Exercise 5.22.  Exercise 3.12 of section 3.3.1 presented an append 
procedure that appends two lists to form a new list and an append! 
procedure that splices two lists together. Design a register machine to 
implement each of these procedures. Assume that the list-structure 
memory operations are available as primitive operations.

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Inputs in registers 'a' and 'b', output in 'val' for both.

Append:

  (assign continue (label append-done))
loop
  (test (op null?) (reg a))
  (branch (label null))
  (save continue)
  (save car)
  (assign car (op car) (reg a))
  (assign a (op cdr) (reg a))
  (assign continue (label after-loop))
  (goto (label loop))
after-loop
  (restore car)
  (assign val (op cons) (reg car) (reg val))
  (restore continue)
  (goto (reg continue))
null
  (assign val (reg b))
  (goto (reg continue))
append-done

Append!:

  (assign val (reg a))
loop
  (assign a-cdr (op cdr) (reg a))
  (test (op null?) (reg a-cdr))
  (branch found-last-pair)
  (assign a (reg a-cdr))
  (goto (label loop))
found-last-pair
  (perform (op set-cdr!) (reg a) (reg b))