[Coco] assembly language question.

[jdaggett at gate.net]

Gene

The operation is always

R - M 

where R is the register and M is either a second register or memory. CCR is set according to 
the results and addressing mode. 

In your case the MPU would take $02FA and subtract $000A. The subtraction instruction will 
affect the N, Z, V or C bits. The BLO instruction tests the C bit and will issue a branch if and 
only if the CBIT = 1. In this case there is no borrow so the CBIT will be zero and no branch 
taken. 

james

On 3 Feb 2010 at 21:10, Gene Heskett 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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