[Coco] 6809 / General CPU question

jdaggett at gate.net jdaggett at gate.net
Thu Feb 24 13:02:27 EST 2011


I am writing this late in the discussion and have not read all the replies so I do beg forgiveness if this has been answered correctly

The 6809/6809E has NO real internal circuitry to determine whether a software or hardware or a power up reset has occurred if I remember correctly. There needs to be external circuitry to keep the processor in RESET until the on board or external oscillator has stabilized. If this time is not adhered to then erratic start up of the processor will occur. 

After RESET is released then either the 6809 or the 6809E will fetch the first of the two byte reset vector. So yes when the 6809 comes out of reset the first thing it does is a fetch operation. The RESET service routine should have a means of determining a cold restart or warm restart. Interesting note is the 6809 has a restart opcode. Issuing an opcode of $3E will will force the processor to load the RESET vector and start execution again. 

hope this helps


-----Original Message-----
>From: Mike Rowen <mike at bcmr3.net>
>Sent: Feb 19, 2011 7:59 AM
>To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
>Subject: [Coco] 6809 / General CPU question
>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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