[Coco] 6809 / General CPU question

[Stephen H. Fischer]

Hi,

I guess I am not communicating well.

On power on there must be a location to start execution. As nothing is in 
RAM yet it must be ROM code.

I do not have the books handy to check so it might be that the ROM addresses 
point to ROM code that uses RAM locations to continue. But I think that was 
a different 6x0x machine. (KIM, SYM, FLEX 680x ...)

What the CPU sees in it's address space when powered on can be quite 
different in many ways from that controlled by hardware registers later.

When OS-9 is running and some other programs also is all RAM, all 64K. The 
DECB ROMs have been switched out.

SHF

----- Original Message ----- 
From: "William Astle" <lost at l-w.ca>
To: <coco at maltedmedia.com>
Sent: Saturday, February 19, 2011 5:32 PM
Subject: Re: [Coco] 6809 / General CPU question


> On 11-02-19 06:28 PM, Stephen H. Fischer wrote:
>> Hi,
>>
>> But the ROM is running at power on!
>>
>> It is a little tricky switching the FFFE, FFFF area from ROM to RAM but 
>> it is done at power on.
>>
> That turns out not to be the case. Have you ever tried to change the 
> values in the vector table? It's not possible.
>
> Besides, the time under discussion is at CPU startup when nothing has had 
> a chance to copy anything to RAM yet! The vector fetched from FFFE/FFFF 
> MUST be in ROM or something similar or the system could never bootstrap.
>
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