Date   : Thu, 19 Mar 2009 19:40:26 -0000
From   : jumbos.bazzar@... (Mark Haysman)
Subject: Hardware questions

>> 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.

So, can you not just blindly put  64K of SRAM with A8-15 controlled by a 
latch from FCFF, and the lower address and data bus connected to simply give 
you 64K of extra paged RAM? Will having RAM at FDFE and FDFF confuse the OS?

Mark.