[Coco] assembly question

[camillus]

hi my $0.02 worth.

I looked up in the 6809 instructions and DEC B (A) does not change the carrybit.

Also the BCC and/or BGE are testing the carry bit, not the value of the counter in B.  

BCC ( Branch if Carry Clear ) will only branch when the carry is 0.

BGE ( Branch Greater or Equal to zero ) which will always branch because it is either 1 ( greater then 0 ) or 0 ( equal with zero )

So to make this work I would test the content in B with COMB, #0 which will set carrybit when true and then BNE ( Branch Not Equal to zero ) as next instruction, ofcource needs to branch out of the loop in this case.
 So if you really want to have 4 itterations 0-3 then you have to start at 4 because zero would take u out.

The reason that you exit the loop, I think is because the counter B goes down to zero, and I do not know if it can go negative. My guess is that somehow the carry get set by the fact B try's to go lower then zero.

Do not take this info as the holy grail, it was just my thoughts, and I am just human, prone to error or mistakes...lol



cb

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On 8/9/2015 5:59:45 AM, Barry Nelson <barry.nelson at amobiledevice.com> wrote:
Really? Arg. I just wrote some code using bcc and it seemed to work after a decb, at least my code did not loop endlessly. I wonder how it managed to exit my loop? Probably something else inside my loop is setting the carry flag? I guess I had better use bge instead. Thanks for the info. Darn code quirks! You are sure right?

On Aug 9, 2015, at 1:18 AM, coco-request at maltedmedia.com wrote:

> Date: Sat, 08 Aug 2015 21:53:08 -0600
> From: William Astle
> To: coco at maltedmedia.com
> Subject: Re: [Coco] assembly question
> Message-ID:
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> DEC doesn't set C (carry) so any branch that relies on the setting of C
> cannot be used meaningfully after DEC. That is the four "unsigned"
> branches. DEC does, however, set N, Z, and V which means the signed
> branches (which don't test C) do work as expected (since they use V, not C).
>
> The reason INC and DEC do not affect C is so that they can be used for
> iteration counting in multiple precision arithmetic.
>
> On 2015-08-08 21:32, K. Pruitt wrote:
>> I have an assembly question.
>>
>> How is the BCC operand any different than the BGE operand in the context
>> of using it on a counter?
>>
>> Here's the code example:
>>
>> ldb #$03
>> Loop stb Counter
>>
>>
>>
>> decb
>> bcc Loop
>>
>> The idea is to take the counter from 3 to 0 and include 0 in the loop.
>> So four times through the loop.
>>
>> The code above does not work for me. However, this code does:
>>
>> ldb #$03
>> Loop stb Counter
>>
>>
>>
>> decb
>> bge Loop
>>
>> The only difference is the bge instead of the bcc. From the description
>> I read in Leventhal's book, bcc and bge sound functionally identical in
>> this context.
>> But they are not.


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