PET II questions (P500, B128, B256)

From: Marko Mäkelä (
Date: 2001-12-10 11:05:44

Last weekend, I ported the custom transfer routines of my cassette-RS232
adapter to my CBM 720 (also known as High Profile B-256).  Everything else
worked nicely, but I can't relink or run BASIC programs.  I disassembled
the BASIC ROMs (901241-03 and 901240-03) in order to find the following

clr	= $8a27
linkprg	= $869c
stxpt	= $b41a
newstt	= $8752

In the Commodore 64, the corresponding addresses are:

clr	= $a659
linkprg = $a533
stxpt	= $a68e
newstt	= $a7ae

What I found is probably not fully equivalent to the C64 routines, because
once I've loaded a BASIC program to $010003 using this version of prlink
and try to LIST it, the listing continues with rubbish.  Using the machine
language monitor (commands SYS60950, V01 and M0003), I found out that the
last three zero bytes of the program were overwritten (the middle byte was
nonzero).  I verified that the culprit can't be the linkprg call, but I
couldn't figure out which zero page variables in bank 15 could affect

Can anyone tell me how to solve this problem, or to perform a RUN command
in machine language on these computers?

For the P500 (which I assume is the same as PET II series model CBM 500),
I have quite little information.  Judging from the SID note table in the
user's guide (which should be scanned, I know, but I lack the time and the
equipment) and from the schematic diagram, the B-128 and B-256 run at 2 MHz,
but I'm not sure about the P500.  Is it 1.022727 MHz like other NTSC
computers, or 985248 Hz like the PAL C64, or something else?  Also, which
firmware does this machine run?  The firmware hasn't been archived on
FUNET (yet; hint to the P500 owners on this list).

Speaking of the firmware, my 720 has otherwise the same KERNAL as
"kernal.901244-03.swedish.bin" on FUNET, but the $aa padding bytes
at $edf8-$edff have been changed to 62 f2 a5 6d 5e 80 85 e9.  Does anyone
know what this patch could be?  I should write up the Commodore part
numbers on the chips and to read the contents of the character ROM at some
other point.

Please help me create a _portable_ data transfer solution for _all_ CBM
8-bit computers.  (Any PET BASIC 1.0 users out there?)


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