Comment by dspillett | Hacker News Reader

Comment by dspillett | Hacker News Reader

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> In fact, it's not even useful to say it's a "64-bit system" just because it has some 64-bit registers.

Usually the size of general purpose registers is what defines the bitness of a CPU, not anything else (how much memory it can address, data bus width, etc).

For instance, the 80386SX was considered a 32-bit CPU because its primary register set is 32-bit, despite the fact it had a 24-bit external address bus and a 16-bit external data bus (32-bit requests are split into two 16-bit requests, this was done to allow the chip to be used on cheaper motherboards such as those initially designed with the 80286 in mind).

Note that this is for general purpose registers only: a chip may have 80-bit floating point registers in its FPU parts (supporting floating point with a 64-bit mantissa) but that doesn't make it an 80-bit chip. That was a bit more obvious when FPUs where external add-ons like the 8087 (the co-pro for the 16-bit 8086 family back in the day, which like current FPUs read & wrote IEEE754 standard 32- & 64- bit format floats and computed/held intermediate results in an extended 80-bit format).

>Usually the size of general purpose registers is what defines the bitness of a CPU

The Motorola 68000 has 32-bit registers but it's usually considered a 16-bit CPU because it has 16-bit ALU and 16-bit data bus (both internal and external).

Motorola 68k is a curious case because it originally was supposed to be a 16bit cpu, not 32bit, and the 24bit addressing that ignored upper 8 bits didn't help the perception.

Ultimately, 68k being "16bit" is a marketing thing from home computers that upgraded from 8bit 6502 and the like to m68k but didn't use it fully.

That is an odd case.

I'd still call it a 32-bit CPU as it had 32-bit registers and instructions (and not just a few special case 32-bit instructions IIRC). Like the 386SX it had a 16-bit external data bus, but some of its internal data routes were 16-bit also (where the 386SX had the full 32-bit core of a 386, later renamed 386DX, with the changes needed to change the external data bus) as were some of its ALUs hence the confusion abaout its bit-ness.

In a way, the fact that you have home computer market calling it 16bit, while at the same time you have workstation systems that plainly talk about 32bit ISA, shows how much of marketing issue it is :)

Would you call the 6809 a 16 bit device?

I'm not aware of that one off the top of my head. If it naturally operated over 16-bit values internally (i.e. it had 16-bit registers and a primarily 16-bit¹ instruction set), at least as fast as it could work with smaller units, then probably yes.

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[1] So not a mostly 8-bit architecture with 16-bit add-ons. The 8086 had a few instructions that could touch 32 bits, multiply being able to give a 32-bit output from two 16-bit inputs for instance (though the output was always to a particular pair of its registers), but a few special cases like that doesn't count so it is definitely 16-bit.

Well, the 6809 was basically the same in these respects.

Internal registers are 16 bit, with the accumulator (A) being provisioned as two 8 bit registers (A, B) as needed. Index X, Y, Stack, User Stack, PC, are all 16 bit registers.

The Hitachi 6309, adds to that with up to 32 bit register sizes in specific cases.

In any case, the ALU and data transfers are 8 bits and I am not sure I ever saw the 6809 referenced as a 16 bit device.

Maybe 16 bit curious, LMAO.

I'd say that it's a somewhat extended 8bit device because it's still 8bit focused architecture (6800) with extensions towards better handling of 16bit values and certain common parts involved including the zero/direct page are also effectively an increase in flexibility for 8bit code not so much move to 16bit.

That said, "16bit curious" is a great term :D

I'm glad you like it. I've used that term for years because I don't know what to call a chip like the 6809.

It certainly can punch well above its weight class, at least when compared with 6502 z80 and some others.

I really can't call it 16 bit, because of the small address space, and the fact that the ALU is 8-bit. But you can't always go by the ALU because I believe the z80 and 8080 have four bit ALUs. And I don't think there's anyone that would call those chips four bit.

Motorola seemed to design things in a specific way that people really liked, and this pushing the limits of what is an expert design seems to be one of those because even going back to the 6800, the one index register was 16 bit.

And lastly the 68k is an exemplary design, but in the same design language is 32 bit curious.

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