ASM Command of the Week

[chickendude]

I haven't responded yet simply because, to be honest, i had no idea this command even existed I could see it maybe being used for a fixed-width font text-wrapping routine or something, though that might not be quite so simple as you'd have to check for a space character and EOS character.

[Xeda112358]

In that case, you can play with CPI >.>

[Hot_Dog]

Here's the next one:

CPL

[Xeda112358]

CPL is a great command for logic. Pretty much, it inverts the A register. The most common thing I use it for, is with drawing sprites and pixels with erasing logic. For example, to display a sprite by erasing, take the sprite data and invert it with cpl, then use AND logic on the screen. Also, you can use it to invert the graph buffer:

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ld hl,9340h      ld bc,3        ld a,(hl)        cpl        ld (hl),a        inc hl        djnz $-4        dec c        jr nz,-7      ret

[calc84maniac]

CPL can also be used to subtract from 255. This is useful to subtract A from any number N by doing this:

cpl
add a,N+1

Of course, if N=254, 255, or 0, replace the add with a dec a, nothing, or inc a respectively.

[Xeda112358]

Building off of what calc said, NEG is like CPL \ INC A, except NEG will modify the c flag.

[chickendude]

Like others have said, I use cpl for a quicker/smaller xor $FF, for example in my tilemapper to get the mask  when  rotating the screen:

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updateRotation:     ld a,(xOff)         ;what is the xOffset?     and 7               ;we're only interested in the lower three bits (essentially xOff % 8)     ld hl,gbufMask     ld e,a     ld d,0     add hl,de           ;pointer to the rotation mask     ld a,(hl)     ld hl,maskLeft     ld (hl),a     ld hl,maskRight     cpl                 ;xor $FF     ld (hl),a ;...     ret gbufMask: .db %11111111   ;0 display all of the first byte, none of the second .db %11111110   ;1 after rotating left once, we only want the 7 leftern most bits of byte 1, and rightmost bit of byte 2 .db %11111100   ;2 .db %11111000   ;3 .db %11110000   ;4 .db %11100000   ;5 .db %11000000   ;6 .db %10000000   ;7

...which, btw, is based off an idea calc84maniac mentioned on IRC a few years ago of doing the rotation in the fastcopy routine, though i remember you saying that your main fastcopy loop was much faster than mine

[Hot_Dog]

The next one is

SCF

Normally, this is used for math (do share )

Sometimes I find the need to use the carry flag for conditions beyond what it is normally used for (detecting overflows or underflows).  For example, I could theoratically use C or NC to see if an object is collidable or not.  Whatever the situation, I would use SCF to set the carry flag if a condition is true. 

[ralphdspam]

Was I not updating the topic weekly?  Whoops.  
Anyways, there is a very nice use of SCF in Axe's sound routine.  Instead of using it for math, it is used for conditional branching.

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xor a __FreqOutLoop1: push bc ld e,a __FreqOutLoop2: ld a,h or l jr z,__FreqOutDone dec hl dec bc ld a,b or c jr nz,__FreqOutLoop2 ld a,e xor %00000011 scf __FreqOutDone: pop bc out ($00),a ret nc jr __FreqOutLoop1

[Xeda112358]

I often like to use it to return whether a function was successful or not. For example, the c flag here means the pixel was in bounds, so its location was computed normally:

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GetPixelLoc: ;Input: ;     b is X ;     c is y ;Output: ;     HL points to byte ;     A is the mask ;     nc if not computed, c if computed        ld a,c        cp 64 \ ret nc        ld a,b        cp 96 \ ret nc        ld l,c        ld h,0        ld b,h        add hl,hl        add hl,bc        add hl,hl        add hl,hl        ld b,a        rrca \ rrca \ rrca        and 0Fh        ld c,a        ld a,b        ld b,0        add hl,bc        ld bc,(BufPtr)        add hl,bc        and 7        ld b,a        inc b        ld a,1          rrca          djnz $-1        scf        ret

Sometimes I need to correct an addition or subtraction by also adding or subtracting one:

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OP1NameLength: ;Returns the name length in HL         ld hl,8478h NameLength:         xor a \ ld b,a \ ld c,a         cpir         ld h,a \ ld l,a         scf \ sbc hl,bc         ret

(There is a better version of that)
Also, sometimes there are some very specific adjustments. In this case, you will see scf is used only when the end of the file is met, so an extra 1 must be subtracted to get the line length:

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SearchLine: ;Inputs: ;     HL points to the start ;     BC is the number of bytes to search ;     DE is the line number ;     A is the line byte ;Outputs: ;     A is not changed ;     BC is the number of bytes left for the search ;     DE points to the line ;     HL is the length of the line ;===============================================================      dec de \ inc d \ inc e      or a      ld (TempWord1),hl SearchLoop:      cpir                       ;21X-5      jp po,EndSearchLoop+1      dec e \ jr nz,SearchLoop-3      dec d \ jr nz,SearchLoop-3 EndSearchLoop:      scf      ld de,(TempWord1)      sbc hl,de      ret

[Hot_Dog]

OR

In bit manipulation, you can use this to set bits

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ld a, %10000000 or %10000001   ;The first and last bits will be set no matter what.  All others will not be affected.

OR A is a wonderful substitue to "CP 0."  Also, OR A is a great way to reset the carry flag.  Whenever you want to subtract something from HL, you have to use the instruction SBC, and in a lot of cases, you want to make sure that the carry flag is reset, or your subtraction result could come out wrong.

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ld hl, 10000 ld de, 5000 or a sbc hl, de  ;Now it's guranteed that HL = 5000 instead of 4999.

[calc84maniac]

OR can be used to check if a multi-byte value is zero. For example, to check if BC is zero, use
ld a,b
or c
jr z,Value_Is_Zero

If you have a 32-bit value in DEHL, you can similarly use
ld a,d
or e
or h
or l
jr z,Value_Is_Zero

[Hot_Dog]

BIT

This will test a bit in a register to see if it's 1 or 0.  I find this instruction especially useful for (hl) and A  (If I don't want to lose the value that I have stored in register A with bit manipulation)

[Xeda112358]

In a similar vein, you can do some soft-core SMC to speed up a routine such as pixel-testing or even turning the pixel on or off:

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;=============================================================== PlotPixel: ;=============================================================== ;Inputs: ;     B is the x-coordinate ;     C is the y-coordinate ;     D=Method ;        1=Test ;        2=Off ;        3=On ;     (BufPtr) points to the buffer to draw to ;Outputs: ;     BC is equal to (BufPtr) ;     DE not modified ;     HL points to the byte the pixel is drawn to ;     When using the pixel test method: ;       z means pixel off ;       nz means pixel on ;Destroys: ;     A ;===============================================================      ld a,3Fh            ;7      srl b \ rra         ;12    19      srl b \ rra         ;12    31      srl b \ rra         ;12    43      cpl                 ;4     47      or d                ;4     51      rrca \ rrca         ;8     59      ld (Method),a       ;13    72      ld a,b              ;4     76      ld b,0              ;7     83      ld h,b \ ld l,c     ;4     87      add hl,bc           ;11    98      add hl,bc           ;11   109      add hl,hl           ;11   120      add hl,hl           ;11   131      ld c,a              ;4    135      add hl,bc           ;11   146      ld bc,(BufPtr)      ;20   166    BufPtr points to the buffer      add hl,bc           ;11   177      .db $CB             ;15   192    First byte of BIT Method:      .db 46h             ;            second byte of the BIT instruction      ret                 ;10   202  37 bytes

[Quigibo]

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1 2	bit 5,a bit 3,a

These will set the hidden flag in the F register in the same way as, and in addition to, the z flag.  Obsucre? Yes.  Useful?