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QuickSort in 6502 assembler

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    QuickSort in 6502 assembler

    Originally posted by carlsson View Post
    The first attempt failed, but after changing to a different algorithm, I got QuickSort working
    Kind of. I was about to post the code, but I just found if the data to be sorted is too versatile (too big interval between smallest and largest) and at the same time already sorted, my algorithm recurses so many steps I run out of stack space and computer crashes.

    I tried to come up with a hackery solution, but so far I came up with no real good workaround. One way would be to take a copy of the whole stack elsewhere, and start a new one, but it is a bit clumsy. I tried to store only the low byte of the return address as long as I stay within the same memory page. It helped a bit, but not fully. Since the recursive calls come from only two places, maybe I can store some counter how many times the return address is valid instead of storing each return address:

    Now: ret1 ret1 ret1 ret1 ret2 ret2 ret2 ret1 ret2 ret2 ret1 end (12 bytes)
    Idea: 4 ret1 3 ret2 1 ret1 2 ret2 1 ret1 end (11 bytes)

    I could probably align the code in a way so I can smart-code the counter into the return address or at least distinguish between the two, but I'd look for a general solution. This is nothing I need to use right now, so it is far from priority, just annoying.

    Anyway, this is what I have for now. It would assemble with DASM and execute on a Commodore 64. Address $0400 equals screen matrix, so you can see the data being sorted.

    #processor 6502
    #seg main
    array eqm $0400 ; $c100
    larec eqm $c200
    rarec eqm $c300
    crec eqm $fa
    ktmp eqm $fb
    lval eqm $fc
    left eqm $fd
    right eqm $fe
              org $0900
              lda #$00
              sta crec
              sta larec,x
              lda #$ff
              sta rarec,x
              jsr quick
    quick:    ldx crec
              lda rarec,x
              sta right 
              lda larec,x
              sta left
              inc crec
              tax                 ; left
              cpx right
              bcs qout$           ; if left >= right exit
              ldy right           
              lda array,y
              cmp array,x         ; if d[left] > d[right]
              bcs bprep$
              jsr swap
    bprep$:   lda array,x         ; repeat
              sta lval
    prep$:    lda lval
    loop$:    cpx right
              beq loop2$
              cmp array,x         ;   repeat j=j+1 until d[left] <= d[j]
              beq loop2$
              bcs loop$
    loop2$:   cpy left
              beq uloop2$
              cmp array,y         ;   repeat k=k-1 until d[left] >= d[k]
              bcc loop2$
    uloop2$:  sty ktmp
              cpx ktmp            ;   if k >= j
              bcs uloop1$
              jsr swap
    uloop1$:  plp
              beq prep$
              bcc prep$
    uprep$:   ldx left
              jsr swap
              ldx crec
              lda left
              sta larec,x
              cpy left
              sta rarec,x
              bcc no1st$
              beq no1st$
              jsr quick           ; jsr
    no1st$:   ldx crec
              ldy rarec,x
              sta larec,x
              lda rarec-1,x       ; lda right
              sta rarec,x
              jsr quick
    qout$:    dec crec
    ;; swap takes x/y as index to the two values to swap
    swap:     lda array,x
              lda array,y
              sta array,x
              sta array,y
    To make it worse (?), I have another code written by someone else that is about the same in size, but functions better. It overcomes the recursive calls, which is something I'd like to do as well. I have some iterative implementation of QuickSort in a C book, but I never was successful to convert it into 6502. Maybe that's the desired approach anyway; stay away from recursion on small platforms.
    Anders Carlsson