10 Aug
2004
10 Aug
'04
12:43 p.m.
Hi Dave
A common failure item in these old machines is the RAM.
Any failure of the RAM makes things like subroutines fail
quickly.
Dwight
From: "Dave Dunfield"
<dave04a(a)dunfield.com>
Thanks guys - that helps a lot.
+++
Does anyone have "inside" information on exactly what the PET firmware does
when
Deos
ftp://ftp.funet.fi/ still exist? That used to be a good place for
Commodore schematics and technical info,
Yup, FUNET's still alive. <ftp://ftp.funet.fi:/pub/cbm> for the CBM stuff.
There's also a web-based version at <http://nic.funet.fi/pub/cbm/index.html>.
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).
Regards,
Dave
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Vintage computing equipment collector.
http://www.parse.com/~ddunfield/museum/index.html