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Communication Between BASIC and Assembly Language
If an ICALL statement is executed with overlapped processing, it is possible that a BASIC
variable in the common area may be "busy" when the routine wants to access it. (The busy bits
of variables passed as arguments are checked - and are non-busy - before the ICALL is
executed.) Although it is 'still possible to access the variable without regard to the status of the
busy bits, frequently that is not a desirable programming approach. You may on occasion want
to check the value of the busy bits when you suspect the user of. the routine may be using
overlapped processing.
Busy bits are checked from an assembly program using the "Busy" utility to be described
shortly. If you are checking the bits for a busy condition, and the busy condition is set, it
remains set throughout the time you are in the assembly routine. For it to become un-busy, you
must exit the routine and permit the operating system a chance to perform the 1/0 operation
and clear the busy bits.
For example -
340
IF Bus) THEN 330
If the Sort routine exits, setting Busy to 0 if a busy condition is not encountered, and to
non-zero otherwise, this is a tight loop which keeps trying to execute Sort until the common
variables which are busy become un-busy and it can proceed on its way. By exiting the routine
after each unsuccessful attempt, the operating system is given an opportunity to perform the
110 operation which has the variable(s) tied up.
UTILITY: Busy
The Busy utility checks the status of the busy bits of a variable in BASIC's common area. It is
not necessary to check the busy bits of a variable passed as an argument since all arguments are
checked upon calling a routine (and the call is executed only when all the arguments are not
busy).
General Procedure: The utility is given the location of the common declaration for the vari-
able. It returns the value1(}f the busy bits for that variable into the A register.
Special Requirements: This utility should only be used for variables in common.
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