+ (let* ((instr (core-get ptr))
+ (A-pointer (eval-operand (instr 'A-mode) (instr 'A-num) ptr))
+ (B-pointer (eval-operand (instr 'B-mode) (instr 'B-num) ptr))
+ (modifier (instr-modifier instr)))
+ (case (instr-opcode instr)
+ ((DAT) '()) ;Game over, man, game over!
+ ((MOV)
+ (case modifier
+ ((A))
+ ((B))
+ ((AB))
+ ((BA))
+ ((F))
+ ((X))))
+ ((ADD)
+ (case modifier
+ ((A))
+ ((B))
+ ((AB))
+ ((BA))
+ ((F))
+ ((X))))
+ ((SUB)
+ (case modifier
+ ((A))
+ ((B))
+ ((AB))
+ ((BA))
+ ((F))
+ ((X))))
+ ((MUL))
+ ((DIV))
+ ((MOD))
+ ((JMP))
+ ((JMZ))
+ ((JMN))
+ ((DJN))
+ ((SEQ CMP))
+ ((SNE))
+ ((SLT))
+ ((SPL))
+ ((NOP) (list (addr+ ptr 1)))
+ (else
+ (error "Unrecognised opcode" (instr-opcode instr))))))
+
+(define (eval-operand mode num ptr)
+ (addr+ ptr
+ (case mode
+ ((immediate) 0)
+ ((direct) num)
+ ((indirect-A) (addr+ num ((core-get (addr+ ptr num)) 'A-num)))
+ ((indirect-B) (addr+ num ((core-get (addr+ ptr num)) 'B-num)))
+ ((pre-indirect-A)
+ (let ((aux-instr (core-get (addr+ ptr num))))
+ (instr-set-A-num! aux-instr (addr+ -1 (aux-instr 'A-num)))
+ (addr+ num (aux-instr 'A-num))))
+ ((pre-indirect-B)
+ (let ((aux-instr (core-get (addr+ ptr num))))
+ (instr-set-B-num! aux-instr (addr+ -1 (aux-instr 'B-num)))
+ (addr+ num (aux-instr 'B-num))))
+ ((post-indirect-A)
+ (let* ((aux-instr (core-get (addr+ ptr num)))
+ (old-A-num (aux-instr 'A-num)))
+ (instr-set-A-num! aux-instr (addr+ 1 (aux-instr 'A-num)))
+ (addr+ num old-A-num)))
+ ((post-indirect-B)
+ (let* ((aux-instr (core-get (addr+ ptr num)))
+ (old-B-num (aux-instr 'B-num)))
+ (instr-set-B-num! aux-instr (addr+ 1 (aux-instr 'B-num)))
+ (addr+ num old-B-num)))
+ (else
+ (error "Unrecognized mode" mode)))))