Yesterday someone in a CPC board posted this question, and as..
1.) I never used these Z80 commands before
2.) I couldn't find infos in this MSX board via the search function
..I would be quite interested in the question, if anybody has found a good use for RLD & RRD?

RLD / RRD? you mean the rotating bit?

it is not?

RL D
RR D

what you mean exactly about usague?

(I not understand the question because they are very common)

I guess you mean besides their intended usage in multiplication and division of BCD numbers?

I've seen it used to scroll screens by 4 pixels in some Spectrum games, but that's about it. And I've never used them either.

Quote:

Yesterday someone in a CPC board posted this question, and as.. 1.) I never used these Z80 commands before 2.) I couldn't find infos in this MSX board via the search function ..I would be quite interested in the question, if anybody has found a good use for RLD & RRD?

WOW!, sincerely I always though that was clones of RR D / RL D..... I re-checked the Z80 doc, now I see your point......

Aplications:

Thinks, In a ROM and by that an UNCRACKEABLE GAME!

just use RRD / RLD in everywhere you need to load the lower nibble of a RAM address.... If the program runs in RAM, it will work one time, the second time the RAM will be modified and IT will not work!.

By example, some graphics load in screen 5. The second time that they will be loaded to VRAM, all graphics corrupted by sure.!

But, without doubt the original task is the load of BCD digits.!

Here is for example compact HEX print routine, that uses RLD: (Data to be printed is pointed by HL)

	xor	a
	rld
	call	Nibble

Nibble	push	af
	daa
	add	a,F0h
	adc	a,40h

	call	a2h	;CHPUT

	pop	af
	rld
	ret

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Here is for example compact HEX print routine, that uses RLD: (Data to be printed is pointed by HL) xor a rld call Nibble Nibble push af daa add a,F0h adc a,40h call a2h ;CHPUT pop af rld ret

Very good... I rember the "daa + add a,F0h" trick from GuyverR800's code...

Hey that was my code

You can also use RRD or RLD to access to the nibbles of a table in RAM. I use one of these instructions to apply changes in an SC2 color table decrunched to RAM before transfer it to VRAM.

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Hey that was my code

Oh,! Sorry O.o)

Really impressive.
So this code

or a
daa
add a,F0h
adc a,40h

will convert 0-15 (in A) into "0"-"9" and "A"-"F"...
Really cool, I never had a deeper look at the DAA command!

I totally miss these instructions! I think that some of my programs would include them very soon

msxbanzai.tni.nl/dev/faq.html Here is routine which converts a binary number to hexadecima using DAA

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Thinks, In a ROM and by that an UNCRACKEABLE GAME! just use RRD / RLD in everywhere you need to load the lower nibble of a RAM address.... If the program runs in RAM, it will work one time, the second time the RAM will be modified and IT will not work!.

In fact there is one rom that does it: Ashguine 3 (from Microcabin). It was a hard time to find what was going wrong with the disk version Anyway, I don't think they did that for protection purposes, but just for coding convenience.