[Coco] 6809 / General CPU question
6809er at srbsoftware.com
Sat Feb 19 11:43:41 EST 2011
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.
> Coco mailing list
> Coco at maltedmedia.com
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