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From: dg@wrs.UUCP (David Goodenough)
Newsgroups: comp.misc
Subject: Re: Assembly Language
Message-ID: <309@wrs.UUCP>
Date: Tue, 18-Aug-87 14:22:18 EDT
Article-I.D.: wrs.309
Posted: Tue Aug 18 14:22:18 1987
Date-Received: Fri, 21-Aug-87 07:29:56 EDT
References: <892@edge.UUCP> <7359@think.UUCP> <4180@ncoast.UUCP>
Reply-To: dg@wrs.UUCP (David Goodenough)
Organization: Wind River Systems, Emeryville, CA
Lines: 94

In article <4180@ncoast.UUCP> robertd@ncoast.UUCP (Rob DeMarco) writes:
>    However, logically speaking, an assembler program is difficult to follow
>and grasp. You would HAVE to "comment fiendishly" so the person can see what
>it is doing. Logically on the human mind, a statement like this:
>
>    IF A = 8 THEN GOTO 5000
>
>is a little easier to follow then this:
>
>    CPI 8H
>    JZ 0C00H

I also do a fairly large amount of work in assembler, and I've found a very
clean, simple method of commenting & explaining what's going on.

Step 1:		Write & debug the code in C

Step 2:		Copy the C source to a .Z file (assembler source)

Step 3:		Put a ';' (comment start character) in front of each line

Step 4:		Produce the opcodes to do what the C does

Step 5:		Debug (99% of the time with a listing in hand)

So typical C code might be:

strncpy(dest, source, count)
char *dest, *source;
 {
    while (count-- && (*dest = *source))
     {
	dest++;
	source++;
     }
    *dest = 0;
 }

And the output:

;strncpy(dest, source, count)
.var	#dest	6		; offset from ix of dest
.var	#source	8		; offset from ix of source
.var	#count	6		; offset from ix of count
;char *dest, *source;
.extern	_strncpy
_strncpy:
	call	#csv		; set up frame pointer
	ld	c,(ix + #count)	; get count to bc
	ld	b,(ix + #count + 1)
	ld	e,(ix + #source); get source to de
	ld	d,(ix + #source + 1)
	ld	l,(ix + #dest)	; get dest to hl
	ld	h,(ix + #dest + 1)

/* as an aside, I use macros for the three ld pairs above:
	load	b,c,#count
 * which expands to the first load pair */
; {

loop:
	ld	a,b
	or	c
	jr	z,break		; exit loop if count == 0
	dec	bc		; count--

	ld	a,(de)
	ld	(hl),a		; copy a byte
	or	a		; and test for zero
	jr	z,break		; exit if zero byte at end of string

;    while (count-- && (*dest = *source))
;     {
	inc	de
;	 dest++;
	inc	hl
;	 source++;
	jr	loop
;     }
break:				; we always hit here with zero in a
	ld	(hl),a		; so save a zero byte to end
;    *dest = 0;
	jp	#cret		; clean up stack & return
; }

Now any one who can't read and understand that with a minimal knowledge
of z80 assembler needs to go back to school.
--
		dg@wrs.UUCP - David Goodenough

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