; File: multiply.asm
; Written by: Deepak Kumar
; Date: March 31, 2024
; Purpose:
; Write a MULT subroutine to multiply two numbers B = A * B (>=0)
; Parameters:
;   R0: A
;   R1: B
; Result is returned in R1 as well
; Therefore MULT is R1 = R1 * R)
; Other Registers used:
;   R2
;
; Algorithm:
; R2 = R1 (copy B into R2)
; R1 = 0 (initialize result)
; while R2 != 0 do
;   R1 = R1 + R0
;   R2 = R2 -1
;        
; The main program
;
        .ORIG       x6000
A       .FILL       3
B       .FILL       5
        .END
        .ORIG       x600A
START   LD      R0, A
        LD      R1, B
        JSR     MULT
        HALT
;
; MULT
; Expects A in R0 and B in R1
; returns result (A * B) in R1
; Uses R2 to store B
;
MULT    ST      R2, SaveR2  ; Save R2
        ADD     R2, R1, #0  ; R2 = R1
        AND     R1, R1, #0  ; initialize result
        ADD     R2, R2, #0  ; Trick to set condition flags
; while R2 != 0
LOOP    BRz     DONE
; do
        ADD     R1, R1, R0
        ADD     R2, R2, #-1
        BR      LOOP
DONE    LD      R2, SaveR2  ; Restore R2
        RET
SaveR2  .BLKW   1
        .END