Commodore VIC-20 - Page 50

WAIT
Format: Abbreviation: Scraen Display:
I
WAIT location, Wp.}:!i-|A W [*]
maski [, mask2]
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 specified way. This is used for arcane 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 maski. If there is
a mask2 in the statement, the result of the first operation is
exclusive-ORed with mask2. This sounds confusing, but there's an
easier way to look at it. The maski value "filters out'r any bits that
we don't want to test. Where the bit is 0 in maski, the corresponding
bit in the result will always be 0. The mask2 value will flip any bits, so
we can test for an off condition as well as on. Any bits being tested
for a 0 should have a 1 in the corresponding position in mask2.
EXAMPLE:
WAIT 36868 , 144 , 16
What are we testing for here? Here's a binary look at our two
masks:
144 = 10010000
16 = 00010000
This WAIT statement will halt the program until either the 128 bit
is on or the 32 bit is off.
i
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