[Coco] Clearing condition code register

[jdaggett at gate.net]

Mike

According to the programming manual the COM/NEG instructions does as 
I mentioned. 

Does not 100% guarentee that I am correct though. The manual can be 
wrong. Has happened before.


The COM instruction actually inverts the bits and adds zero to the 
memory location. the result is stored in the memory location. 

The NEG does a twos compliment. 


james


On 28 Mar 2005 at 21:45, KnudsenMJ at aol.com wrote:

From:           	KnudsenMJ at aol.com
Date sent:      	Mon, 28 Mar 2005 21:45:51 EST
Subject:        	Re: [Coco] Clearing condition code register
To:             	coco at maltedmedia.com
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> 
> In a message dated 3/27/05 3:17:47 PM Eastern Standard Time,  
> jdaggett at gate.net writes:
> 
> >The  COM instruction will always set the Carry bit. The NEG will set
> >the  carry bit if there is a borrow, otherwise it is  cleared.
> 
> 
> 
> Are you sure?  ISTR that the COM instruction just inverts all the 
> bits, and should never set the Carry or Overflow bits.  
> 
> NEG does a COM and then adds 1, which will set the Carry bit if the 
> register was 0 or held the largest negative number (which has no
> positive  representation).
> 
> But if you've got the 6809 manual handy, then you're right.  --Mike 
> K.
> 
> 
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