[Coco] 6809 / General CPU question

William Astle lost at l-w.ca
Sat Feb 19 20:08:49 EST 2011

On 11-02-19 05:52 PM, Stephen H. Fischer wrote:
> Hi,
> If I remember correctly, the first thing that ROM does is to copy the 
> ROM addresses to RAM and switch the RAM into the previously ROM 
> occupied addresses.

Which, of course, does not answer his question since the ROM is not 
running yet at power on when the CPU fetches the reset vector.

Usually the hardware will be configured to direct accesses to FFFE and 
FFFF to a ROM location. In the coco1 and coco2, FFFx is redirected to 
BFFx in ROM by the hardware. On the coco3, FFFx is forced to refer 
always to the top 16 bytes of the internal ROM.

> ----- Original Message ----- From: "Mike Rowen" <mike at bcmr3.net>
> To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
> Sent: Saturday, February 19, 2011 3:46 PM
> Subject: Re: [Coco] 6809 / General CPU question
>> 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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