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WAIT
Abbreviation: W <SHIFT+A>
TYPE: Statement
FORMAT: WAIT <location>,<mask-1>[,<mask-2>]
Action: The WAIT statement causes program execution to be suspended
until a given memory address recognizes a specified bit pattern.
In other words WAIT can be used to halt the program until some
external event has occurred. This is done by monitoring the status
of bits in the input/output registers, The data items used with
WAIT can be any numeric expressions, but they will be converted
to integer values. For most programmers, this statement should
never be used. It causes the program to halt until a specific
memory location's bits change in a specific way. This is used
for certain I/O operations and almost nothing else.
The WAIT statement takes the value in the memory location and
performs a logical AND operation with the value in mask-1. If
there is a mask-2 in the statement, the result of the first operation
is exclusive-ORed with mask-2. In other words mask-1 "filters
out" any bits that you don't want to test. Where the bit
is 0 in mask-1, the corresponding bit in the result will always
be 0. The mask-2 value flips any bits, so that you can test for
an off condition as well as an on condition, Any bits being tested
for a 0 should have a I in the corresponding position in mask-2.
If corresponding bits of the <mask-1> and <mask-2>
operands differ, the exclusive-OR operation gives a bit result
of 1. If corresponding bits get the same result the bit is 0.
It is possible to enter an infinite pause with the WAIT statement,
in which case the <RUN/STOP> and <RESTORE> keys can
be used to recover. Hold down the <RUN/STOP> key and then
press <RESTORE>. The first example below WAITs until a key
is pressed on the tape unit to continue with the program. The
second example will WAIT until a sprite collides with the screen
background.
EXAMPLES of WAIT Statement:
WAIT 1,32,32
WAIT 53273,6,6
WAIT 36868,144,16
(144 & 16 are masks. 144=10010000 in binary and 16=10000
in binary. The WAIT statement will halt the program until the
128 bit is on or until the 16 bit is off)
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