ASM Command of the Week

[ralphdspam]

The ASM board hasn't been as active as it should be, so I'm starting a weekly discussion called "ASM Command of the Week."  Basically, we will discuss hacks, routines, optimizations, or alternatives to the command.  

It's basically going to be like these topics, except this one is going to be weekly.
DJNZ:
http://www.omnimaga.org/index.php?topic=10972.0
RLD and RRD:
http://www.omnimaga.org/index.php?topic=10161.0
IX and IY:
http://www.omnimaga.org/index.php?topic=10036.0

We'll start with a really easy one.  This week's command is:
XOR

[calc84maniac]

Possibly one of the best uses of XOR is in combination with AND, to act as a bitwise multiplexer.

For example, this uses the bits of C to select bits from A and B: if the bit in C is 1, that bit is selected from A, and if the bit in C is 0, that bit is selected from B.
xor b
and c
xor b

[Hot_Dog]

xor a is probably the most common use, faster than "ld a, 0"

xor is also commonly used to "flip" bits in register A.

[ZippyDee]

You can also use xor to test whether two bits within a byte are the same.

and MASK
jr z, bitsAreTheSame
xor MASK
jr z, bitsAreTheSame
;... otherwise, bits are different
where MASK is a mask of the two bits to compare


With this concept, you can use it to swap two bits within a byte:
ld a, (addr)
and MASK
jr z, doneSwapping
xor MASK
jr z, doneSwapping
ld a, (addr)
xor MASK
ld (addr), a

[calc84maniac]

and MASK
jp pe,bitsAreTheSame

* calc84maniac runs

[ZippyDee]

Explain how that works please...>.< I've never understood how to use parity.

Edit: After thinking about it that makes sense, but that only allows you to test two bits, whereas my method could be used to test any number (I think). Of course the swapping would only work for two bits at a time...

[DrDnar]

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1

AND mask

masks out all bits except the two in question. If the remaining two bits are the same, either there will be 8 zeros and no ones, or 2 ones and 6 zeros. Thus, the parity will be even. However, if the remaining two bits are different, 1 will be zero, and 1 will be one. Thus, the parity will be odd.

[chickendude]

Possibly one of the best uses of XOR is in combination with AND, to act as a bitwise multiplexer.

For example, this uses the bits of C to select bits from A and B: if the bit in C is 1, that bit is selected from A, and if the bit in C is 0, that bit is selected from B.
xor b
and c
xor b

What could you/would you want to use this for?

I think this is an interesting topic and would like to see it continue

Like Hot_dog, i often use xor to flip bytes. I think i'm pretty unimaginative though when i code.

[ralphdspam]

Possibly one of the best uses of XOR is in combination with AND, to act as a bitwise multiplexer.

For example, this uses the bits of C to select bits from A and B: if the bit in C is 1, that bit is selected from A, and if the bit in C is 0, that bit is selected from B.
xor b
and c
xor b

What could you/would you want to use this for?

You can use it for a lot of graphics stuff.  ie: masking sprites, grayscale, etc.
I also use it for preserving certain flags in a register while changing others.

[chickendude]

Ok, i'm not really too interested in grayscale, but i'd like to see how masking sprites might work out.

Here's some basic code to draw a masked byte to the gbuf:

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;ix=masked byte to draw ;ix+1=byte mask ;hl=location in gbuf to draw to  ld a,(ix+1)  ;load mask into a  and (hl)   ;and the mask onto the byte in gbuf  or (ix)    ;draw the byte  ld (hl),a  ;update gbuf

So that would become:

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ld a,(ix)  xor (hl)  and (ix+1)  xor (hl)  ld (hl),a

Which is an extra byte larger. I've always drawn my masks like this:
sprite:
%00011000
%00111100
%01111110
%11111111
mask:
%11100111
%11000011
%10000001
%00000000...a 1 being a byte to preserve in the gbuf (whereas i suppose with the xor method it'd be inverted).

If you want, i've got some code from a masked sprite routine i wrote a while back for an rpg i was working on that we could try to optimize I don't like how i went about it, first rotating/loading the mask onto the gbuf and updating the gbuf, then rotating/loading the sprite onto the gbuf (drawing twice to the gbuf).

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drawPlayerLoop:    push bc drawPlayerRow    ld d,(ix+32)     ;sprite mask    ld e,(ix+33)    ld a,$ff         ;a 1 means the gbuf bit will be preserved    dec b \ inc b \ jr z,skipMaskClip    ;if b=0, no need to rotate    scf \ rr d \ rr e \ rra \ djnz $-6   ;rotate mask b bits skipMaskClip:    and (hl)         ;a = mask to the furthest right    ld (hl),a        ;store masked gbuf    dec hl           ;next byte...    ld a,(hl)    and e    ld (hl),a    dec hl           ;last byte    ld a,(hl)    and d    ld (hl),a    pop bc           ;recall b=x offset, how many bits to rotate    inc hl           ;return hl to starting gbuf position    inc hl    push bc          ;store bc again    ld d,(ix)        ;sprite    ld e,(ix+1)    xor a            ;empty a to receive flowover from shifting    cp b    jp z,skipSpriteClip    srl d \ rr e \ rra \ djnz $-5   ;rotate sprite skipSpriteClip:    or (hl)          ;load sprite into masked gbuf    ld (hl),a        ;update gbuf    dec hl           ;next byte, etc.    ld a,(hl)    or e    ld (hl),a    dec hl    ld a,(hl)    or d    ld (hl),a    inc ix           ;next sprite row    inc ix    ld de,14         ;jump to next row in gbuf, +2 (draw rightmost byte first)    add hl,de    pop bc    dec c    jp nz, drawPlayerLoop

[Art_of_camelot]

By the way, are you still working on that RPG any? I always thought it had a really nice look to it. =)

[chickendude]

Well, my laptop busted (or rather, i busted my laptop) but i had made a fairely recent backup of it (the code on yaronet). Right now i'm a bit spread out but especially once i get Monopoly taken care of (a project that's been biting at my feet for maybe 8 years now) it's next on my hitlist. Actually, a lot of the work i've done with the Monopoly text/menu engine will probably be carried over into the RPG. One of the big things i need to do now is work on the story a bit. Thanks, btw

Does anyone have any suggestions for the next "Command of the Week"?

[Xeda112358]

Okay, time for a new command, ay? Well here goes:
CPIR
CPIR can be an extremely useful and powerful command, especially if you are doing a search.
What it does:
It performs cp (hl) then it decrements BC and increments HL. If the byte was a match, the loop stops and PE is the flag state of the parity/overflow flag and the other compare flags are affected normally. If BC reaches 0, the parity flag is PE.

As an example, if you want to find and jump to a certain line in an executing program, you can do something like this:

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;Inputs: ;     DE is the line to jump to ;     (965Bh) points to the start of the program ;     (965Fh) points to the end of the program ;Notes: ;     The two addresses above are set accordingly while a BASIC program is running      push de      ld de,(965Bh)     ;This is the start of the currently running program      ld hl,(965Fh)      ;This is the end of the currently running program      or a      sbc hl,de           ;HL is now the size of the program      ex de,hl      ld b,d      ld c,e      pop de      dec hl ;HL points to the start of the program ;DE is the line to jump to ;BC are the size of the program ;A is the newline token SearchLoop:      dec de      ld a,d \ or e      jr z,StoLine      ld a,3Fh      cpir      ret nz                  ;this means BC hit 0. In other words, the line does not exist. ;the byte is 3Fh, but we need to make sure that it is not part of a two-byte token      dec hl \ dec hl      ld a,(hl)      inc hl \ inc hl      cp 7Eh \ jr z,SearchLoop+5      cp $AA \ jr z,SearchLoop+5      cp $BB \ jr z,SearchLoop+5      cp $EF \ jr z,SearchLoop+5      cp 5Fh \  jr z,SearchLoop      sub 5Ch \ jr c,SearchLoop      sub 7 \ jr c,SearchLoop+5      jr SearchLoop StoLine:      dec hl      ld (965Dh),hl                  ;this loads the address to the BASIC program counter      ret

It can probably be optimised a little, but I am sure you get the idea

[ralphdspam]

Nice code, but I can't think of any other uses of the command.  Can you think of any hacky type usage?

[Xeda112358]

You can use cpir to determine the length of a string or name. For example, if you want the length of a program name in OP1 with a maximum limit of 9:

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ld bc,9      ld hl,OP1      xor a      cpir      jr nz,$+3                ;This means the string ended at maximal length        ccf      ld a,9 \ sbc a,c      ld c,a

In fact, you can turn that into a general code where you can find the length of a zero-terminated string pointed to by HL with a maximum size:

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push bc      xor a      cpir      pop hl      jr nz,$+3                ;This means the string ended at maximal length        ccf      sbc hl,bc           ;the c flag was reset by 'xor a', so hl is the length      ret

There are other ways that I have used it, too, I just need a moment to think of some more uses I was actually hoping some other coders had neat hacks.

EDIT: Another use for this is when you want to display a string, but only up to x digits.