A controller instruction in our register-machine language has one of the following forms, where each $input$$_i$ is reg($register$-$name$) or constant($constant$-$value$).

These instructions were introduced in section 5.1.1: assign($register$-$name$, reg($register$-$name$)) assign($register$-$name$, constant($constant$-$value$)) assign($register$-$name$, list(op($operation$-$name$), $input$$_1$, $\ldots$, $input$$_n$)) perform(list(op($operation$-$name$), $input$$_1$, $\ldots$, $input$$_n$)) test(list(op($operation$-$name$), $input$$_1$, $\ldots$, $input$$_n$)) branch(label($label$-$name$)) go_to(label($label$-$name$))

The use of registers to hold labels was introduced in section 5.1.3: assign($register$-$name$, label($label$-$name$)) go_to(reg($register$-$name$))

Instructions to use the stack were introduced in section 5.1.4: save($register$-$name$) restore($register$-$name$)

The only kind of $constant$-$value$ we have seen so far is a number, but later we will also use strings and lists. For example, constant("abc") is the string "abc", constant(null) is the empty list, and constant(list("a", "b", "c")) is the list list("a", "b", "c").