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
