Common machine code sequences

[cmal]

DJNZ ...
DEC C
JR NZ, ...

Same size but much faster. However you need to initialize BC differently: B=n%256 and C=1+(n/256)

Ops! My math was wrong, it should be B=n%256 and C=(n+255)/256

This is a great improvement. If you compare the two methods, you're basically using half the amount of T-states (13 for djnz) for most iterations instead of 26 for every iteration. Do a bit of loop unrolling on top of that and you'll really cut down on time. Thanks for sharing.

Traditional method:
DEC BC 6
LD A,B 4
OR C 4
JR NZ, ... 12
total 26 t-states

Faster method:
DJNZ ... 13
DEC C 4
JR NZ, ... 12

total 29 t-states, but only 13 on most iterations

[cmal]

I think this could be a very interesting project and particularly worthwhile for:
(1) people coming to learn speccy Z80 for the first time (me!)
(2) people interested in preserving the age-old wisdom of ace coders who actually programming top spectrum games back in the 80s (and also more recently)

Any thoughts?

P.S. I could probably set up the above described website What's the general interest level in the community?

I like the idea and I don't think such a site exists today. There are other Z-80 sites with routines and stuff but not Spectrum-specific from what I've seen.

[cmal]

Here's a small one-liner that's commonly used:

XOR A

instead of:

LD A,0

[ketmar]

Code: Select all

  and  a,#0F
  cp   a,10
  sbc  a,#69
  daa

prolly the only snippet i've ever seen with "daa". ;-)

ah, yep, this converts low 4 bits of A to printable hex digit.

[Bedazzle]

I wonder should we compile a ZX Spectrum Z80 coding cookbook for common and useful ZX Spectrum coding routines.

Wiki that can live forever seems a good choice for this.

[uglifruit]

A couple of good links for this kind of thing (to get started)
(That I have bookmarked, plus the associated parent URLs)


http://z80-heaven.wikidot.com/optimization
http://z80-heaven.wikidot.com/math

https://wikiti.brandonw.net/index.php?title=Z80_Optimization
https://wikiti.brandonw.net/index.php?title=Z80_Routines:Math:Division

[Einar Saukas]

ROUTINE TITLE: Quaxel scrolling

ROUTINE DESCRIPTION: Scroll bitmap image 4 pixels sideways

CODE SAMPLE 1: Scroll entire screen 4 pixels to the right (compact version)

Code: Select all

    ld hl,16384
    ld bc,24
    xor a
loop:
    rrd
    inc l
    djnz loop
    inc h
    dec c
    jp nz,loop

CODE SAMPLE 2: Scroll entire screen 4 pixels to the right (unrolled loop version)

Code: Select all

    ld hl,16384
    ld b,192
    xor a
loop:
REPT 31
    rrd
    inc l
ENDR
    rrd
    inc hl
    djnz loop

CODE SAMPLE 3: Scroll entire screen 4 pixels to the left (compact version)

Code: Select all

    ld hl,16384
    ld bc,24
loop:
    dec l
    rld
    djnz loop
    inc h
    dec c
    jp nz,loop

CODE SAMPLE 4: Scroll entire screen 4 pixels to the left (unrolled loop version)

Code: Select all

    ld hl,16384
    ld b,24
    xor a
loop:
REPT 32
    dec l
    rld
ENDR
    inc l
    dec l
    jp nz,loop
    inc h
    djnz loop

NAME OF SUBMITTER: Einar Saukas

[Bedazzle]

A couple of good links for this kind of thing (to get started)

It is not useful for most of people, because in Russian, so via help of google translate:
https://translate.google.com/translate? ... %23codeopt