### Author Topic: Using SRA (HL) or SRL (HL)  (Read 4074 times)

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#### AMSDOS

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##### Re: Using SRA (HL) or SRL (HL)
« Reply #25 on: 12:47, 06 May 16 »
It wont work - you did 3 rotation for lsb and 1 rotation for msb in h and did not mask unnecessary bits.
The main idea for TFMs code is to rotate bits in a byte instead of shifting. In case of shifting you do sla and rl 5 times to get the result like in the example I gave earlier.
In case of rotation, you rotate the content by 3 bits in the opposite direction. Madrams example code does 8bit multiplication by 32 with 16bit result.
If you are looking for 16bit*32 multiplication with rotation, the code would probably look as follows:

ld a, (ix)
rrca
rrca
rrca
ld h,a
and 0xe0
ld l,a
xor h
ld h,a

ld a,(ix+1)
rrca
rrca
rrca
and 0xe0
or h
ld h,a

It takes 90 tstates which is similar to rld;sla code for 16bit values I posted earlier and consumes x2 more memory...

Oh well, that's a bit slower than the add hl,hl.

Was just trying something based on ones comments, but missed the masking components. But as you can probably tell, I've had little to do with rotation.

Incidentally I was poking around on CPCRULEZ and forgot about Math Programming they had there. This page looks interesting, was just wondering if "Doc n°3" could be useful? It uses RRA, ADD HL,DE SLA E RL D and a couple of loops in play.
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#### AMSDOS

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##### Re: Using SRA (HL) or SRL (HL)
« Reply #26 on: 13:04, 06 May 16 »
Try this:

xor a
ld hl, var

rld
sla (hl)
inc hl
rld
rl (hl)

var: dw 0

If you have your variable in ix,  replace ld hl, var with push ix; pop hl

Sorry I only just noticed this. Hmm, well the code I'll be putting into my game, which is being written in Hisoft Pascal. I managed to get it to work with the ADD HL,HL routine earlier, so in HP a function is defined:

FUNCTION row(xpos : integer) : integer;

that Function name and Variables defined are accessible from Index Registers like this:

Code: [Select]
`ld l,(ix+02)ld h,(ix+03)`

for xpos, and the value can be put back into row like this:

Code: [Select]
`ld (ix+04),lld (ix+05),h`

But am interested if another way is possible, but will probably get confused!
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#### Docent

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##### Re: Using SRA (HL) or SRL (HL)
« Reply #27 on: 22:06, 06 May 16 »

Oh well, that's a bit slower than the add hl,hl.

Was just trying something based on ones comments, but missed the masking components. But as you can probably tell, I've had little to do with rotation.

Incidentally I was poking around on CPCRULEZ and forgot about Math Programming they had there. This page looks interesting, was just wondering if "Doc n°3" could be useful? It uses RRA, ADD HL,DE SLA E RL D and a couple of loops in play.

I had a quick look at the page you linked - and the code there is incorrect. It claims to multiply DE by A, but it does only partial calculation - look at  ld ,b 6 - it should be ld,b 8. Here's the correct one:

ld hl,#0000
ld b,#08
mul8loop:
rrca
jr nc,mul8skip
add hl,de
mul8skip:
sla e
rl d
djnz mul8loop

This does generic 16bit*8bit multiplication, but it will be much slower than the dedicated solution for *32 provided earlier, because the code needs to iterate through all 8 bits of the multiplier. Only the mandatory part (rrca, sla e, rl d, djnz) will take more than 250 tstates, so in your case, if you don't need the generic multiplication,  it is better to stick to add hl,hl trick.

#### AMSDOS

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##### Re: Using SRA (HL) or SRL (HL)
« Reply #28 on: 01:03, 07 May 16 »
I had a quick look at the page you linked - and the code there is incorrect. It claims to multiply DE by A, but it does only partial calculation - look at  ld ,b 6 - it should be ld,b 8. Here's the correct one:

ld hl,#0000
ld b,#08
mul8loop:
rrca
jr nc,mul8skip
add hl,de
mul8skip:
sla e
rl d
djnz mul8loop

This does generic 16bit*8bit multiplication, but it will be much slower than the dedicated solution for *32 provided earlier, because the code needs to iterate through all 8 bits of the multiplier. Only the mandatory part (rrca, sla e, rl d, djnz) will take more than 250 tstates, so in your case, if you don't need the generic multiplication,  it is better to stick to add hl,hl trick.

Thanks for explaining that. I'm unsure if the code on CPCRULEZ has come from a magazine, normally they give the magazine source in those cases.
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