diff --git a/ex-5_31-xx.scm b/ex-5_31-xx.scm
index 2a51196..6441cde 100644
--- a/ex-5_31-xx.scm
+++ b/ex-5_31-xx.scm
@@ -74,7 +74,7 @@ ev-appl-did-operator-no-restore
         (display (car xs) port)
         (display "\n" port)
         (write-list-to-port (cdr xs) port))))
-  (if #f ; #t means write to file; #f means don't write to file
+  (if #t ; #t means write to file; #f means don't write to file
     (let* ((compile-result (compile code target linkage))
            (assembly-insts (statements compile-result))
            (port (open-output-file file-name)))
@@ -391,13 +391,6 @@ ev-appl-did-operator-no-restore
 
 (display "\nex-5.38 - optimize-procedure-application\n")
 
-; a.  The open-coded primitives, unlike the special forms, all need their
-; operands evaluated. Write a code generator spread-arguments for use by all
-; the open-coding code generators. Spread-arguments should take an operand list
-; and compile the given operands targeted to successive argument registers.
-; Note that an operand may contain a call to an open-coded primitive, so
-; argument registers will have to be preserved during operand evaluation.
-
 (define (spread-arguments operand-list)
   (define (compile-operands operand-list operand-number)
     (cond
@@ -414,33 +407,45 @@ ev-appl-did-operator-no-restore
          (compile-operands (cdr operand-list) (+ operand-number 1))))
       (else
        (error "Only two arg registers supported -- SPREAD-ARGUMENTS"))))
-
   (let ((operand-codes (compile-operands operand-list 1)))
     operand-codes))
 
-(define (display-lines xs)
-  (define (display-line x)
-    (display x) (newline))
-  (map display-line xs))
+(define (primitive-procedure? exp)
+  (define primitive-procedures '(= * - +))
+  (and (pair? exp)
+       (= (length exp) 3) ;; only support two args for now
+       (memq (car exp) primitive-procedures)
+       ))
 
-(display-lines (third (spread-arguments '(1 1))))
-(newline)
+(define (compile-primitive-call op target linkage)
+  (end-with-linkage linkage
+    (make-instruction-sequence
+      '(arg1 arg2)
+      `(,target)
+      `((assign ,target (op ,op) (reg arg1) (reg arg2))))))
 
-(display "b. CONTINUE HERE\n")
+(define (compile-primitive exp target linkage)
+  (let ((proc-code (compile-primitive-call (operator exp) target linkage))
+        (operand-codes (spread-arguments (operands exp))))
+    (preserving '(env continue)
+     operand-codes
+     proc-code)))
 
-; b.  For each of the primitive procedures =, *, -, and +, write a code
-; generator that takes a combination with that operator, together with a target
-; and a linkage descriptor, and produces code to spread the arguments into the
-; registers and then perform the operation targeted to the given target with
-; the given linkage. You need only handle expressions with two operands. Make
-; compile dispatch to these code generators.
+(compile-to-file
+ '(define (factorial n)
+    (if (= n 1)
+        1
+        (* (factorial (- n 1)) n)))
+ 'val 'next "factorial-opt.sicp-asm")
 
-; c.  Try your new compiler on the factorial example. Compare the resulting
-; code with the result produced without open coding.
+; c. The optimization saves 39 instructions
+; λ symposium sicp → λ git master* → wc -l factorial.sicp-asm factorial-opt.sicp-asm
+;   79 factorial.sicp-asm
+;   40 factorial-opt.sicp-asm
+;  119 total
 
-; d.  Extend your code generators for + and * so that they can handle
-; expressions with arbitrary numbers of operands. An expression with more than
-; two operands will have to be compiled into a sequence of operations, each
-; with only two inputs.
+(display "[answered]\n")
 
 (display "\nex-5.39\n")
+
+(display "\nex-5.40\n")
diff --git a/misc/ch5-compiler.scm b/misc/ch5-compiler.scm
index c27c010..d2dd2c7 100644
--- a/misc/ch5-compiler.scm
+++ b/misc/ch5-compiler.scm
@@ -34,6 +34,8 @@
                            target
                            linkage))
         ((cond? exp) (compile (cond->if exp) target linkage))
+        ((primitive-procedure? exp)
+         (compile-primitive exp target linkage))
         ((application? exp)
          (compile-application exp target linkage))
         (else
@@ -46,6 +48,8 @@
 (define (empty-instruction-sequence)
   (make-instruction-sequence '() '() '()))
 
+;; Implemented in 5.38.
+(define (primitive-procedure? exp) #f)
 
 ;;;SECTION 5.5.2