MaxCoderz

Posted: **Mon 25 Apr, 2005 3:16 pm**

Man, now I wanna know what dwedit said. anyway, this a clearscreen routine, it was said somewhere else in prgm help. I figure it's good to put here too.

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;instead of

	ld hl,gbuf
	ld (hl),0
	ld de,gbuf+1
	ld bc,767
	ldir

; try this

Clearbuf:
	di	 	 ;becuase so many of you don't believe me when
	ld hl,0	; I say it is safe to have interrupts enabled.
	ld d,h
	ld e,l
	add hl,sp
	ld b,32
	ld sp,gbuf+768
CBloop:
	push de
	push de
	push de
	push de 
	push de
	push de
	push de
	push de
	push de
	push de
	push de
	push de
	djnz CBloop
	ls sp,hl
	ret

The ret can be removed if you are only clearing the screen once in you prgram, but if not call it to clear the screen.

Posted: **Mon 25 Apr, 2005 4:09 pm**

To check if a 16 bit value equals zero:

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ld a,h
or l

Posted: **Mon 25 Apr, 2005 4:49 pm**

Comparing two 16-bit values without destroying them:

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ld a,L1
sub L2
ld a,H1
sbc a,H2

This will also work with less arithmetic-friendly register pairs like BC and DE.

Posted: **Mon 25 Apr, 2005 6:50 pm**

Jim e wrote:anyway, this a clearscreen routine, it was said somewhere else in prgm help. I figure it's good to put here too.

I believe that it's CoBB's routine, though I may be mistaken. Quite ingenious.

Posted: **Mon 25 Apr, 2005 6:55 pm**

I use 8 pushes in mine, but it's indeed practically the same.

Posted: **Mon 25 Apr, 2005 7:01 pm**

I don't who first came up with it, but i know I've used that for more than 3 or 4 years.

Posted: **Mon 25 Apr, 2005 7:44 pm**

I guess lots of people. It's not the most original idea around, really...

Posted: **Mon 25 Apr, 2005 8:01 pm**

Normally, you would use the following to increment a variable:

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ld a,(var)
inc a
ld (var),a

But if hl is not tied up and all you do is check flags, use indirection:

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ld hl,var
inc (hl)
ld a,(hl)   ;Optional

@AndySoft: I think you were confusing immediate numbers and addresses... well, here is how I keep it straight:

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ld hl,$5647     ;loads $56 -> h and $47 into l

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ld hl,86*256+71 ;also loads $56 -> h and $47 into l

Straight and simple. But lets say you want to store 0 -> (penCol) and 56 -> (penRow) with a 16-bit register; what I do is "reverse" them by imagining an X was drawn. So, by drawing the 'X', penCol should be first in the formula:

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ld hl,56*256+0         ;56 comes first
       \   /
        \ /
         X
        / \
       V   V
ld (penCol),hl

It would also be handy to remember that penCol comes before penRow and curRow comes before curCol. Hope this helps anyone who gets confused

Posted: **Mon 25 Apr, 2005 8:30 pm**

Gambit wrote:
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ld hl,86*256+71 ;also loads $56 -> h and $47 into l

Note that TASM will NOT compile that - you'd need to do it like this:

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ld hl,(86*256)+71 ;also loads $56 -> h and $47 into l

Posted: **Mon 25 Apr, 2005 8:38 pm**

I never had problems doing that. Though I know tasm doesn't do order of operations, it just goes in order from first to last.

Posted: **Mon 25 Apr, 2005 9:03 pm**

I'm using TASM 3.2, and it won't parse it correctly for me...

Posted: **Mon 25 Apr, 2005 9:14 pm**

so am i.

Tasm wrote:Version 3.2b Feb, 2000

Posted: **Mon 25 Apr, 2005 9:34 pm**

Gambit wrote:
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ld hl,$5647     ;loads $56 -> h and $47 into l
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ld hl,86*256+71 ;also loads $56 -> h and $47 into l

But the opcode for that is $21 $47 $56, not $21 $56 $47, for those of us who are on-calc-inclined.

Posted: **Mon 25 Apr, 2005 10:18 pm**

You might want to check out http://www.shiar.org/ticalcs/z80optim.php for many optimizations.

Fast ways to divide A by powers of 2:

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div_2:
srl a
ret

div_4:
rra
rra
and %00111111
ret

div_8:
rra
rra
rra
and %00011111
ret

div_16:
rra
rra
rra
rra
and %00001111
ret

div_32:
rlca
rlca
rlca
and %00000111
ret

div_64:
rlca
rlca
and %00000011
ret

div_128:
rlca
and %00000001
ret

Thanks to CoBB for pointing out the div_8 to me.

Posted: **Mon 25 Apr, 2005 10:22 pm**

Yea i didnt know the ordering :-/ . You might have to switch it around to make it work correctly :p

well another optimization! : if you need to do (in either direction

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 ld h,d
ld l,e

instead use

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 ex hl,de

also a useful instruction if you use either in a routine and need to save the other.

MaxCoderz

[TI ASM] Optimizations