At 11:49 AM 9/17/2001 -0500, Brian Wheeler wrote:
>On Mon, 2001-09-17 at 11:20, Gregor N. Purdy wrote:
> > > > It would also be helpful to have jump_ic and absjump_ic, rather than
> > > > having to load these things into registers.
> > >
> > > You can use branch_ic for this. absjump is probably a bad idea since it
> > > appears the return value of the opcode function is really a memory
> > > address of the host machine. If an absolute value (relative to the
> > > beginning of the opcode stream) was used, then it would have to be
> > > relocated at load-time and wouldn't be able to remain read-only.
> >
> > My mental model for this thing is "Parrot is a CPU architecture" and
> > a lot of what I'm trying to make it do is behave as much as possible
> > in that way. So, if we are talking about a CPU architecture (ISA),
> > lacking the full complement of relative and absolute jumps, register
> > and constant comparisons, etc. is a bummer.
>
>I think we're ok on relative ops, but you're right, we're short of
>absolute.
FWIW, all the absolute ops should be done against register values, not
constants. There'll be ways to get absolute addresses of things like
subroutines and PMCs.
> > Mucking with things like memory addresses of host machines, etc. is
> > important, but only at the interpreter level, not at the ISA design
> > level (IMHO).
> >
> > I don't see a relocation issue anyway, since the way I look at the
> > interpreter code is:
> >
> > PC === (code - start_code)
> >
> > So if an instruction says "absjump 43", then I know that I need
> > to do:
> >
> > code = (start_code + 43)
> >
> > and if an instruction says "(rel)jump 43" then I know that I need
> > to do:
> >
> > code += 43
> >
> > neither of these is a problem in my eyes.
Both of those ought to be plain relative jumps. The absolute jump should be
resolved by the assembler to a relative offset. ;)
>Ok, they're not really problems, per se, but the way the interpreter is
>constructed, the ops *always* return the offset to the next PC.
Nope. The ops always return the absolute next PC. The un-preprocessed
versions generally return relative values which the preprocessor munches
(the AUTO_OP ones) but that's just a preprocessor thing.
Dan
--------------------------------------"it's like this"-------------------
Dan Sugalski even samurai
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