Subversion Repositories zipcpu

;
;       DIVS
;
;       Given R0,R1, computer R0 = R0/R1 and R1 = R0%R1 for signed R0,R1.
;       We'll call R0 (input) x, R1(input) y, result is such that
;       R0 * y + R1 = x.  Now let's work through our signs with an example
;       where x = +/- 22, and y = +/- 4:
;
;       x y
;       + +     No change, just call divu
;       - +     (x=-22,y= 4,R0=-5,R1=-2)
;       + -     (x= 22,y=-4,R0=-5,R1= 2)
;       - -     (x=-22,y=-4,R0= 5,R1=-2)
;
;
;
divs:   ; Given R0,R1, computer R0 = R0/R1 and R1 = R0%R1 for signed R0,R1
        SUB     2,SP
        ; (stall)
        STO     R3,2(SP)
        ;
        CLR     R3              ; Keep track of resulting sign in R2
        TST     -1,R0           ; Is R0 negative?
        MOV     ret_div32s(PC),R2
        LLO.LT  3,R3            ; If so, resulting sign will be negative, and
        NEG.LT  R0              ; then we negate R0 (R0 = ABS(R0))
        MOV.LT  divu_divs_return(PC),R2
        TST     -1,R1           ; Is R1 negative?
        XOR.LT  1,R3            ; If so, result will be opposite sign of before
        NEG.LT  R1              ; Now we get R1=ABS(R1)
        MOV.LT  divu_divs_return(PC),R2
        STO     R2,1(SP)
        BRA     divu            ; Do our unsigned multiply
        ; JSR   divu            ; Do our unsigned multiply
divu_divs_return:
        TST     1,R3            ; Check resulting sign
        NEG.NE  R0              ; Need to flip the sign of our result
        TST     2,R3            ; Now, if R1 was originally negative
        NEG.NE  R1              ; Then negate R1
ret_div32s:
        LOD     2(SP),R3
        ADD     2,SP
        RETN