Subject: Re: tf_pc value
To: Ben Harris <firstname.lastname@example.org>
From: Chris Gilbert <email@example.com>
Date: 03/11/2001 00:57:37
Eeks, when did port-arm mailing list appear? Checks archive, odd the mail
from ben to arm26 and arm32 never made it...
On Saturday 10 March 2001 4:07 pm, Ben Harris wrote:
> Right. I think I've got something approaching a conclusion:
> My reason for not having the assembler stubs for the SWI and undef handler
> wind back tf_pc is that in order to do so, they need to know whether the
> processor was in Thumb state (since the PC is advanced by one instruction,
> rather than by a fixed number of bytes). At the obvious point to do the
> winding back, there are no spare registers to MRS into, so since the C
> handlers will need to know what state the CPU was in anyway, they may as
> well wind back tf_pc at the same time.
the swi they could be done in asm see page A9-12 of the ARM ARM, it's shows
how to get the right number out for either thumb or arm, so with the same
text you can move the tf_pc appropriatly.
Something of note is that it shows that thumb only allows for 256 swi codes,
I believe that netbsd has 512 syscalls at the mo, looks like some kind of
magic might be needed to make thumb cope with that....