[Coco] assembly language question.

[William Astle]

Arthur Flexser wrote:

> The example seems a bit unusual in that you're comparing a 16-bit quantity

> (the X register) with an 8-bit one (due to the use of the 'less than'

> sign).  I'm guessing that that would cause only the LSB of X ($FA) to be

> compared to <$000A (=$0A) with the outcome that the BLO no branch since $FA

> is greater than $0A by an unsigned comparison.


Not correct, actually. That's "direct page" addressing. That has no 
impact on the operand size. Only the address the operand is fetched from.


> 

> Art

> On Wed, Feb 3, 2010 at 9:10 PM, Gene Heskett <gene.heskett at verizon.net>wrote:

> 

>> Greets all;

>>

>> I've read through the moto programmers manual for the 6809 for the cmp

>> functions, and am still confused.  The cmp function is a subtract function

>> where the answer is thrown away, only affecting the CC flags so that a

>> following beq or blo will take the correct jump.  But somehow, it isn't

>> clear

>> which is subtracted from what.

>>

>> Given that regs.x contains $02FA, and that the integer memory location

>> referenced by LblName contains $000A, will the blo take, or be ignored

>> here?

>>

>>        cmpx  <LblName memory address in DP that holds the $000A above

>>        blo     somewhere else

>>

>> Thanks.

>>

>> --

>> Cheers, Gene

>> "There are four boxes to be used in defense of liberty:

>>  soap, ballot, jury, and ammo. Please use in that order."

>> -Ed Howdershelt (Author)

>>

>> <cesarb> Damn, every time I spawn, qf-client-x11 locks hard

>> <Zoid> Don't die?

>> <Knghtbrd> good incentive.

>>

>> --

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>>

> 

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-- 
William Astle
lost at l-w.ca