Re: Instruction w/ prefix

[Doug Evans <dje@sebabeach.org>]

Dmitry Eremin-Solenikov wrote:
> Hello,
>
> Doug Evans wrote:
>> Dmitry Eremin-Solenikov wrote:
>>> Hello,
>>>
>>> I'm using cgen to create binutils port for the CISC architecture
>>> (M68HC(S)08 if somebody cares). I've stumbled upon one problem: the is
>>> an instruction prefix which can change meaning
>>> of the next instruction (basically it then uses SP register instead 
>>> of IX one).
>>>
>>> What is the most clean way to do this w/ cgen? I can of course
>>> duplicate instruction fields, counting another byte in the beginning,
>>> instructions, etc.. This will permit me to catch illegal combinations.
>>> OTOH it doesn' look like completely clean idea.
>>>
>>> What would you suggest?
>>>
>>>   
>>
>> Hi.  Sorry for the slow reply.
>>
>> I think the right solution (which I wouldn't wait for) is to extend 
>> cgen to handle instruction prefixes.
>
> Do you have any ideas how to implement (or at least design) this?
>
> As for me, the prefix just changes the addressing (prefixed instruction
> just uses SP as a base instead of IX register).

My current thinking is that they wouldn't be that different from 
individual instructions, except that they'd have a flag (attribute?) 
that says "I'm a prefix", and instructions that take prefixes would 
specify which prefixes are valid, and semantics would have some way of 
querying/specifying what prefixes are present/required.
[There's more details like whether multiple prefixes are valid, if 
there's a required order of prefixes (I guess), but that's the high 
order bit, I think.]

>
>> In the meantime, I would treat the prefix as its own instruction.
>
> Yep, I've tried this approach. However I've stumbled upon the fact that
> cgen just sums all specified values, thus leading me into problems.
>
> I'm currently looking into how to overcome this. Maybe you can suggest 
> a  sample which I can look upon?
>

Can you elaborate on "just sums all specified values"?  If all the 
prefixes are their own instructions, what values are summed up?

P.S. If one needed a simulator here, I'd create a register, named say 
h-use-sp-for-base or some such, that specified whether the "use-sp-base 
prefix" was present, reset it at appropriate times, have the use-sp-base 
prefix set it, and query the register in appropriate locations.  Then 
you could either choose between SP and IX in every instruction that 
needs it by querying h-use-sp-base, or you could create a virtual 
register that returns one of SP or IX depending on the value of 
h-use-sp.  I haven't tried it, but I think that would work.  [If it 
doesn't, I think it *should* work so let's fix it if it doesn't.]