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Index > Main > Program works, but slowly (beginner question)

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aviv0404



Joined: 07 Feb 2020
Posts: 5
aviv0404
Hey, I started learning assembly this week from the a udemy course that uses FASM for teaching assembly.

One of the assignments is to write a program that takes a number N and prints back all the pairs (x,y) such that x < y < N.

So first I wrote the program in python to make things easier:
[img]https://imgur.com/SDktZuB[/img]
Code:
n = 4
for i in range (n):
        for j in range(n):
                if (i < j):
                        if (j < n):
                                print ((i,j))

    



Then I wrote it in assembly following the python verison so my code won't be considered as "spaghetti code".
And it looks like this:
[img]https://imgur.com/iEvQZa9[/img]
Code:
;unsigned: JA JB JAE JBE
;signed: JL JG JLE JGE

format PE console
entry start

include 'win32a.inc'

section '.text' code readable executable

start:
        call    read_hex
        mov     edi, eax ; n
        mov     esi, 0h ;i for loop 1
        mov     ecx, 0h ;j for loop 2
main_loop:
        for_1:
                for_2:
                        if_1:
                                ;condition
                                cmp     esi, ecx
                                jae     end_if_1
                                if_2:
                                        ;condition
                                        cmp     ecx, edi
                                        jae     end_if_2
                                        ;print numbers
                                        mov     eax, esi
                                        call    print_eax       
                                        mov     eax, ecx
                                        call    print_eax
                                end_if_2:
                        end_if_1:
                        ;increasing j and checking if its the end of the loop
                        inc     ecx
                        cmp     edi, ecx
                        jnz     for_2
                end_for_2:
                ;increasing i and checking if its the end of the loop
                inc     esi
                cmp     edi, esi
                jnz     for_1
        end_for_1:

end_process:
        push    0
        call    [ExitProcess]

include 'training.inc'
    


The teacher's solution looks like this:
[img]https://imgur.com/myC7eRt[/img]
Code:
; 1. Write a program that takes the number n as input, and prints back all the
;    pairs (x,y) such that x < y < n.
;

format PE console
entry start

include 'win32a.inc'

; ===============================================
section '.text' code readable executable

;       Make a loop that loops from x to y and prints all combinations. Place that loop
;       inside another loop that goes from y to n and also prints all combinations.
start:
    call    read_hex

        mov     ebx, eax        ; save user input
        mov     edx, 1          ; y counter

loop_y:
        mov     ecx, 0          ; x     counter

loop_x:
print_x_y:
  mov   eax, ecx
        call    print_eax       ; print x

  mov   eax, edx
        call    print_eax       ; print y

next_x:
        inc     ecx

        cmp     ecx, edx
        jnz   loop_x            ; if x !== y

next_y:
        inc     edx

        cmp     edx, ebx
        jnz   loop_y            ; if y !== n

    ; Exit the process:
        push    0
        call    [ExitProcess]

include 'training.inc'

    


The logic of my code works. but it's a lot slower than the teacher's code.
When I assemble my code and run it, it prints immediately the first set of numbers (first loop of for_2), but then the next loops are delayed (it also seems like the delay is of the same interval).

Am I using too many labels? Or did I end up really writing a spaghetti code? I don't see my loop getting stuck anywhere for any reason... I'd appreciate any help!

For example for 4 as the input ill get:
0
1
0
2
0
3
delay for some reason (1~3 seconds)
1
2
1
3
delay
2
3
delay
end
Post 07 Feb 2020, 08:32
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17348
Location: In your JS exploiting you and your system
revolution
If we rewrite the python code then you might see the problem:
Code:
n = 4
for i in range (n):
        for j in range(i+1,n):
                print ((i,j))    
You don't need any "if" statements. Start the j loop from the i value.
Post 07 Feb 2020, 09:20
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aviv0404



Joined: 07 Feb 2020
Posts: 5
aviv0404
revolution wrote:
If we rewrite the python code then you might see the problem:
Code:
n = 4
for i in range (n):
        for j in range(i+1,n):
                print ((i,j))    
You don't need any "if" statements. Start the j loop from the i value.


Alright so I rewrote it and it's smooth now
Code:
;unsigned: JA JB JAE JBE
;signed: JL JG JLE JGE

format PE console
entry start

include 'win32a.inc'

section '.text' code readable executable

start:
        call    read_hex
        mov     edi, eax ; n
        mov     esi, 0h ;i for loop 1
        mov     ecx, 0h ;j for loop 2
main_loop:
        for_1:
                mov     ecx, esi
                inc     ecx
                for_2:
                        mov     eax, esi
                        call    print_eax       
                        mov     eax, ecx
                        call    print_eax
                        ;increasing j and checking if its the end of the loop
                        inc     ecx
                        cmp     edi, ecx
                        jg              for_2
                end_for_2:
                ;increasing i and checking if its the end of the loop
                inc     esi
                cmp     edi, esi
                jg              for_1
        end_for_1:

end_process:
        push    0
        call    [ExitProcess]

include 'training.inc'
    

I still don't understand why the delay happened when the conditions where part of the code though
Post 07 Feb 2020, 11:48
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17348
Location: In your JS exploiting you and your system
revolution
The problem is that you didn't reset the ECX counter inside loop1. So ECX spins all the way to 2^32 before wrapping back to zero.
Post 07 Feb 2020, 12:18
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aviv0404



Joined: 07 Feb 2020
Posts: 5
aviv0404
revolution wrote:
The problem is that you didn't reset the ECX counter inside loop1. So ECX spins all the way to 2^32 before wrapping back to zero.


Oh now I see it!
The first loop was fine, ecx got to 4 and exit the loop but at the second loop it increased to 5 and then like you said there was a wraparound

Thanks a lot! I took this concept for granted since I'm used to high level languages like python. I'll definitely pay attention to this in the future!
Post 07 Feb 2020, 13:13
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17348
Location: In your JS exploiting you and your system
revolution
I highly recommend the use of a debugger. Especially a GUI debugger like Ollydbg. For beginners they can make things a lot clearer as you watch the execution process in action.
Post 07 Feb 2020, 13:21
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aviv0404



Joined: 07 Feb 2020
Posts: 5
aviv0404
revolution wrote:
I highly recommend the use of a debugger. Especially a GUI debugger like Ollydbg. For beginners they can make things a lot clearer as you watch the execution process in action.


Yeah a debugger will be really handy, I'll download Ollydbg right now!
Thank you once more.
Post 07 Feb 2020, 13:55
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7736
Location: Kraków, Poland
Tomasz Grysztar
You can find some basic tips on how to work with fasmw + OllyDbg in my video guides.
Post 07 Feb 2020, 14:05
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aviv0404



Joined: 07 Feb 2020
Posts: 5
aviv0404
Tomasz Grysztar wrote:
You can find some basic tips on how to work with fasmw + OllyDbg in my video guides.


Thanks, I'll definitely look into it!
Post 07 Feb 2020, 14:42
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