[Coco] 6809 / General CPU question

[Stephen H. Fischer]

Hi,

You are expanding my understanding. Thanks.

>So the RESET vector (and all
> Interrupt vectors) are always read from ROM even when the CoCo is in
> ALL RAM mode.

As he SAM has the entire address it can do this.

This is quite different from many of the computers I own (6502 / 6809) which 
were much more simple.

SHF

----- Original Message ----- 
From: "Darren A" <mechacoco at gmail.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Saturday, February 19, 2011 7:24 PM
Subject: Re: [Coco] 6809 / General CPU question


> 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