[Coco] 6809 / General CPU question

[Darren A]

On 2/19/11, Stephen H. Fischer wrote:
>
> The CoCo 2 manual says "if the CPU reads FFFF it will actually read the
> contents of BFFF".
>
> If the ROM is switched out will BFFF come from RAM?
>
---

Yes and no.  If the SAM is put in ALL RAM mode then reading directly
from BFFE and BFFF will read the RAM (assuming the CoCo has 64K RAM).
But when the CPU reads from FFFE and FFFF, the SAM will always output
a device code to select the ROM.  So the RESET vector (and all
Interrupt vectors) are always read from ROM even when the CoCo is in
ALL RAM mode.

A cartridge can override the internal Reset and Interrupt vectors by
decoding those addresses and asserting SLENB* to prevent selection of
the internal ROM.

Darren