[Coco] lwtools assembler difference

[David Ladd]

William, that is very good to know that.  I myself didn't know that.
Though I normally don't look at the assembled code byte per byte to compare
stuff either. :O

Now that I know such things like that happen I will have to be careful to
make sure I use ,x rather than forcing 0,x

On Tuesday, September 1, 2015, William Astle <lost at l-w.ca> wrote:

> If I recall correctly, EDTASM uses the 5 bit offset if there is any offset
> specified, even a literal 0.
>
> What lwasm does is more complicated than you have observed.
>
> If you write 0,x explicitly, it assumes you really want 0,x. There are
> reasons you might (timing critical loops, for instance). And yes, I have
> seen at least one actual real chunk of code that required a 5 bit 0 offset.
>
> However, if you write any expression that is not a literal "0", it will
> evaluate it and if it evaluates to 0, it will use the no offset version.
>
> That is, unless you have pragma "noindex0tonone" in force, in which case
> it will assemble using the 5 bit offset.
>
> Of course, if you force 8 or 16 bit offsets, you'll get those.
>
> The reason a literal "0,x" is treated as "yes, I want the 5 bit mode" is
> this: if you're writing a literal 0 offset, you know for sure what the
> offset is so you could just as easily write ",x" instead of "0,x".
>
> As you noted, the actual outcome is not affected. The resulting code is
> still operationally correct and the same size. Only the cycle count changes
> with 5 bit offsets being 1 cycle slower.
>
>
> On 2015-09-01 13:16, Dave Philipsen wrote:
>
>> Well, in my comparison of the code generated by the lwtools assembler
>> and the stock OS9 assembler I have found a quirk of the lwtools
>> assembler.  I won't call it an error because it appears that the outcome
>> is not affected and it seems to be a minor thing.  On every other
>> assembler I've used including AS6809, CSC6809, and the stock OS9
>> assembler the following assembly source:
>>
>> ldb  ,x
>>   or
>> ldb 0,x
>>
>> is assembled in machine code as:
>>
>> E6 84
>>
>> which is the indexed addressing mode with no offset
>> However, the lwtools assembler (evidently) assembles in machine code as:
>>
>> E6 00
>>
>> which is the indexed addressing mode with a 5-bit offset with the offset
>> being zero.
>>
>> I dont have a copy of EDTASM anymore so I can't say how it would
>> assemble it.  This just seems to be the case of a smart assembler which
>> determines the given offset to be zero and thus translates it as a "no
>> offset" postbyte as opposed to a "5-bit offset" postbyte.
>>
>> Dave Philipsen
>>
>>
>>
>
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