mult.mac

                    .TITLE  Resursive Multiply
                    .BLKW   1000.
        ANS:        .BLKW   2
        M:          .BLKW   1
        N:          .BLKW   1
                    
        START:      MOV #START, SP      ; R6 gets set to this address
                    SUB #10., SP        ; R6 -= 10
                    MOV N, 8.(SP)       ; 
                    MOV M, 6(SP)        ; 2(SP) AND 4(SP) CONTAIN DP ANSWER
                    MOV #NEXT, 0(SP)    ;
                    JMP RMUL            ; Go to recursive "function"
        NEXT:       MOV 2(SP), ANS
                    MOV 4(SP), ANS+2
                    ADD #10., SP
                    
                    
        ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
        ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
        ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
        
        RETURN:     CLR 2(SP)
                    JMP @0(SP)
        
        RMUL:       TST 8.(SP)
                    BLE RETURN          ; N <= 0 ?    
                    ADD 6(SP), 4(SP)    ; Low Order of Answer gets added by multiple M
                    ADC 2(SP)           ; ADC to high order if overflow    
                    DEC 8.(SP)          ; N -= 1
                    
        JMPSTUFF:   MOV SP, R5
                    SUB #10., SP
                    MOV #RETMULT, SP
                    MOV 6(R5), 6(SP)
                    MOV 8.(R5), 8.(SP)
                    
        RETMULT:    ADD 6.(SP), 4(R5)
                    ADC 2(R5)           ; Use register instead of SP - faster
                    ADD 2(SP), 2(R5)
                    ADD #10., SP