Guarded Execution
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`as' supports the full range of guarded execution directives for
each instruction. Just append the directive after the instruction
proper. The directives are:
`/tx'
Execute the instruction if flag f0 is true.
`/fx'
Execute the instruction if flag f0 is false.
`/xt'
Execute the instruction if flag f1 is true.
`/xf'
Execute the instruction if flag f1 is false.
`/tt'
Execute the instruction if both flags f0 and f1 are true.
`/tf'
Execute the instruction if flag f0 is true and flag f1 is false.