[Coco] 6809 / General CPU question

[Steven Hirsch]

On Sat, 19 Feb 2011, Bruce W. Calkins wrote:

>
> ----- Original Message ----- From: "Mike Rowen"
>
>> Basic theory question here. When a 6809 is powered up, does it 
>> automatically
>> begin fetching instructions at a specific location? I have been looking
>> through some books, but I have yet to find any document that describes the
>> boot process from the CPU perspective.
>> 
>> Regards,
>> -Mike
>
> ===============================
>
> My understanding is that all CPUs start looking at a specific location.  The 
> information at that location determines where the next instruction comes 
> from.  I don't know the address for the 6809s start off the top of my head 
> though, but I do remember reading about it in my past.

That's true.  But, mere application of power is not generally sufficient. 
Most CPUs also have a timing requirement for their hardware reset line 
during the power-up / start phase.  Without conditioning RST, you can end 
up in an "undefined" (read: non-functional) state.



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