22 Jan
2008
22 Jan
'08
3:08 a.m.
Date: Mon, 21 Jan 2008 12:41:31 -0600
From: trixter at oldskool.org
To: General at mail.mobygames.com
CC:
Subject: Re: Subject: Re: div by 10 on Z80
Chuck Guzis wrote:
Most likely not for the 800/10 problem but things like 28/3 might be
small enough to be worth the clock cycles. Most any power of two
problem can be done faster in code than the divide instruction.
The one we have now, on a Z80, looks like 428 to 463 clock cycles
to deal with the zero to 799 divided by 10.
Dwight
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div10:
xor a
ld de,0fd80h ; largest power of 2 times 10 less then 800d
ld b,07 ; only seven loops needed to finish divide 80
divloop:
add hl,de ; trial subtract
jr c,div10s1 ; carry means trial passed
sbc hl,de ; undo previous add, carry is clear
div10s1:
rl a
add hl,hl ; shift number to reuse same constant, 640 decimal
djnz divloop
add hl,hl ; push full remainder in HL
ret
Again, I haven't tried it out, but it should work.
Every time I see code like this for processors without a native DIV, I
wonder if the same code ported to x86 would indeed outperform the native
DIV. Would it? I know that on a 286 or higher, where MUL and DIV were
greatly optimized to about 12 cycles, no; but what about on the original
808x, where MUL/DIV could take as much as 144 cycles?