gnusim8085

Shouldn't this code set the zero flag with CMP?

Asked by BlackCow

Not sure if I found a bug or if its my lack of understanding but shouldn't this code set the zero flag on the CMP command?

mvi a, 0dh
sta 08000h
lxi b, 08000h
cmp b ;Shouldn't zero flag be set here?
hlt

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Debjit Biswas
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Best Debjit Biswas (debjitbis08) said : 		#1

Hi,

Per my understanding, this what your code does,

;store 0dh in register A
mvi a, 0dh
;store the value stored in A into memory location 8000h
sta 08000h
;Load 8000h into BC
lxi b, 08000h
;compare value of registers A and B
;register A has 0dh and B has 80h so comparison is non-zero
cmp b
hlt

Maybe you wanted to do this

MVI A, 0dh
STA 08000h
LXI H, 08000h
CMP M
HLT

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BlackCow (blackcow99) said : 		#2

Ohh, so you can't use the BC or DE registers to read an 8 bit value out of memory, only the HL registers? So then what is BC and DE for?

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BlackCow (blackcow99) said : 		#3

Thanks Debjit Biswas, that solved my question.

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Debjit Biswas (debjitbis08) said : 		#4

For register pair addressing almost all 8085 instructions use the HL registers except for two, the LDAX and STAX instructions. They use use either B and C or D and E registers to address memory.