[A89] Re: change the SR

To: <assembly-89@lists.ticalc.org>
Subject: [A89] Re: change the SR
From: "Niklas Brunlid" <e96nbr@efd.lth.se>
Date: Tue, 29 Jan 2002 18:05:07 +0100
References: <OE63PBjxPGsKsJbB8qr00002cfa@hotmail.com>
Reply-To: assembly-89@lists.ticalc.org


> Hi,
> 
> I want to change the SR with a move ..,SR instruction but i got a privileged
> violation error, because the processor is in user-mode. How do I get into
> supervisor mode? I know, when an interrupt is called, the S/U bit is
> switched, but when it returns from interrupt, it's switched again. Do I have
> to write my own interrupt code that doesn't restore the SR when it returns
> back to normal code? Isn't there an easier way(ROM call?) to get the
> processor in Supervisor mode, so I can change the Interrupt Mask in the
> SR???

Trap 1 swaps the contents of the D0 register into the SR register, and bit 2
of 0x600001 controls the low memory protection. This code should work:

  move.w  #0x0700,d0
  trap  #1
  move.l  (0x00000064),old_int1
  ...
  move.l  (0x000000bc),old_trap15
  bclr.b  #2,(0x00600001)

  move.l  #MyInt1,(0x00000064)
  ...
  move.l  #MyTrap15,(0x000000bc)
  bset.b  #2,(0x00600001)
  trap  #1

Not all the replacement interrupts and traps are needed, of course. Also, it
is generally a good idea to restore the SR like it was (i.e. store or don't
change D0 before Trap 1 is called again).

--
 / Niklas Brunlid
Check out Prosit for the TI-89 / TI-92+ at http://prosit.ticalc.org
Random PQF v5.1 Quote follows:

The Archchancellor's most important job, as the Bursar saw it, was to sign
things, preferably, from the Bursar's point of view, without reading them
first.
        -- Middle management explained
           (Terry Pratchett, Moving Pictures)

Follow-Ups:

[A89] Re: change the SR

From: "Gijs Leegwater" <g_leegwater@hotmail.com>

References:

[A89] change the SR

From: "Gijs Leegwater" <g_leegwater@hotmail.com>