The GBA normally runs in system mode, so you're free to change CPU
modes whenever you want. Just use an mrs or msr (don't remember which
is which).
What is it you're trying to do? The only thing I can think of is
messing with the FIQ regs, which I have sucessfully tested on
hardware from the normal mode.

--- In gbadev@yahoogroups.com, Michael Reid - CSCI/P2003
<m6reid@s...> wrote:
> hi,
>
> i'll start with the obligatory, "i'm new to this" bit. i've only
been
> playing around w/ programming the gba for about a month.
>
> anyhow, i would like to be able to add my own system calls, via the
swi
> instruction, however, the little info i have found about this
implies that
> the system calls are BIOS only, and that there is no way to add any
> user-defined calls.
>
> does anyone know if this is true or not? has anyone been able to
define
> their own syscalls?
>
> if not, then can anoyone think of a way to force a hardware
interrupt
> (enter irq mode)?
>
> thanks