Date   : Thu, 19 Mar 2009 18:54:21 +0000 (GMT)
From   : info@... (Sprow)
Subject: Hardware questions

In article <4218cdf7708d7400cf9b29a4c7a09e17@...>,
   Richard Gellman <splodge@...> wrote:
> On Thu, 19 Mar 2009 16:08:55 -0000, "Mike" <profpep@...> wrote:
> > &FDFE and &FDFF are the Test software indirection locations."
>
> Ah... now this is different again. So the top two bytes are indirections.
> Would I be correct in saying that as long as no test software is loaded,
> these can be used for data? I'm assuming the MOS itself doesn't use these
> otherwise it would be jumping to random data.

The MOS does inspect &FDFE/FDFF. 

During the startup sequence it zeros the IRQ-copy-of-A with interrupts
disabled and briefly reenables them. If IRQ-copy-of-A is corrupted it
assumes the test hardware is plugged in and blindly jumps indirect to &FDFE,
Sprow.