[Coco] assembly language question.

[Gene Heskett]

On Wednesday 03 February 2010, William Astle wrote:
>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
>
>The subtraction in CMP is exactly equivalent to SUB. That is, the
>operand is subtracted from the register. In this case, the subtraction
>is $02FA-$000A giving $02F0 with C=0, N=0, V=0, Z=0.
>
>In this case, the condition will be false so no branch will be taken.
>(BLO is the same as BCS or branch if C=1).
>
>Thus, in the above, if X is less than the value at LblName (unsigned),
>the branch is taken; otherwise it's not.
>
Which is the general understanding I had too, but was getting confused about 
because stuff isn't working quite right in sc6551.dr.  Thanks William.

-- 
Cheers, Gene
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