[Coco] 6809 / General CPU question

[Mike Rowen]

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?

On Feb 19, 2011, at 11:43 AM, Steve Bjork <6809er at srbsoftware.com>  
wrote:

> In the era of the 6809, it was up to the circuit designer to hold  
> the reset line of the CPU low till the system was stable. Not only  
> did the power have to be stable but the other chips and circuits  
> need to ready for the CPU to do its work.
>
> Some CPUs would just start running at address zero but the 68XX type  
> of chips would use the Reset Vector (store at address $FFFF) as the  
> location to start with.  The 6809 would also turns off its ability  
> to service interrupts so the hardware and RAM can be initialize so  
> the system will run correctly.
>
> The 6809 was design that the RESET line as cold (power on) reset.   
> The CoCo design takes this one step further by using the Reset Line  
> as both cold and warm reset.  Since the memory is fill with random   
> data with power on (cold) reset code looks at a byte in memory to  
> decide if it should do a cold or warm restart.  If byte at $71 hold  
> the value of $55 then it will attempt a warm reset else it will do a  
> cold reset initialize the computer's memory with all the stuff  
> needed for BASIC to run.
>
> As I said, the reset code will attempt a warm start. The next step  
> is for the code to grab the warm reset vector at $72.  But before  
> the code starts at the new address, it must make sure the code  
> starts with a NOP ($12) instruction to verify this is good code and  
> the vector is not pointing to random address.
>
> The bottom line is the CoCo always does a power on reset whenever  
> the Reset Line is activated, be it power on or someone hitting the  
> reset button.  Its the power-up/reset code and the status of $71  
> (Reset Flag) / $72 (Vector address) controls what it should do.
>
> Steve (Zaxxon) Bjork
>
> On 2/19/2011 4:59 AM, Mike Rowen wrote:
>> 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
>>
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