From: cr88192@gmail.com

On 12/18/2025 12:20 PM, Rosario19 wrote:
> 8 bit cpu for access memory other than 0..255 location has need at
> last one 16 bit register and 16 bits operations, so i think that even
> a 8 bit cpu has to have int in C language as 16 bits

There are no "true" 8 bit systems in this sense, as pretty much every
existing 8-bit CPU has had support for 16-bit operations in some way,
though often by using register pairs.

So, in this sense, it makes more sense to call them 8/16 instead, as the
exact line between 8-bit machines and 16-bit machines is often a little
fuzzy (and both tend to look basically similar as far as C is concerned;
for the machines big enough to where running C on them is viable).



One could then ask, what would a minimal 8-bitter look like.
   Might make sense to go with fixed-length 16-bit instructions.
   Likely 8x8b registers;
   ...

Say:
   AL, AH
   BL, BH
   CL, CH
   SP, LR

If you want a 16-bit ADD, you do two:
   ADD, ADC

Then, say:
   RCL/RCR (Rotate Carry Left/Right, 1-bit)
   CLC, STC (Clear/Set Carry)
   CMPLTZ (Compare Less Than Zero, Set/Clear Carry)
     Essentially "Copy sign bit to Carry".
   CMPEQZ (Set Carry if Src==0)
   BT/BF/BRA/BSR (Disp8)
   JMP reg  ("RTS"=="JMP LR")
   LI (2RI, Copy 8-bit value to register)
   LB/SB (Load/Store Byte)
   MOV, AND, OR, XOR (2R)
   ADD / ADC, SUB / SBB
   ...

Could probably implement most of C in this. Lots of stuff is going to
need to be implemented as runtime functions though.
SUB/SBB, not strictly necessary but would save a lot of instructions.


For extra fun, could maybe try to use an 8-bit address space, but more
likely just 16-bit with register pairs, say:
   AX, BX, CX, SP
     Except LB/SB Src/Dst: AX, BX, CX, LR
With addressing modes, say:
   [SP+AX] / [BX+AX] / [BX] / [CX]
   [BX+Disp2*2+Rd[0]] //4x 16-bit words
   [SP+Disp3*2+Rd[0]] //8x 16-bit words
Allows fitting every possibility into 4 bits.


So, for example, if you do:
   if(x>y) { ... }
     LW AX, [SP+2]
     LW CX, [SP+4]
     SUB CX, AX
     CMPLTZ CX
     BF .L0
     ...
These being mostly pseudo-instructions that crack into byte ops.

Could maybe go "more minimal", but then the number of instructions
needed to do "pretty much anything" would get a bit absurd (would be
less useful to have an 8-bitter where pretty much no useful program
could fit into a reasonable-sized ROM space).

Then again, maybe someone could come up with something both less bad and
more minimal (well, for those who feel things like the 6502 were maybe a
little too feature-rich).

...

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