Re: [f-cpu] Manual 0.2.6

[Michael Riepe <michael@stud.uni-hannover.de>]

On Thu, Jul 25, 2002 at 01:52:17PM +0200, Yann Guidon wrote:
[...]
> > >       7.1.7 `syscall'/`trap':
> > >               It's still not clear what the difference is.
> > >               I suppose both instructions perform an SRB.
> the difference is that "syscall" uses CPU cycles in a shared library
> or something like that, on behalf of the calling task. A trap is
> used for exceptional conditions, not common functions.
> But that old question could still change a bit...

Shared library calls usually are ordinary function calls.

Two different instructions only make sense if one of them

	- doesn't switch to supervisor mode
	- doesn't perform an SRB

(that is, works in the context of the caller). But how are you going
to return from it? `rte' won't work because it assumes that a context
switch has been performed. The only form that would make sense is

	syscall imm16, r1

which performs

	r1 := pc + 4
	pc := table_from_some_SR[imm16]

(plus range checking) but nothing else.

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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