Few Z80 instructions change flag

[mrtinb]

Coming from 6502 it seems like Z80 has a lot fewer instructions that change the flags.

My code was

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LD HL,(x)
JR NZ y

But LD HL,(x) does not change the flags. I assumed Z-flag was set if content of address x was $0000.

So I think the code could be

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LD HL,(x)
ADC HL,HL
JR NZ y

but that will set Z-flag if content of address x is $0000 and $8000 which is not what I'm looking for.

[XavSnap]

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LD HL,(x)
LD DE,FFFF; SET UPPER VALUE
ADC HL,DE ; Add HL+DE
JR C y ; TEST IF <>0 goto y

[sirmorris]

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ld a,h
or l
jr z,...

[mrtinb]

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ld a,h
or l
jr z,...

[1024MAK]

Coming from 6502 it seems like Z80 has a lot fewer instructions that change the flags.

My code was

Code: Select all

LD HL,(x)
JR NZ y

But LD HL,(x) does not change the flags. I assumed Z-flag was set if content of address x was $0000.

A lot of what the Z80 does in relation to flags is because the Z80 was made code compatible with the Intel 8080 CPU.

For the instructions like LoaD (LD) that don't change the flags, this can be an advantage in some situations.

Mark

[PokeMon]

Typically a load (LD) (or comparable mov - MOV) does not change flags at all processors I know (didn't use 6502 yet).
This behavior is normally unwanted and only arithmetic or logic operations do change flags.
The flag behavior is also different form instruction to instruction.

INC sets flags only for 8 bit register but not for the 16 bit registers.
ADD HL,reg does not change zero or sign flag while ADC HL,reg does.
The carry flag can be reset with any AND/OR instruction.
Typically AND A or OR A is used to check the accumulator contents and sets flags respectively without changing the value.
This can be used in programs effectively and keep flag states for a longer time.

This is the best reference for instructions and behaviors:
http://www.zilog.com/appnotes_download. ... 5Ca1pnPT0=

Fortunately they updated the document in september 2016 as the older version has several copy and paste bugs. I documented them Zilog a long while ago - probably the had time to correct these bugs.