigned char without loss of precision.
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