[Coco] assembly question

[L. Curtis Boyle]

Art, I always did the same thing (including in Nitros9). I only used bcc/bcs mnemonics when the program flow was checking the carry bit; if I was dealing with unsigned math, I would use bhi/bhs/blo/bls, as it just made more sense to me. 

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> On Aug 9, 2015, at 2:44 PM, Arthur Flexser <flexser at fiu.edu> wrote:
> 
> I always found it much clearer to use the mnemonic BLO rather than BCS
> (both are equivalent, opcode $25), and BHS rather than BCC (both opcode
> $24).  Makes the outcome less likely to be unexpected.
> 
> Art
> 
>> On Sun, Aug 9, 2015 at 4:30 PM, camillus <camillus.b.58 at gmail.com> wrote:
>> 
>> 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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