And modern X86 chips are in fact NOT 64 bit anymore, but hybrids that handle tasks with 256 bits routinely, and some even with 512 bits, with instruction extensions that have become standard on both Intel and AMD
On a note of technical correctness: That’s not what the bitwidth of a CPU is about.
By your account a 386DX would be an 80-bit CPU because it could handle 80-bit floats natively, and the MOS6502 (of C64 fame) a 16-bit processor because it could add two 16-bit integers. Or maybe 32 bits because it could multiply two 16-bit numbers into a 32-bit result?
In reality the MOS6502 is considered an 8-bit CPU, and the 386 a 32-bit one. The “why” gets more complicated, though: The 6502 had a 16 bit address bus and 8 bit data bus, the 368DX a 32 bit address and data bus, the 368SX a 32 bit address bus and 16 bit external data bus.
Or, differently put: Somewhere around the time of the fall of the 8 bit home computer the common understanding of “x-bit CPU” switched from data bus width to address bus width.
…as, not to make this too easy, understood by the instruction set, not the CPU itself: Modern 64 bit processors use pointers which are 64 bit wide, but their address buses usually are narrower. x86_64 only requires 48 bits to be actually usable, the left-over bits are required to be either all ones or all zeroes (enforced by hardware to keep people from bit-hacking and causing forwards compatibility issues, 1/0 IIRC distinguishes between user vs. kernel memory mappings it’s been a while since I read the architecture manual). Addressable physical memory might even be lower, again IIRC. 2^48^B are 256TiB no desktop system can fit that much, and I doubt the processors in there could address it.
Buffalox@lemmy.world 3 days ago
No that’s not true, it’s way way more complex than that, some consider the data bus the best measure, another could be decoder. I could also have called a normal CPU bitwidth as depending on how many cores it has, each core handling up to 4 instructions per cycle, could be 256 bit, with an average 8 core CPU that would be 2048 bit.
There are several ways to evaluate, but most ways hover around the 256 bit, and none below 128 bit.
sugar_in_your_tea@sh.itjust.works 3 days ago
There absolutely is, and the person you responded to made it incredibly clear: address width. Yeah, we only use 48-bit addresses, but addresses are 64-bit, and that’s the key difference that the majority of the market understands between 32-bit and 64-bit processors. The discussion around “32-bit compatibility” is all about address size.
And there’s also instruction size. Yes, the data it operates on may be bigger than 64-bit, but the instructions are capped at 64-bit. With either definition, current CPUs are clearly 64-bit.
But perhaps the most important piece here is consumer marketing. Modern CPUs are marketed as 64-bit (based on both of the above), and that’s what the vast majority of people understand the term to mean. There’s no point in coming up with another number, because that’s not what the industry means when they say a CPU is 64-bit or 32-bit.
Buffalox@lemmy.world 3 days ago
That’s just stupid, no CPU has ever been called by the width of the address bus EVER.
sugar_in_your_tea@sh.itjust.works 3 days ago
Yes they have, and that’s what the vast majority of people mean when they say a CPU is 32-bit or 64-bit. It was especially important in the transition from 32-bit to 64-bit because of all the SW changes that needed to be made to support 64-bit addresses. It was a huge thing in the early 2000s, and that is where the nomenclature comes from.
Before that big switch, it was a bit more marketing than anything else and frequently referred to the size of the data the CPU operated on. But during and after that switch, it shifted to address sizes, and instructions (not including the data) are also 64-bit. The main difference w/ AVX vs a “normal” instruction is the size of the registers used, which can be up to 512-bit, vs a “normal” 64-bit register. But the instruction remains 64-bit, at least as far as the rest of the system is concerned.
Hence why CPUs are 64-bit, all of the interface between the CPU and the rest of the system is with 64-bit instructions and 64-bit addresses. Whether the CPU does something fancy under the hood w/ more than 64-bits (i.e. registers and parallel processing) is entirely irrelevant, the interface is 64-bit, therefore it’s 64-bit.