Re: PIC question

[der Mouse <mouse%Rodents-Montreal.ORG@localhost>]

[Martin, sorry for the accidental direct send of a former version of
this; it was my mistake.]

> I'm new to vax asm [...]

Welcome!

> [...want PIC version of:]

> It does kindof Duff's device in asm - there is an unrolled loop
> starting at .Loop and a jump with offset into the loop is calculated
> in %r5, then the code does:

>       jmp     .Loop(%r5)
> .Loop:        ...

> What is the pic way to do this?

Probably something like "jmp (pc)[r5]".  jmp's operand is a mode ab
operand - <a>ddress of <b>yte - so that's conceptually right, and I'm
moderately sure it's one of the allwoed uses of pc.  (That's DEC
syntax, because I learned VAX assembly under VMS; adjust as necessary
to appease the assembler.)

> Some trick?  There doesn't seem to be a br + register offset
> instruction.

There isn't.  The VAX is very CISC; it doesn't put that kind of thing
in the opcode.  You just use JMP and turn the full power of the
addressing modes loose on the operand.  (The brb/brw instructions are
just optimizations for the very common cases of a small fixed offset;
the branch instructions are among the very few instructions don't have
full addressing mode capability in the operands.)

> Or along the lines of:
> 
>       addl2   %pc,%r5
>       add     .Loop - ., %r5
>       jmp     (%r5)
> .Loop:        ...
> 
> (but the .Loop-. is not accepted by gas)

You're thinking too RISC. :)  Something along those lines could be made
to work, but it's a good deal more complex than necessary.

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