CMP.12:5.3 - Preserve which operation is executed

Let ifzero(test,a,b) evaluate test and then only a when the result is zero, otherwise only b. Let division by zero be a defined execution error. The expression ifzero(x,0,10/x) returns 0 at x=0 and 5 at x=2.

A translation that evaluates all three parts and then selects a value raises the unwanted division error at x=0. Instead generate fresh labels and conditional control:

LOAD x
JZ zero
PUSH 10; LOAD x; DIV
JMP end
zero: PUSH 0
end:

JZ pops the test and jumps when it is zero; otherwise execution continues. JMP transfers control unconditionally. DIV pops right then left and divides left by right, using rational values here. Resolve each fresh label to its instruction position before execution. At x=0 the division instructions are skipped; at x=2 they append 5. Either path preserves earlier stack entries and appends one result.

If the branches instead emit commands, the same construction retains which command occurs. If the source is deliberately changed to evaluate both branches, the preservation claim and translation must change with it. The control rule, rather than a particular programming language’s spelling, determines this choice.