PET startup sequence?? (was LF: Commodore PET schematics, troubleshooting info)

From: Dave Dunfield <>
Date: Tue Aug 10 19:17:14 2004

>> Deos still exist? That used to be a good place for
>> Commodore schematics and technical info,
>Yup, FUNET's still alive. <> for the CBM stuff.
>There's also a web-based version at <>.

Thanks guys - that helps a lot.


Does anyone have "inside" information on exactly what the PET firmware does when
it starts up?

I am still working on the same pair of SuperPET 9000's (one works, one does not).
Both machines have been stripped to just the base 6502 board.

The "bad" machine plays it's tune and clears the screen - then appears to hang.
(If you reset it with the monitor warmed up, you can see the screen fill with
"garbage" and then clear - just like the working one).

Code appears to be running:

 - If I remove the KERNAL rom, the startup beep does not occur, and the screen
   never initializes, so it looks like this ROM is executing.

 - After it appears to "hang", the code still seems to be running. Looking at
   the 74LS154 decoder which shows accesses within 4k boundaries, I see lots of
   accesses to 0xxx (probably page0 RAM), no accesses to intervening locations
   and lots of access to the ROM's, which follow a clearly identifiable pattern.
   Removing any ROM prevents this from happening, with the machine just "flailing"
   until it gets a HLT - so it looks like it may actually be running ROM code.

 - I can see accesses to the keyboard scanner.

On the working machine, after accessing all ROM's as BASIC starts (and prints it
welcome message), it settles into a keyboard scan which appears to exist within
the KERNAL and EDIT rom's - the BASIC ROM's are not accessed unless some function
is activated.

The non-working machine appears to hang during the phase where it is accessing all
ROM's - it continues to do this indefinately.

It looks like the system is hanging during the initialization of BASIC - it never
prints the welcome message (hangs with blank screen).

So far I have been unable to determine what it is doing exactly - any info on the
startup sequence would be MOST appreciated!

I have verified that the ROM's contain exactly the same code as the other machine
(I've seen several PET's fail with bad ROM's, however this does not appear to be
the case here).

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools:
com             Vintage computing equipment collector.
Received on Tue Aug 10 2004 - 19:17:14 BST

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