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•
Figure 2
someone familiar with the
PET
montior
could adapt
its
program adjustment
software.
It works on normal programs
and it sure is fast.
PET
uses the routine
whenever new lines are added or old
lines deleted.
If variable pointers are the
same for all programs and if assignment
statements are used to initialize all pro·
grams, then several programs might
be
able to share the variable working area. I
haven't tried a lot of this, but
it
does
work in simple cases. This technique will
allow FORTRAN like passed variable
subroutines,
support
BLOCK
type
statements and conserve a
Jot
of
memory.
Shifted programming has several
advantages but there are also some pit·
falls. I'm sure that I haven't found them
all. I'll tell you about those that I've
fallen into, and Murphy will find some
new ones for you.
As
a first example,
let's take the case where shifted pro-
grams are loaded
in
under keyboard con-
trol. When this is done, all memory
above
1024
is reinitiallzed. Any shifted
programs already in memory are 36'd
out. The only way
to
prevent this is
to
ad·
just the top
of
memory pointer so that it
points below the existing shifted pro-
grams. This must
be
done before atemp-
ling
to
LOAD from the keyboard. Shifted
(or normal) programs LOADed under pro-
gram control do not
36
out memory. But
the first part
of
memory may
be
set
up
to
receive BASIC.
In
addition, pointers
aren't changed.
I
Another pitfall I is the tendency for
PET
to
"go
away". Any error in pointer
setup will usually
~use
this problem. It
is the rule rather th n the exception. Ver·
sion 2 ROMs are umored
to
allow a
warm reset.
Unfort~nately,
they aren't
available for the
ot~
8K PETs yet.
A third pitfall is ireally just the result
of careless programming.
The
available
space within any
lrogram
should be
reduced as much a possible. Program
space includes v riable and string
space. Although
mf
PET
has 16K of
memory (half in
BET
I),
I've found it easy
to
over-run memory or
to
overlap pro-
grams.
If
multiple
~ASIC
programs are
to
coexist, a mem
ry
map and some
planning are neces ary. I don;t have a
dynamic adjustmen, routine. Perhaps
So
much for the pitfalls, here's
some
of
the good news. The shifted pro-
gram technique can
be
used for BASIC
programs
to
coexist with Commodore's
tape machine language monitor. Sure,
you'll
be
able to buy a new set of ROMs
that have the
monitor-someday.
But
you can have nearly the
same-thing now.
You
may need
an
additional routine to
transfer the bottom of
p(ige one
(OA-22
hex) memory back and forth between
machine language monitor and BASIC
usage. Both BASIC and the machine
language monitor want this part
of
memory for scratch pad.
What else can
be
done with shifted
BASIC programs?
ROM
BASIC pro-
grams,
truly
modular
development,
library routines, and lots more. Now that
BASIC programs can
be
placed wherever
you want them, your imagination is the
only limit.
)
T.J.a.
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