CODEWITHSUNDEEP
arrayssortingassemblyx86-16
Bethan Huish
Bethan Huish

Reputation: 43

How to sort an array based on pairs in 8086 assembly

Say I have an array defined by:

array DW 1,1,3,0,3,3,4,4,-1

The array is terminated by -1, how would I be able to sort the array in pairs of descending order based on the first number in the pair (if first number is the same then it's sorted by the second number) as such:

4, 4; 3, 3; 3, 0; 1, 1;

Upvotes: 0

Views: 395

Answers (1)

Sep Roland
Sep Roland

Reputation: 39166

array DW 1,1, 3,0, 3,3, 4,4, -1

The first number in each pair of word-sized numbers is the most significant for your task.
Each of these pairs can be seen as a dword, but on x86 (little endian) the first word will be the least significant. That's just the opposite of what you need. What if you temporarily swapped the words? Then you could sort the array as normal dwords.

  1. Swap
  2. Sort these dwords normally. (Beware terminator is still word)
  3. Swap.

This could be the swap procedure (32-bit):

Swap:
    mov ebx, array
    jmp First
Next:
    rol dword [ebx], 16
    add ebx, 4
First:
    cmp word [ebx], -1
    jne Next
    ret

This could be the swap procedure (16-bit):

Swap:
    mov  bx, array
    jmp  First
Next:
    xchg ax, [bx+2]
    mov  [bx], ax
    add  bx, 4
First:
    mov  ax, [bx]
    cmp  ax, -1
    jne  Next
    ret

A solution where you do these pre-swap and post-swap operations within the dword sorting algorithm would be just as easy.

Upvotes: 1

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