The PET BASIC interpreter can be used to locad
an
assembly language
program into the selected area of memory. The process
is
a rudimentary one.
consisting of POKEing the decimal equivalents of the
6502
machine
language instructions.
To get the instructions
in
decimal. vou can
write
vou pro-
gram
in
6502
assembly language (reference manuals are listed
in
an
appendixl.
hand code it into hexadecimal. and then convert the hexadecimal codes
to
decimal. Commodore's Terminal Interface
Monitor
(available on cassette
from
Commodore if it
is
not built into your
PET)
stores the hexadecimal codes directly.
However,
with
the Monitor vou must load the assembly language routine sepa-
rately
from
the BASIC program, whereas by POKEing vou can load the assembly
language routine
as
part of executing the main program written
in
BASIC. (The
PET
is
suitable
for
implementing only small portions
of
the program
in
assembly
language.)
DATA
statements are used to define the machine language codes.
which can
be
subsequently READ into the program and passed
to
a
POKE
loop.
Control
is
transferred to
an
assembly language program
in
one of
two
ways: the SYS command
or
the USR function. They are more or less in-
terchangeable.
but
SYS
is
geared to turning control over
to
an
assembly language
program. whereas
USR
is
a true
function
reference
that
allows a value to
be
sent
to
the called assembly language routine and a value returned by it
to
the main
program.
The assembly language program must return control back to BASIC via
a Return-From-Subroutine (RTS) instruction.
SYS
SYS
is
a system
function
that
transfers program control
to
an
independent
subsystem.
Format:
SYS
(add
ress)
where:
address
is
a numeric constant. variable. or expression
representing the starting addess at
which
ex-
ecution of the subsystem
is
to
begin. The value
must
be
in the range 0
~
address
~
65535.
Unlike other functions.
SYS
can
be
specified alone in a
direct
or program
statement.
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