Re: The integral type 'byte' (was Re: Suggested method for returning a string from a C program?)

On 27/03/2025 01:10, Waldek Hebisch wrote:

bart <bc@freeuk.com> wrote:

On 26/03/2025 18:09, Janis Papanagnou wrote:
>

, primitive CPU architectures,

>
8-bit architectures were fine, just a bit short of registers and with
limitation instruction sets. But that is to be expected with only 27,000
transistors on a chip or whatever it was for Z80.

 All sources that I found say that Z80 is about 8000 transistors.
You probably took number from 8086 which was climed to have 27000
transitors.

OK, then with 8000 it makes the capabilities even more remarkable. Current processors have billions of transistors.

Z80 (like earlier 8080) in "general" instructions required
accumulator as one of arguments, that severly limited utility
of registers.
 

(BTW, have a go at emulating such a processor in sofware; tell me in 3
months how you got on.)

 Lone processor is not very interesting.  I do not think coding
simple emulator would take a lot of time.  Z80 instruction set
is a bit bulky, due to prefixed instructions, but decode could
be done quite naively using 5 tables with 256 positions each
(or a bit smarter using 3 tables)

I don't think this is the one I saw, but the z80.c file here:
https://github.com/redcode/Z80/tree/master/sources
is nearly 3000 lines. There is a lot of stuff consider!
In my case, I wanted something performant, but I could see 90% of emulation time being spent in multiple flags most of which would never be tested.
(Of course, with emulation you need to consider the system that the CPU is connected to as well.)
But, yeah, 3 months was being generous. The point is, it wouldn't be as trivial as some might think.

Looking at short description of Z8000, it does not look nice:
it has ugly "segmentation" scheme to address more than 64kB
of memory.

That's the thing I didn't like either. But everything else: register layout and instruction set, looked great. I think the Z80000 version took care of the memory addressing, but that didn't get off the ground.
In the meantime, the 80386 came along in 1985, although the protected mode 32-bit OS to allow you to easily use the flat address space took another decade.