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ASM: compare two characters?

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ASM: compare two characters? A place for discussing programming languages not covered in specific forums such as Assembler, COBOL, etc. - you get the idea.

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  #1  
Old May 21st, 2006, 10:36 PM
n000b n000b is offline
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ASM: compare two characters?
Hi,

I have a character and I need to know whether it is greater than or less than some other characters (for example, I need to know if the character is greater than/less than the letter 'a').

How do I do this for the Motorola 68K processor?
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  #2  
Old May 22nd, 2006, 05:06 AM
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Quote:
Originally Posted by n000b
Hi,

I have a character and I need to know whether it is greater than or less than some other characters (for example, I need to know if the character is greater than/less than the letter 'a').

How do I do this for the Motorola 68K processor?


This is a very beginner question, perhaps you should go read a tutorial on 68k asm.

There also isn't much experience with asm for that architecture here.
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Old May 22nd, 2006, 05:27 AM
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I have tried Googling for a while with no result, could you please point me in the right direction?
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Old May 22nd, 2006, 05:54 AM
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I can't seem to find anything either, other than a tip to try find a manual on ebay.
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Old May 22nd, 2006, 06:00 AM
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I'll have a look for something like that. Is it just one command that I need? If it is just one command then could you please tell me what it is and I'll go see how to use it
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Old May 22nd, 2006, 06:07 AM
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Hrm, i just found an old book on writing a compiler that uses 6800..

Quote:
On the 68000 the condition flags are set whenever any data is
moved or calculated. If the data is a 0000 (corresponding to a
false condition, remember), the zero flag will be set. The code
for "Branch on zero" is BEQ. So for our purposes here,


BEQ <=> Branch if false
BNE <=> Branch if true
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Old May 22nd, 2006, 06:11 AM
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Thanks mate, I'll check in to it
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Old May 22nd, 2006, 08:39 PM
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I thought that this might be how you do it but it doesn't work:

Code:
	org	$1000
	move.l	#$7ffe,sp
	
	move.b	#$64,d2
	move.b	#$68,d3
	
	sub.b	d3,d2
	bne	aaa
	bra	bbb


	
; loop through and display a string ('smaller')
aaa	move.l	#a,a4

anext	move.b	(a4)+,d1
	
	cmp.b	#null,d1
	beq	finish
	
	move.b	d1,d0
	
	move.b	#248,d7
	trap	#14

	bra	anext
	
	
	
; loop through and display a string ('bigger')
bbb	move.l	#b,a4

bnext	move.b	(a4)+,d1
	
	cmp.b	#null,d1
	beq	finish
	
	move.b	d1,d0
	
	move.b	#248,d7
	trap	#14

	bra	bnext
	
	
	
	
	
finish	move.b	#228,d7
	trap	#14

	
	org	$1500
a	dc.b	'smaller',null
b	dc.b	'bigger',null
null	equ	00	
	end


I'm basically subtracting one from the other and then using the branch if equal/not equal codes, but it's not working. What am I doing wrong?
Last edited by n000b : May 22nd, 2006 at 08:50 PM.
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Old May 22nd, 2006, 11:57 PM
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I think I worked it out
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Old May 23rd, 2006, 08:23 AM
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Could you please post your solution then? We do get a fair few 68k assembler questions, and there isn't a whole lot of expertise on it in here. It might help a few other people.
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Old May 23rd, 2006, 08:34 AM
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I had to use the cmp.b instruction, and then use blt (less than), bgt (greater than), beq (equal) etc.

Like this:

Code:
; checks to see if d2 is greater than d1 and branches to "someplace" if it is
cmp.b d1,d2
bgt someplace
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LinuxPenguin agrees: Thanks for coming back to post your solution, it's a great help to those who bother searching.
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