; File: mult.asm
; Written by: Deepak Kumar
; Date: March 25, 2025
; Purpose:
;       Multiply two integers A > 0, and B (>=0)
; Main Program to test
; Does R1(35) <- R0(7) * R1(5)
        .ORIG   x3000
START   AND     R0, R0, #0  ; R0 <- 0
        ADD     R0, R0, #7  ; R0 <- 7(A)
        AND     R1, R1, #0
        ADD     R1, R1, #15  ; R1 <- 5
        JSR     MULT        ; R1 <- R0 * R1
        HALT
        .END
;
; MULT Subroutine for A(R0) <- A(R0) * B(R1)
;
; Algorithm:
;   Given A, B
;   result <- 0
;   while B > 0 do
;       result <- result + A
;       B <- B - 1
;
; Registers:
; R0 : A, and result
; R1: B
;
; R2 <- R1 (Save B)
; R1 <- 0
; while R2 != 0 do
;   R1 <- R1 + R0
;   R2 <- R2 - 1
; NOTE: uses R0 and R2, so save and restore
        .ORIG   x3020
MULT    ST      R2, SaveR2
        ADD     R2, R1, #0  ; R2 <- R1
        AND     R1, R1, #0  ; R3 <- 0 (result)
        ADD     R2, R2, #0  ; R2 <- R2+0 (to set flags)
; while R2 != 0
LOOP    BRz     MULDone    
; do
        ADD     R1, R1, R0  ; R1 <- R1 + R0
        ADD     R2, R2, #-1 ; R2 <- R2 - 1
        BR      LOOP
MULDone LD      R2, SaveR2
        RET
SaveR2  .BLKW   1
        .END