Commodore VIC-20 - Page 153

the previous instruction(s), but by internally examining the status
register.
As was just mentioned, there is a "zero result" flag in the status
register. The "BEQ" instruction branches if the "zero result" flag
(known as "Z") is set. Every branch instruction has an opposite
branch instruction. The BEQ instruction has an opposite instruction
"BNE" ("branch on result NOT equal to zero," i.e., "Z" not set).
The index registers have a number of associated instructions
which modify their contents. For example, the "IMX" instruction will
"increment the X index register." If the X register contained $FF
before it was incremented (the maximum number the X register can
contain), it will "wrap around" back to zero. If we wanted a program
to continue to do something until we had performed the increment
of the X index that pushed it around to zero, we could use the BNE
instruction to continue "looping" around, until X became zero.
Apart from INX, there is "DEX", which will decrement the X index
register. If it is zero, it will wrap around to SFF. Similarly, there are
"INY" and "DEY" for the Y index register.
But what if a program didn't want to wait until X or Y had reached
(or not reached) zero? Well there are comparison instructions,
"CPX" and "CPY", which allow the machine language programmer
to test the index registers with specified values, or even the
contents of memory locations. If we wanted to see if the X register
contained $40, we would use the instruction:
CPX #$40 compare X with the "value" $40.
BEQ (some other branch to somewhere else in the program, if
part of the this condition is "true."
program)
The compare and branch instructions play a major part in any
machine language program.
The operand specified in a branch instruction when using
VICMon is the address of the part of the program the branch should
go to, if taken. However, the operand is only, in fact, an "offset" from
where the program currently is. to the address specified. This offset
is just one byte, and therefore the range that a branch instruction
can branch to is limited from 128 bytes backward, to 127 bytes
forward; this is a total range of 255 bytes, which is, of course, the
maximum range of values one byte can contain. VICMon will tell
you if you branch out of range, by refusing to "assemble" that
instruction. It is unlikely that you will be doing such huge branches
for quite a while anyway. For nearly all situations this is adequate
anyway. The branch is a "quick" instruction by machine language
standards because of this "offset" principle as opposed to an
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