Assembly Z80, best way to divide by 16 | MSX Resource Center (Página 2/2)

Paragon (1830)

20-09-2022, 23:10

Ah, indeed, I disallowed numeric constants to prune the space. I can try allowing 8 bit constants, but that'll take forever haha.

And indeed, I'll try it in the unpacker! If it's already been optimized, I doubt anything would be found, but let me give it a try!

Por santiontanon

Paragon (1830)

21-09-2022, 04:42

Ah, yes, allowing constants: 0, 1, 2, 3, 4, 7, 8, 15, 16, 31, 32, 63, 64, 127, 128, 240, 255

I get this in about 10 seconds, which is pretty much albs_br's second solution (other variations are found, but they are all equivalent):

ld a, h
rlca
rlca
rlca
rlca
and 15

Allowing for all constants between 0 - 255, I left it running for 5 minutes and it had not yet finished so, I stopped it hehe

Por [WYZ]

Champion (451)

21-09-2022, 09:09

~~are RRD or RLD useful here?~~

Ops, already thought by TheNestruo & GDX.

Por Micha

Expert (110)

21-09-2022, 10:20

The fastest way I can think of would be utterly impractical and bizarre, but takes only 16 cycles (machine + T1) :

ld l,0
ld a,(hl)

Disadvantages:
- it needs an absurd "lookup table" scattered through the complete 64k of memory; the values of the table will be 256 bytes apart from eachother at &0000, &0100, &0200, .... &FF00
- register l is not preserved

I would personally go for the lookup table as proposed in the first post of this topic...

Por theNestruo

Champion (429)

21-09-2022, 10:54

Micha wrote:

The fastest way I can think of would be utterly impractical and bizarre, but takes only 16 cycles (machine + T1) :

ld l,0
ld a, (hl)

Disadvantages:
- it needs an absurd "lookup table" scattered through the complete 64k of memory; the values of the table will be 256 bytes apart from eachother at &0000, &0100, &0200, .... &FF00
- register l is not preserved

I would personally go for the lookup table as proposed in the first post of this topic...

I see your utterly impractical and bizarre way and raise the bet (i.e.: it's faster, but even more impractical!):

ld l, h
ld a,(hl)

The LUT would be now at $0000, $0101, $0202, ..., $fefe, $ffff (oops! you cannot divide $ff)

Por bore

Master (182)

21-09-2022, 11:57

While we are looking at impractical solutions, how about changing the way the value is represented?

	ld	a, h	; convert to 4.4 fixpoint and divide by 16
	;and	$f0	; drop fractional bits

As a bonus it can keep the fractional bits.

Por Micha

Expert (110)

21-09-2022, 16:28

theNestruo wrote:

I see your utterly impractical and bizarre way and raise the bet (i.e.: it's faster, but even more impractical!):

ld l, h
ld a,(hl)

The LUT would be now at $0000, $0101, $0202, ..., $fefe, $ffff (oops! you cannot divide $ff)

Great ! another 3 cycles shaved off....!

Por santiontanon

Paragon (1830)

21-09-2022, 19:02

Haha, awesome solutions! And some are not that crazy haha. Those spread out LUTs might even be feasible in some demos with constrained values

Por Grauw

Ascended (10819)

23-09-2022, 21:13

Haha, those are some great solutions indeed! Bore takes the win in my book . Approaching the problem from a different angle like that can definitely lead to much faster algorithms, for sure I wouldn’t dare call it impractical without knowing the context of the precise intended application.

Por albs_br

Champion (499)

01-10-2022, 15:38

theNestruo wrote:

If you are using Z80 Assembly meter, there is a z80-asm-meter.platform setting. Set it to msx.

Thanks @theNestruo, it worked like a charm. No standard Z80 times anymore!