Vector Basic Interpreter Basic 80 Reference Manual - page 153
All versions of BASIc-ao
have
prov~s~ons
for
interfacing
with assembly language subroutines.
The USR Function allows
assembly language subroutines to be called in the
same
way
BASIC's intrinsic functions are called.
The addresses
of
the
DEINT,
GIVABF,
MAKIN'l' and
FRCINT
routines
are stored in
loca-
tions
that
must be
supplied
individually for different im-
plementations of BASIC.
Memory space must be set
aside
for
an
assembly
language
subroutine- before it can be loaded.
During initialization,
enter the highest memory location minus the amount of memory
needed
for the assembly language subroutine(s).
BASIC uses
all memory available from its starting location up, so
only
the
topmost
locations
in memory can be set aside for user
subroutines.
When an assembly language subroutine is
called,
the
stack
pointer
is set up for 8 levels (16 bytes) of stack storage.
If more stack space is needed, BASIC's stack
can
be
saved
and
a
new
stack
set
up for use by the assembly language
subroutine. BASIC's stack must be restored, however,
before
returning from the subroutine.