Thinking about operand evaluation.

diff --git a/mars.scm b/mars.scm
index c77e2d3..2f4db9d 100644 (file)
--- a/mars.scm
+++ b/mars.scm
@@ -22,7 +22,13 @@
 ;;; Instructions
 ;;
 
-(define (make-instr opcode modifier addrA modeA addrB modeB))
+(define (make-instr opcode modifier A-mode A-num B-mode B-num))
+(define (instr-opcode instr) (list-ref instr 0))
+(define (instr-modifier instr) (list-ref instr 1))
+(define (instr-A-mode instr) (list-ref instr 2))
+(define (instr-A-num instr) (list-ref instr 3))
+(define (instr-B-mode instr) (list-ref instr 4))
+(define (instr-B-num instr) (list-ref instr 5))
 
 
 ;;; Players
@@ -46,20 +52,23 @@
 
 (define (run players step)
   (cond
-   ((> step max-steps) players) ;Tie between remaining players
-   ((null? players) '())        ;Somehow we have no players remaining
+   ((> step max-steps) players)      ;Tie between remaining players
+   ((<= (length players) 1) players) ;There's only one player left who thus wins
    (else
     (let ((player (car players))
-          (other-players (cdr players)))
-      (if (null? ptrs)
-          (run other-players (+ step 1))  ;Player is out
-          (let* ((ptrs (player-ptrs player))
-                 (new-ptrs (execute-instr (car ptrs))))
-            (player-set-ptrs! (append (cdr ptrs) new-ptrs))
-            (run (append other-players (list player)) (+ step 1))))))))
+          (other-players (cdr players))
+          (ptrs (player-ptrs player)))
+      (let ((new-ptrs (execute-instr (car ptrs))))
+        (if (null? new-ptrs)
+            (run other-players (+ step 1))
+            (begin
+              (player-set-ptrs! (append (cdr ptrs) new-ptrs))
+              (run (append other-players (list player)) (+ step 1)))))))))
 
 (define (execute-instr ptr)
-  (let ((instr (vector-ref core ptr)))
+  (let* ((instr (vector-ref core ptr))
+         (A-operand (eval-operand (instr-A-mode instr) (instr-A-num) ptr))
+         (B-operand (eval-operand (instr-B-mode instr) (instr-B-num) ptr)))
     (case (instr-opcode instr)
       ((DAT))
       ((MOV))
@@ -79,3 +88,13 @@
       (else
        (error "Unrecognised opcode" (instr-opcode instr))))))
                
+(define (eval-operand mode num ptr)
+  (case mode
+    ((immediate))
+    ((direct))
+    ((indirect-A))
+    ((indirect-B))
+    ((pre-indirect-A))
+    ((pre-indirect-B))
+    ((post-indirect-A))
+    ((post-indirect-B))))