Exercise 5.2.  Use the register-machine language to describe the 
iterative factorial machine of exercise 5.1.

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(controller
    (assign c (constant 1))
    (assign p (constant 1))
  test_counter
    (test (op >) (reg c) (constant n))
    (branch fac_done)
    (assign p (op mul) (reg c) (reg p))
    (assign c (op add) (reg c) (constant 1))
    (goto test_counter)
  fac_done)

I explicitly initialized c and p, and treated n as a 'constant', since it 
is not altered.  This corresponds to the diagram.

Assuming c, p, and n are all registers, and are already initialized, as 
in the example in the text:

(controller
  test_counter
    (test (op >) (reg c) (reg n))
    (branch fac_done)
    (assign p (op mul) (reg c) (reg p))
    (assign c (op add) (reg c) (constant 1))
    (goto test_counter)
  fac_done)