May June 1980 - Commodore Computers

MAY/JUNE, I JUNE. 1980. 198O. ISSUE 4. COMPUTE. "5 115
Relocate PET Monitor
Almost Anywhere
The Monitor program provided on tape to owners
of PETs with the old ROMs occupies locations (in
hex) $0400 to 5076B, S076B, right where the PET stores
BASIC programs' programs! If you're only interested in working
with 6502 machine language routines, that location
for the Monitor poses no problem. .
Unfortunately for me, I wanted to have the
Monitor available at the same time as BASIC programs.
Despite a variety of attempts, I was unable
to find a combination that would allow me to enter
and run BASIC programs while having the Monitor
available in its original location.
On the other hand, if the Monitor could be
moved somewhere very near the highest available
memory location with which the PET is equipped,
then the end-of-memory pointer could be reset to
protect the Monitor. Then a BASIC program could
be run, and it would not interfere with the
Monitor.
Myy problem appeared to be solved when I found
an article by Professor Harvey B. Herman of the
University of North Carolina ("Move It", MICRO,
September 1979). . Although a critical figure seemed
to be missing from the article, it was not too difficult
to puzzle out, since the technique was based on
an article by Jim Butterfield in the PET User Notes
(Issue I, 1, Vol. 2). Incidentally, Jim has been a key
figure in unravelling the secrets of the PET, both
publishing information himself and pointing others in
the right direction. .
Although a relocation technique is summarized
in this article, the discovery that got me excited
was a simple e but powerful extension to make ii it do
many relocations, after the first one was completed.
Why would anyone want to relocate the MonitorM to many different locations? The answer lies in the
earlier observation that it would be desirable to
locate it near the top end of available memory.
With a 4k PET, a good location for the MonitorM would be starting at the 3k location. On an 8k PET,
you would want to locate it from 7k up. If a 4k PET
had ana n 8k memory extension attached, giving a total
of 12k, the Monitor should be located at the Ilk
point. Other combinations of PETs and external
memories would lead us to want Monitors located
ata t 15k, 19k, 23k, 27k and 31k.
.
With the approach described here, all of those
Monitors can be created in little more time than it
takes to SAVE each one.
First, we must create one working Monitor
relocated to a position where it does not conflict
with the e original. Here's one way to do it:
1. I. LOAD the e Monitor (Prim (Print STATUS; PEEK
(630) to insure that the LOAD was good ...
Roy Busdiec ker
Roy Busdiecker
Micro Software Systems
PO P.O. Box 1442
Woodbridge, VA 22193
PET should print 0 0). Rewind the tape.
2. Type SYS62894, then press RETURN. You
will get the message PRESS PLAY ON
CASSETTE #1. Insert the Monitor tape
tape and press PLAY. The tape will run
until the FOUND MONITOR message
appears, then it will report READY. At
this point the tape header has been found
and loaded into locations 634 through 638,
but the Monitor itself has not started loading.
Here is the meaning of bytes 635
through 638.
635
636 637 638
10 lo byte
First location that
program tape will
load into.
hi byte 10 lo byte Last location that
program tape will
load into.
hi byte
3. By changing the values in these locations, we
can make this second copy load into a
different area of memory. For this exercise,
let's put it into locations 3k (decimal 3072,
hex OCOO) 0C00) and up. To do that,
POKE 636, 12:POKE 638,15:SYS 62403
The tape will run and PET will report
LOADING until the MonitorM is in. . Rewind
the tape and set it aside. At this point,
you will have two identical copies of the
Monitor. M One is located from $04000 to
S076A ($ (S indicates a hex value), as shown
in Figure 1. The other copy is in locations
S0COO SOCOO to $0C6A SOC6A (see Figure 2 for the first
128 locations). The second copy, of course,
will not work because its addresses point to
the wrong locations.
4. In order to make the $0C00-0C6A $OCOO-OC6A copy
of the Monitor usable, you must alter
(patch) the contents of 84 memory locations.
There area several ways to do this, but the
straight-forward approach I used (outlined
below) is about as easy as any other.
a. TypeT SYS 1039 and a press RETURN.
This transfers control to the $0400- S0400-
$076B Monitor, M which will display
PC SR AC XY YR SP
C6ED 30 33 39 00 FE
b. b . Consult Figure 1I to see which values
are to be changed (the circled numbers).
A circled value in line 0410 calls for a
change in the corresponding position in
line 0C10. OC Line 0500 corresponds to
OD 10, 0600 to OEOO , and 0700 to
0D10, 0600 to 0E00, and 0700 to