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From: wittig@gmdzi.UUCP (Georg Wittig)
Newsgroups: comp.os.cpm
Subject: Re: Z80 algorithms
Message-ID: <1335@gmdzi.UUCP>
Date: 17 Oct 89 13:24:58 GMT
Organization: GMD - German National Research Laboratory for Computer Science
Lines: 62


Someone mailed to me the following memory fill algorithm:

>
>	ld	hl,buffer		; point at buffer
>	ld	de,buffer + 1		; point at next byte
>	ld	bc,count - 1		; number of bytes minus one
>	ld	(hl),xxx		; save the first byte
>	ldir				; replicate through rest of buffer
>
>is the fastest buffer fill I know on the Z80.

There exists a much faster faster algorithm for that. Let's see:

The central statement in your solution is `ldir'. It takes 21 T states per
byte. For 16 bytes this is 336 T states.

Using the push statement is much faster:
	Set D = E = the byte to be filled in;
	let SP point 1 byte after the end of the area to be filled;
	B contains the number of 16 byte blocks to be filled.
Then use "push DE"s, and you're finished very quickly.

	DI			; CP/M must not interrupt, because SP will be
				; misused
	LD	(sp_save),SP	; save current SP value
	LD	SP,HL		; assuming HL points to <end+1>
L:	PUSH	DE		; 8 times, so 16 bytes are filled
	PUSH	DE
	PUSH	DE
	PUSH	DE
	PUSH	DE
	PUSH	DE
	PUSH	DE
	PUSH	DE
	DJNZ	L		; a 16 byte portion has been processed.
	LD	SP,(sp_save)	; restore the SP
	EI			; done

This way you can fill up to 4096 (256*16) bytes. If more bytes are to be
filled, build a loop around it. If the number of the bytes to be filled isn't a
multiple of 16, the bytes 1 to 15 can be filled straight forward with a
traditional algorithm.

The timing of that algorithm: The central loop starts at "L:" and ends with
"DJNZ". "push DE" needs 11 T states; DJNZ needs 13 ones. So for 16 bytes to be
filled you get: 

	8 * 11 + 13 = 101

	101 / 336 = 30 %

Relatively fast, isn't it? And -- it works!

PS: The idea isn't mine, I found it some time ago in a journal. I'm sorry I
don't remember which one it was.

-- 
Georg Wittig   GMD-Z1.BI   P.O. Box 1240   D-5205 St. Augustin 1 (West Germany)
email: wittig@gmdzi.uucp   phone: (+49 2241) 14-2294
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