Re: [M100] Z-80 Parity Fix

[Stephen Adolph]

George, update

This patch works:
var_fix:
LDA 0FB65H
CPI 08H
DCR A
DCR A
DCR A
JC var_carry
ORA A
RET
var_carry:
ORA A
STC
RET

but this patch does not:
LD A,($40AF) SUB 3 CP 8-3 JR NC,_25E5 OR A SCF RET _25E5: OR A RET

have not yet figured out why!

On Thu, Dec 19, 2024 at 3:13 AM George Phillips <gp2...@shaw.ca> wrote:

> As Stephen Adolf discovered the Z-80 will compute overflow instead of
> parity when doing arithmetic operations.  Thus BASIC's 8085 RST 5
> subroutine won't work on the Z-80 as the 8085 computes parity on DCR A
> while the Z-80 computes overflow.
>
> Note here that the routine gets Carry from the CPI and Sign, Zero and
> Parity from the last DCR A.
>
> LDA FB65H
> CPI 08H
> DCR A
> DCR A
> DCR A
> RET
>
> Adding a "OR A" will almost fix it as that will compute parity and
> recompute S and Z.  But that will clear the carry.  A little branching
> will handle this correctly:
>
> LDA FB65H
> CPI 08H
> DCR A
> DCR A
> DCR A
> JR C,has_carry
> OR A
> RET
> has_carry:
> OR A
> SCF
> RET
>
> It the occurred to me that, rather like the old "replacing the step
> stone" joke, Microsoft had to solve this problem before.  Indeed, in the
> TRS-80 Model 1 ROM we see:
>
> LD A,($40AF)
> CP $08
> JR NC,_25E5
> SUB $3
> OR A
> SCF
> RET
> _25E5:
> SUB $03
> OR A
> RET
>
> I feel confident my solution will work and is even a byte shorter than
> what Bill or Paul (or some other Microsoft employee) came up with.
>
> Before learning the purpose I did go into the weeds a bit looking at
> compact subroutines to compute parity.  I'll save that for a separate
> message.
>
>                          -- George
>
>