Basic Expression Algorithm

The basic algorithm for expressions is easy:

• Operand (leaf node): get a register; generate a load; return the register.

• Operator (interior node): generate operand subtrees; generate op; return result register.

(defun genarith (x)
  (if (atom x)              ; if leaf,
      (genload x (getreg))  ;   load
      (genop (op x)         ; else op
             (genarith (lhs x))
             (genarith (rhs x))) ) )

>(genarith '(* (+ a b) 3))
   LOAD  A,R1
   LOAD  B,R2
   ADD   R1,R2
   LOAD  3,R3
   MUL   R2,R3
R3