James McCarron <jmccarro@world.std.com> wrote:
+---------------
|   '((assign ,target (op lookup-variable-value) (const ,exp) (reg env))))))
| 
| ... In particular, I don't see why the code in the last list is well-formed.
| I expected there to be a "perform" instruction wrapping the
| (op lookup-variable-value) part. Is there any chance that this is a typo?
| (More likely, though, I just don't understand the "assign" instruction.)
+---------------

The latter. IIRC [I don't have it handy], SICP2 changed the format of the
"assign" instruction from just (assign reg <value>) -- where <value> could
be any functional call, such as (cons (reg a) (reg b)) -- to imply an
implicit "perform" on the tail of the "assign" to produce the value.
So things like this [grep'd out of "ch5.scm"] are perfectly legal in
the SICP2 register machine:

	(assign proc (op lookup-variable-value) (const +) (reg env))
	(assign env (op compiled-procedure-env) (reg proc))
	(assign val (op compiled-procedure-entry) (reg proc))

as well as these simpler forms:

	(assign val (const 2))
	(assign val (reg n))
	(assign continue (label print-result))


-Rob

-----
Rob Warnock, 8L-855		rpw3@sgi.com
Applied Networking		http://reality.sgi.com/rpw3/
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