.ORG    $0

start   LW      r1, r0, $3000   ; r1 = factor 1
        LW      r2, r0, $3002   ; r2 = factor 2
        MV      r3, r0          ; product

        LI      r4, $0080       ; -128
        LI      r5, $1          ; stores 1 for parity check
        LI      r6, $0          ; counter for the loop

; special case for -128
        SLT     r0, r2, r4      ; r2 == -128?
        BRZ     special

; sign extend r1
        SLLI    r1, r1, $8
        SRAI    r1, r1, $8

; negate r2
        AND     r7, r2, r4      ; r2 < 0?
        BRZ     loop
        SUB     r2, r0, r2      ; negate r2

; actual multiplication loop
loop    SLTI    r0, r6, $8      ; loop guard
        BRZ     reneg

        AND     r0, r2, r5      ; check parity (1 if add)
        BRZ     shift

        ADD     r3, r3, r1

shift   SRL     r2, r2, r5      ; step 4
        SLL     r1, r1, r5
        ADD     r6, r6, r5
        BRA     loop

reneg   SLT     r0, r7, r0
        BRZ     done
        SUB     r3, r0, r3      ; negate
        BRA     done

special SLLI    r3, r1, $8      ; r3 = r1 * 128
        SUB     r3, r0, r3      ; r3 = -r3
done    STOP

; max - 518 cycles
; min w/o special case - 499 cycles
; special case - 74 cycles