[Coco] lwtools assembler difference

[Dave Philipsen]

Thanks for the explanation, William.  I imagine whoever assembled the 
Boot module that I have had the pragma "noindex0tonone" enforced then 
for some reason because the source code I have seen for the Boot file 
does not use a literal "0" but a symbol that represents "0".

I'm only bringing this up because I'm trying to get the stock OS9 
assembler to output the exact same module that is in the repository and 
when comparing I came up with this difference.  I don't even have lwasm 
installed yet....just trying to make it all work natively from NitrOS9.

Dave Philipsen


On , William Astle 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
>> 
>>