Simulator: base_insn and insn in decode.c

[Ronald Hecht]

I switched to the SID framework. I found it easier to create a new 
target. The disassemby works better as it calls this disassembler code 
in src/opcodes. But I'm still in trouble with decode.cxx. It looks the 
same as in sim. I'n my opinion the entire_insn should alwasy be left 
aligned and not right_aligned as it is now. Base_insn should have the 
same alignment. I think this way it is easier to support variable sized 
instructions.

Best Regards
Ronald

Dave Brolley wrote:

> I forgot to address your other question which was about the difficulty 
> of setting entire_insn before decoding. It's a bit of a chicken and 
> egg situation isnt'y it? You can't set entire insn properly until you 
> know something about the insn, but you can't call decode without 
> setting entire_insn properly.
>
> Your method of calling decode twice won't work because of this.
>
> Most existing ports determine the insn length by looking at a few bits 
> in base_insn and use this information to fill entire_insn.
>
> I'm wondering if the decoder shouldn't be redesigned so that all it 
> takes is base_insn, which should be all that is necessary to identify 
> the insn. The extractors for each format would then read any 
> additional bytes as needed. There is already support for this for ISAs 
> with insns which are longer than an insn word.
>
> Dave
>