I am shocked and apalled [Z80]

Post by **Dwedit** » Wed 02 May, 2007 9:07 am

I've just observed this ungodly quirk in the wild:
* People actually use the RLC A instruction! (CB 07)

This is unbelievably freaky and shocking.

Post by **tr1p1ea** » Wed 02 May, 2007 9:16 am

Who uses it? Where?!

Maybe they dont care about wasting cycles?

.

threefingeredguy · Post by **threefingeredguy** » Wed 02 May, 2007 9:32 am

Where?

Post by **Dwedit** » Wed 02 May, 2007 10:17 am

Technically, this was for the Gameboy's GBZ80, not the regular Z80. I ran the Goomba emulator for an hour, playing various GB/GBC games in it. I was logging every instruction the program executed.

The RLC A instruction was executed 137,027 times total. Don't know which game used it. Yes, the GBZ80 does have the RLCA instruction.

For no reason, here are the Top 16 GBZ80 instructions:

Code: Select all

80,972,151	JR NZ,*
65,096,871	LD A,(nnnn)
63,428,950	JR Z,*
44,654,086	LD A,($FF00+nn)
32,027,560	AND A
28,065,301	LDI A,(HL)
27,762,895	LD (nnnn),A
26,706,384	CP #nn
24,199,159	OR A
22,119,619	AND #nn
20,076,686	RET
18,791,319	CALL $nnnn
18,340,846	LD ($FF00+nn),A
16,734,913	LD HL,#nnnn
16,002,933	LDI (HL),A
15,967,845	CP (HL)

threefingeredguy · Post by **threefingeredguy** » Wed 02 May, 2007 10:28 am

Good to know.

calc84maniac · Post by **calc84maniac** » Thu 03 May, 2007 4:06 am

I wonder how many appearances of SCF\CCF there were...

Post by **Dwedit** » Thu 03 May, 2007 6:28 am

SCF: 381,473
CCF: 328,272

Post by **benryves** » Thu 03 May, 2007 10:59 am

I assumed that was in reference to there not being a "clear carry flag" instruction and using SCF/CCF.

calc84maniac · Post by **calc84maniac** » Thu 03 May, 2007 12:21 pm

I meant instead of "or a".

Post by **benryves** » Thu 03 May, 2007 12:27 pm

OR A and SCF/CCF both do different things (what if you wanted to preserve S, Z and PV flags?)

Same goes for using LD A,0 rather than XOR A.

CoBB · Post by **CoBB** » Thu 03 May, 2007 5:46 pm

And the same applies to RLCA in the other direction. What if you want the sign or parity of the resulting value while also needing the carry? RLC A is your friend then.

anykey · Post by **anykey** » Fri 04 May, 2007 2:13 am

can somebody explain to me in layman's terms what's so shocking about this?!

threefingeredguy · Post by **threefingeredguy** » Fri 04 May, 2007 3:43 am

anykey wrote:can somebody explain to me in layman's terms what's so shocking about this?!

They used RLC instruction with the register A (two bytes and 8 tstates) when there is an instruction called RLCA which is a 1-byte instruction that does the same thing in 4 tstates.

calc84maniac · Post by **calc84maniac** » Fri 04 May, 2007 1:36 pm

Aw CoBB, you stole my post!

hop · Post by **hop** » Fri 04 May, 2007 3:18 pm

Maybe the logger doesn't make a distinction between the two?