Truly shocking code (z80 asm)

Post by **Dwedit** » Fri 08 Jun, 2007 3:12 pm

Technically, the NEG instruction isn't needed either, since you can just compliment and increment, but I think those instructions may possibly affect the flags differently.

King Harold · Post by **King Harold** » Fri 08 Jun, 2007 4:28 pm

It affects the S, P/V and Z flags differently, and imo NEG should have been a 1byte instruction to be useful, or if it has to be 2 bytes then an operant is useful. As it stands now, NEG is even less useful since it trashes the carry flag (it almost always sets it).

calc84maniac · Post by **calc84maniac** » Fri 08 Jun, 2007 4:35 pm

Well, cpl resets the carry flag, so it's an even tradeoff.

DigiTan · Post by **DigiTan** » Fri 08 Jun, 2007 8:37 pm

King Harold wrote:I never said one should use XOR A, it zero's A!.

Who's saying you said it?

King Harold · Post by **King Harold** » Fri 08 Jun, 2007 8:57 pm

No one, but I thought you talked about XORing A?
It looked to me like you were saying I was defending XOR A.. though I could be mistaken. XOR A has it's uses, as do most instructions (the z80 is a RISC processor after all..) and can be used to reset the carry, but often you might want to preserve the contents of A..
(why am I talking about that?)

I'll go for that even tradeoff (but with NEG you have 1 in 255 change that the carry won't be set, when A was zero..)