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Programming Techniques 13–7
File name 32sii-Manual-E-0424
Printed Date : 2003/4/24 Size : 17.7 x 25.2 cm
Conditional Instructions
Another way to alter the sequence of program execution is by a conditional
test, a true/false test that compares two numbers and skips the next program
instruction if the proposition is false.
For instance, if a conditional instruction on line A05 is
º/@
(that is, is x
equal to zero
?
), then the program compares the contents of the X–register
with zero. If the X–register does contain zero, then the program goes on to the
next line. If the X–register does not contain zero, then the program skips the
next line, thereby branching to line A07. This rule is commonly known as "Do
if true."
.
.
.
Do next if true.
º/@
2
Skip next if false.
1
!
2
!
.
.
.
1
.
.
.
The above example points out a common technique used with conditional
tests: the line immediately after the test (which is only executed in the "true"
case) is a branch to another label. So the net effect of the test is to branch to a
different routine under certain circumstances.
There are three categories of conditional instructions:
Comparison tests. These compare the X– and Y–registers, or the
X–register and zero.
Flag tests. These check the status of flags, which can be either set or clear.
Loop counters. These are usually used to loop a specified number of
times.