Non-Stop PETS, Old And New
Contributed by Micro Software Systems
This note provides a method for disabling the STOP key on either old (2001-4, 2001-8) or new (2001-16, 2001-32) PETs with a single algorithm, even though the locations and contents to be POKED are different. It is based on Len Lindsay's PET-POURRI column in the July 1979 issue of Kilobaud Microcomputing, but provides a correction and avoids a potential problem in the procedure he presented.
In PETs using Version 1 ROMs (models 2001-4 and 2001-8 which have not been modified), the STOP key is disabled with
10 POKE 537, 136
and re-enabled with
20 POKE 537, 133
PETs equipped with Version 2 ROMs (2001-16, 2001-32, and modified 2001-4 and 2001-8) may use the following to disable the STOP.
10 POKE 144, 49
To re-enable the STOP,
20 POKE 144, 46
A composite procedure, which will work on either machine, is based on the contents of memory location (50003) ... a fact which was brought to my attention by Ted Polczynski, attributed again to Len Lindsay. In the old ROM, PEEK (50003) gives a value of 0, while the new ROM returns a value of 1. The following BASIC program segment uses that value to adjust a POKE command for the machine it's running on.
To disable the STOP, use
10 PT = PEEK (50003) : SL = 537-393 * PT : DL = 136-87 * PT : POKE SL, DL
To re-enable the STOP, use
20 PT = PEEK (5003): SL = 537-393 * PT : DL = 133-87 * PT : POKE SL, DL
The advantage of this version of the routine is that it always pokes the same value into the control location, no matter how many times the program is run.
Versions of the form
POKE SL, PEEK (SL) + 3
can cause unpredicted results (including loss of control) if they are executed more than once.
Julian Allason, of Petsoft, is interested in acquiring European rights for programs for PET, TRS 80 and Apple personal computers. For more information, write him at: Applied Computer Techniques Limited, Petsoft Division, 5/6 Vicarage Road, Edgbaston, Birmingham B15 3ES
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