Exercise from an Assembly course I'm enrolled in:

Find closest number.

Add the following into the data section:

nums dd 23h,75h,111h,0abch,443h,1000h,5h,2213h,433a34h,0deadbeafh

This is an array of numbers.

Write a program that receives a number x as input, and finds the dword inside the array nums, which is the closest to x. (We define the distance between two numbers to be the absolute value of the difference: |a-b|).


For the input of 100h, the result will be 111h, because 111h is closer to 100h than any other number in the nums array. (|100h - 111h| = 11h).

Full exercise-sheet on GitHub - xorpd

Here's my approach. Please pay attention to the comments which I've added to explain my ideas.

format PE console
entry start

include 'win32a.inc' 

section '.data' data readable writeable
    nums  dd  23h, 75h, 111h, 0abch, 443h, 1000h, 5h, 2213h, 433a34h, 0deadbeafh

section '.text' code readable executable

    xor         esi,        esi                     ; Current array-index.
    mov         ecx,        (nums_end - nums) / 4   ; Array-length
    mov         ebx,        0x100                   ; Input, Value to compare too. 
    mov         edx,        0x7fffffff              ; Smallest value found until now. Start with the greatest value possible.
    mov         edi,        edi                     ; Index of the element with the smallest value.
    ; -------------------------------------------------------------------------------------------------------------------
    mov         eax,        ebx                     ; Make a copy compare-value.
    mov         ebp,        dword[nums + 4 * esi]   ; Move the value of the current element to ebp.
    cmp         eax,        ebp                     ; If eax is smaller then ebp then we must not subtract ebp from eax.
    jb          eax_is_smaller                      ;   This would result in a negative number. ...
    sub         eax,        ebp
    jmp         subtraction_done
    sub         ebp,        eax                     ; ... subtract eax from ebp instead.
    mov         eax,        ebp                     ; ... and move the result to eax.
    cmp         eax,        edx                     ; Compare the result to the current smallest value.
    jnb         prepare_next
    mov         edx,        eax                     ; If the result is smaller then the current smallest value the result becomes the smallest value.
    mov         edi,        esi                     ; Save the index of the smallest value.
    inc         esi                                 ; Increment the array-index.
    loop        check_next                          ; Check the next array-element.
    ; -------------------------------------------------------------------------------------------------------------------
    mov         eax,        dword[nums + 4 * edi]   ; Use the index of the nearest element to get the value. Then print it to stdout.
    call        print_eax                           ; Print to stdout. Function provided by the tutor.
    ; Exit the process:
    push        0
    call        [ExitProcess]

include 'training.inc'

I've tried it with different compare-values and it provided the expected value every time. So I guess it's generally correct. Nevertheless I'm not completely satisfied. I have used a lot of register.

How can I reduce the number of register? I have found a solution for the problem with getting negative values when subtracting a larger number from a smaller number. It works but it isn't elegant. Is there a better solution?


1 Answer 1

mov edi, edi    ; Index of the element with the smallest value.

An error here. You want to zero this register. Use xor

A better strategy will be to simply subtract without looking at which number is the largest and then later take the absolute value of the difference.

    mov     eax, [nums + 4 * esi]
    sub     eax, ebx
    jnl     IsPositive
    neg     eax
    cmp     eax, edx ; Compare the result to the current smallest value.
    jnb     prepare_next
    mov     edx, eax
    mov     edi, esi

Less code and less register usage!

A further optimization will come from not using ECX as a loop counter. You can determine if the loop is completed by looking at the ESI array index.

    xor     esi, esi         ;Start at index 0
    inc     esi
    cmp     esi, (nums_end - nums) / 4
    jb      check_next

In stead of branching, you could use one of the conditional forms of mov.
Same code size but faster. Having less labels is also nicer.

    cmp     eax, edx ; Compare the result to the current smallest value.
    cmovb   edx, eax
    cmovb   edi, esi

About coding style.

mov eax, dword[nums + 4 * edi]

Does your assembler requires you to write this redundant mention of the size?
Do you get an error if you write it shorter?

    mov     eax, [nums + 4 * edi]
  • \$\begingroup\$ Thanks a lot. Goanna try out the access in brackets-syntax this evening. \$\endgroup\$ Jul 17, 2017 at 1:32

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