Constructive and Creative uses of RLD and RRD

[ralphdspam]

I saw the topic "Constructive and Creative uses of IX and IY," and I liked the idea of brainstorming creative uses for commands. 

Two commands that always perplex me are RLD and RRD.  I cannot think of many practical uses of them. 

[calc84maniac]

I use one of those in TI-Boy to emulate the SWAP (HL) instruction, which swaps the upper and lower nibbles of (HL).
It works like ld a,(hl) \ rrd.

[DrDnar]

As far as I can tell, the instructions are intended for use with BCD. They appear to allow multiplying and dividing by ten. Or sixteen, if you're using binary.

[thepenguin77]

Indeed, the only time I've ever used it was for BCD. It's useful if you need to recover a number from TI's floating points. For instance:

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turnTheEntireOp1ToADecimalStringRegardlessOfSignificateFigures: ld hl, op1+2 ld c, 7 outerLoop: ld b, 2 innerLoop: ld a, $30 rld ld (de), a inc de djnz innerLoop inc hl dec c jr nz, outerLoop xor a ld (de), a ret

[ralphdspam]

I'm glad there are actually uses for that command.  I like the idea of manipulating nibbles, but I don't like the fact that it modifies the source data.  Can you use it on ROM code without any problems? 

[calcdude84se]

I'm glad there are actually uses for that command.  I like the idea of manipulating nibbles, but I don't like the fact that it modifies the source data.  Can you use it on ROM code without any problems? 

You probably shouldn't be manipulating code by nibble Anyway, A contains the correct result even if (HL) can't be changed, IIRC.

[calc84maniac]

* calc84maniac uses OPTIMIZATION

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turnTheEntireOp1ToADecimalStringRegardlessOfSignificateFigures: ld hl, op1+2 ld b, 7 ld a, $30 loop: rld ld (de), a inc de rld ld (de), a inc de inc hl djnz loop xor a ld (de), a ret

Edit:
Ooh, even better!

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turnTheEntireOp1ToADecimalStringRegardlessOfSignificateFigures: ld hl, op1+2 ld bc, 7 loop: ld a, $33 rrd ldi ld (de), a inc de jp pe, loop xor a ld (de), a ret

[Xeda112358]

Ooh, rrd and rld are some of my favorite instructions! When I was describing what it does in my pocket asm tutorial was this:

rrd/rld the two nibbles from (hl) and the last nibble of 'a' make three nibbles. rrd                
           rotates them right and rld rotates them left.

As for a way to use them, I have a few codes that make excellent use of them. This one is from my program BSPRT. It uses hexadecimal sprite data, so instead of converting the hex string to a location in RAM and then copying it to the screen, I just did both at once (here is the sprite display part):

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;Inputs: ;     HL points to the buffer location to draw to ;     DE points to the sprite data      ld bc,080Ch     ;010C08 Loop:      call PutNibble  ;CD****      call PutNibble  ;CD****      ld a,b          ;78      ld b,0          ;0600      add hl,bc       ;09      ld b,a          ;47      djnz Loop       ;10F3      ret             ;C9 PutNibble:      ld a,(de)       ;1A      add $C0         ;C6C0      jr nc,$+4       ;3002        sub 7         ;D607      rld             ;ED6F      inc de          ;13      ret             ;C9

This is my favorite one, though. When I made a graph shifting routine, I made these to shift 4 pixels very quickly:

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ShiftLeft4:      ld hl,GraphBuf+767      ld c,64        xor a        ld b,12          rld          dec hl          djnz $-3        dec c        jr nz,$-9        ret

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ShiftRight4:      ld hl,GraphBuf      ld c,64        xor a        ld b,12          rrd          inc hl          djnz $-3        dec c        jr nz,$-9        ret

I feel pretty proud of these I think the first can be optimised more, but it is still nice, I think

[chickendude]

Unless i'm missing something, the first just says "Array", maybe you pasted the wrong thing?

[Hayleia]

It is because she wrote [code=something].
Here is what she meant

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;Inputs: ;     HL points to the buffer location to draw to ;     DE points to the sprite data      ld bc,080Ch     ;010C08 Loop:      call PutNibble  ;CD****      call PutNibble  ;CD****      ld a,b          ;78      ld b,0          ;0600      add hl,bc       ;09      ld b,a          ;47      djnz Loop       ;10F3      ret             ;C9 PutNibble:      ld a,(de)       ;1A      add $C0         ;C6C0      jr nc,$+4       ;3002        sub 7         ;D607      rld             ;ED6F      inc de          ;13      ret             ;C9

[Xeda112358]

Unless i'm missing something, the first just says "Array", maybe you pasted the wrong thing?

I am not seeing that... On mine, Hayleia and I have the same thing...

[Hayleia]

Unless i'm missing something, the first just says "Array", maybe you pasted the wrong thing?

I am not seeing that... On mine, Hayleia and I have the same thing...

I see "Array" too on yours. Maybe it depends on the browser.
But do you see "BSPRT" next to "code" ? (I don't see it, I had to quote you to see it should be here )

[chickendude]

Well, it doesn't really matter now as i see what code you were talking about in Hayleia's post. Btw, what exactly do you mean by "hexidecimal sprites"? I'm trying to figure out what exactly your code does.

[Xeda112358]

Ah, well with that code, will draw a sprite with hexadecimal data input. By that, I mean . db "3C4281818181423C" as opposed to .db 3Ch,42h,81h,81h,81h,81h,42h,3Ch

The first is what a TI-BASIC string would be like and the program that code is in is designed for TI-BASIC programmers to use.

[Quigibo]

Axe uses RRD and RLD for nibble storing.  I've never used them anywhere else, they are definitely very uncommon. 

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;hl = Nth nibble after address $8000 ;e = Nibble to store there NibSto: scf rr h rr l jr nc,__NibStoHigh rrd ld a,e rld ret __NibStoHigh: rld ld a,e rrd ret __NibStoEnd: