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154
I/O Handling
With the GOTO version there are also two effects, slightly differing -
• The branch is not taken until it has the highest priority of all pending branches .
• The execution of the branch does not lock out any other branches, so that at the end of
the line to which it branches, if there are other pending branches, the highest one of those
is executed.
For example, suppose there are these four statements in effect -
and also suppose that at the end of some BASIC line in the program, an interrupt had been
received from all four of the interfaces involved. Then the process of dealing with them pro-
ceeds like this -
EVENT
Reaches end of current
BASIC line
Finishes Routine 7
NEXT ACTION
SYSTEM PRIORITY
GOSUB Routine 7
Changes from
a
to 15
GOSUB Routine 5
Changes from 15 to 9
Suppose at this point another interrupt is received from select code 7.
EVENT
NEXT ACTION
SYSTEM PRIORITY
Reaches end of current
GOSUB Routine
7
Changes from 9 to 15
BASIC line in Routine 5
Finishes Routine
7
Returns to interrupted
Changes from 15 to 9
point in Routine _5
Finishes Routine 5
GOTO 1000
Changes from 9 to
a
Finishes with line 1000
GOTO Routine 4
Stays at
a
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