CODEWITHSUNDEEP
sortingassembly
justbrianr
justbrianr

Reputation: 73

Sorting a string of characters in assembly

I'm trying to write in assembly language a function sort. It will sort the two dimensional array such that the rows will now contain data in alphabetical order. I have tried many things but it is honestly beyond my current knowledge. here's what I tried so far...

.386
public _Sort
.model flat
.code
_Sort proc

    push ebp
    mov ebp, esp
    push esi
    push edi

    mov edi, [esp + 4]    ; address of destination array
    mov esi, [esp + 8]    ; address of source array
    mov ecx, [esp + 16]   ; # of elements to mov
    cld
    rep movsd
L1:
    mov eax, [esi]
    cmp [esi + 8], eax
    jg L2
    xchg eax, [esi + 8]
    mov [esi], eax
L2: 
    pop edi
    pop esi
    pop ebp

    ret     
_Sort endp
end

Here's the C++ code...

#include <iostream>

using namespace std;

extern "C" int Sort (char [] [20], int, int);

void main ()
    {
    char Strings [10] [20]
                    = { "One",
                        "Two",
                        "Three",
                        "Four",
                        "Five",
                        "Six",
                        "Seven",
                        "Eight",
                        "Nine",
                        "Ten"   };
    int i;
    cout << "Unsorted Strings are" << endl;
    for (i = 0; i < 10; i++)
        cout << '\t' << Strings [i] << endl;
    Sort (Strings, 10, 20);
    cout << "Sorted Strings are" << endl;
    for (i = 0; i < 10; i++)
        cout << '\t' << Strings [i] << endl;
    }

I do realize my assembly does not make sense, sorry about that.

Upvotes: 0

Views: 4703

Answers (1)

Jerry Coffin
Jerry Coffin

Reputation: 490178

You'll want to build your assembly code in functions/procedures, just like you would code in some other language. Much like in C, string comparison, copying, etc., will need to be done in functions. Just for example:

; compares [esi] to [edi], returns +, 0 or - to indicate order
; inputs: esi, edi: addresses of strings
; destroys: esi, edi, edx
;
strcmp_int proc
    jmp short start
loop_top:
    inc esi
    inc edi
start:
    movsx eax, byte ptr [esi]
    movsx edx, byte ptr [edi]
    test edx, edx
    jz @f
    sub eax, edx
    jz loop_top
    ret
@@:
    sub eax, edx
    ret
strcmp_int endp

[warning: this code isn't necessarily intended to be used as-is--just an example of one of the kinds of functions you'll normally need to write to do this sort of job in assembly language. It's been long enough since I wrote much assembly language that you can undoubtedly do better on a modern processor--and at least for things done purely in assembly language, you normally want to put results in flags, rather than a -/0/+ in a register like strcmp (and this) produce. But note that this does return with flags set by the final sub]

See also Why is memcmp so much faster than a for loop check? for some links to optimized implementations (SSE2/AVX2) for explicit-length strings, which can be much faster for medium to long strings. For optimization links in general, see the x86 tag wiki.

Your sort will depend on the sorting algorithm you decide to implement. Obviously, a Quicksort won't look the same as an insertion sort. The main point, however, is simple: don't try to write it as a single, monolithic chunk of code—break it up into pieces that are individually easy to write and understand.

Upvotes: 2

Related Questions