[Coco] [!! SPAM] Re: 6809 / General CPU question

[Mike Rowen]

I think what may have caused me to come up with this question in the first
place is that I am studying the 6809 and 6809 machine language. I was
looking at a memory map of the CoCo and these addresses (FFFE & FFFF) appear
to be in an area called "Dedicated Addresses" and it seems outside of the
addresses mapped to various ROM locations. So I ruled out ROM being mapped
to these the addresses. That may or may not be the case. If it isn't in
ROM,it is likely that it is done through other address mapping on the
hardware, much as John describes. What I have learned through all of the
discussion is that the 6809 indeed needs to get its starting point by
looking at FFFE & FFFF. What is mapped to these addresses appears to be
strictly up to the computer designer. I have a Simon6809 and on that
computer, these addresses (FFFE & FFFF) are clearly ROM locations. From a
computer user perspective, the exact mechanism of this is completed before
you could do anything meaningful on the system. On both a CoCo or Simon6809,
you can't write to these locations.

I'm glad I asked this question as it really helps me appreciate the
complexities of hardware and all of the critical timing involved. It
certainly reveals the elegance of system design. While learning 6809 ML, I
can see how much more one needs to know about all of the combined hardware
to write a Space Intruders or Zaxxon! My hats are off to all of you with
that level of knowledge. Thanks everyone for your replies!

Regards,
-Mike Rowen



On Sun, Feb 20, 2011 at 1:49 AM, John Kent <jekent at optusnet.com.au> wrote:

> I suspect the vector is read into a temporary internal register then
> transferred to the program counter as the program counter is used to fetch
> the vector or at least it can be. The program counter would have to be
> pre-initialized with all 1's in the high address bits and a interrupt vector
> offset in address bits A3 to A1 on reset. I don't know if that is how
> Motorola did it. I was a bit concerned about patents, but I think they have
> all expired, so I don't think I'm violating anyone's patent. I'm certainly
> not making any money out of it.
>
> John.
>
>
>
> On 20/02/2011 10:46 AM, Mike Rowen wrote:
>
>> Thanks everyone for your responses. Ok, so whenever the CPU powers on or
>> experiences a RESET inturrupt, the CPU will read the contents of FFFE & FFFF
>> and load it into the program counter register. It then fetches instructions
>> from that location. Presumably this would be a location in ROM.
>>
>> How does an address get into FFFE & FFFF when the system is initialized?
>> Is this through hardware? Are these RAM locations?
>>
>>
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