... computations¹

For example, the compiler in Chapter 5 generates code in which arguments of procedures are maintained as a list in the argl register. On the other hand, a register machine customized for a procedure of, say, three arguments might usefully keep the arguments in three separate registers.

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... permitted²

For example, (assign val (op +) ((op *) (reg a) (reg b)) (const 1)) is not permitted, since a call to * is nested inside a call to +.

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... code.³

These counts may include a few extra operations needed to run the driver loop itself. This is a small constant overhead that you can ignore when you collect statistics.

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