quicksort in assembly 
Author Message
 quicksort in assembly

hi folks, i've written a quicksort in assembly.  would be glad to
receive any pointers on how i could have made it better.

;;; quicksort implementation

jmp     start

DAT     dw      48, 16, 9, 30, 4, 1, 25, 3, 2, 7, 41, 2, 0
SZ      equ     13

        mov     bp, SZ
        dec     bp
        mov     bx, 0
        push    bp      ; top
        push    bx      ; bottom
        call    qsort

        mov     ah, 4ch
        int     21h

;;; passing parameters via stack
;;; order on stack - lower bound, then upper bound
qsort   proc
        push    bx
        push    bp

        mov     bp, sp
        mov     bx, [bp + 6]    ; get lower bound
        mov     bp, [bp + 8]    ; get upper bound

        ;; terminating condition
        cmp     bx, bp          ; if anchors crossover, we are done
        jge     done

        call    partition

        push    di              ; save pivot for second qsort call

        ;; low end of the array
        dec     di              ; exclude pivot - already sorted
        push    di              ; store upper bound
        push    bx              ; store lower bound
        call    qsort

        pop     di              ; get back stored pivot

        ;; high end of the array
        inc     di              ; exclude pivot - already sorted
        push    bp              ; store upper bound
        push    di              ; store lower bound
        call    qsort
        pop     bp
        pop     bx
        ret     4
qsort   endp

;;; place values that are smaller than pivot on left side
;;; and larger than pivot on right.
partition    proc
        push    bx
        push    bp
        push    cx
        push    si
        push    dx

        ;; adjust size of pointer according to data type
        shl     bx, 1     ; multiple by 2 to access word
        shl     bp, 1

        lea     si, DAT   ; make SI point to our array
        mov     dx, si    ; save first location
        add     dx, bx

        mov     cx, [si + bx]   ; take first item as pivot value
        add     bp, 2           ; compensate for the initial dec BP
        add     bx, 2
        cmp     [si + bx], cx
        jb      up
        sub     bp, 2
        cmp     [si + bp], cx
        jg      down

        cmp     bx, bp          ; two end pointers crossover?
        jge     crossed

;; swap values pointed by 'bx' (moving up)
;; and 'bp' (coming down)
        xchg    [si + bx], [si + bp]
        jb      up

;; swap value pointed by 'down' index with [pivot]
        mov     bx, dx
        xchg    [si + bp], [bx]

        ; convert it back to data-type independence positions
        shr     bp, 1
        mov     di, bp  ; set pivot to be 'down' index

        pop     dx
        pop     si
        pop     cx
        pop     bp
        pop     bx

partition       endp

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Sat, 12 Jan 2002 03:00:00 GMT  
 [ 1 post ] 

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