Mastering flags

Bedazzle · September 2016

I'm little lost when different flags are set/reset after logic operations, and what conditions must be met.

For example,

	ld      a, MODE
	xor     (hl)
	jp      p, xxx

in what situation parity is set? From code around the quoted piece I found, that bit 7 is in play.

Is "Z80 CPU User’s Manual" must read source, or can somebody point me to better resource to get familiar?
Maybe some table with essentials and comments, like

utz · September 2016

The parity flag is set when the result has an even number of bits set.
It's always nice to have a mnemonics table at hand for this kind of stuff. For example, http://www.z80.info/z80flag.htm deals specifically with flags.

Hikaru · September 2016

P/M is the sign flag, not parity. It is checking whether the result of the operation was positive (P, bit 7 = 0) or negative (M, bit 7 = 1)

'P/V' is this:
(PE: P/V=1, PO: P/V=0)
1. In INC r/DEC r, ADD/SUB set to PE if the result crosses the value of #80 (like the carry flag in ADD/SUB when the result crosses 0), otherwise set to PO.
2. 'Long' rotations and logic operations set it to PE if the result has even number of set bits (2, 4...), otherwise PO.
3. In block operations (LDI, CPI etc), PO is set when BC reaches 0, otherwise PE.
4. LD A,I / LD A,R: PE is set if interrupts are enabled, PO if disabled.

utz · September 2016

Damn, well spotted Hikaru ;)

Indeed, the "P/M" condition operates on the S flag, not the P/V flag.

Bedazzle · September 2016

Hikaru wrote: »

2. 'Long' rotations and logic operations set it to PE if the result has even number of set bits (2, 4...), otherwise PO.

You mean

     ld a, %00001111
     sla
     jp p, xxx

and

     ld a, %00000111
     sla
     jp p, xxx

will produce different result? And what is "long rotations"?

Hikaru · September 2016

I suspect these will probably give the same result. Something along the lines of 'insufficient arguments for SLA' :)

Now pay attention:

JP M/JP P: sign flag
JP PE/JP PO: parity flag

So if you have for instance used SLA A: JP PE (or JP PO) then the answer is yes.

Long rotations is what I call the 2-byte rotate/shift instructions that take 8 and more T states. Basically everything that isn't RLA/RRA/RLCA/RRCA (notice that these also have 'long' 2-byte equivalents, typically written as RL A/ RR A/etc).

Mastering flags

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