[Coco] 6809 / General CPU question
Stephen H. Fischer
SFischer1 at Mindspring.com
Sat Feb 19 19:52:35 EST 2011
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
----- 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
> 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
>> 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
>>> begin fetching instructions at a specific location? I have been looking
>>> through some books, but I have yet to find any document that describes
>>> boot process from the CPU perspective.
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