I Need Assembly help w/reversing strings

Newbie Poster

4 posts since Oct 2005

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Here is a solution using inline assembly in c program

#include <iostream>
using namespace std;

int main(int argc, char* argv[])
{
	char s1[] = "Hello World";
	char s2[255] = {0};
	int len = 0;
	_asm
	{
		push	esi
		push	edi
		lea		esi,s1
		lea		edi,s2
		; get length of string s1
		xor		eax,eax	
		mov		ecx,0ffh ; max length of string ?
		repnz	cmpsb	  ;locate 0 byte, if it exists
		jnz		l1		; oops! not a null terminated string
		not		ecx		; make it an integer
		and		ecx,0000000ffh ; mask off high dword
		lea		edi,[len]	; save for output later
		mov		[edi],cx	; 

		;
		;
		; reverse the string
		lea		edi,s2		; set up registers for moving string
		lea		esi,s1
		add		esi,ecx
		dec		esi
		dec		esi
		xor		eax,eax
l2:		; move each byte from ds:esi to es:edi in reverse order
		; would be nice if we could use rep movsb, but we can't
		; so have to do it the lard way.
		mov		al,[esi]
		mov		[edi],al
		inc		edi
		dec		esi
		loopnz	l2
		mov		[edi],0



	l1: nop ; not found
		pop		edi
		pop		esi


	}
	cout << len << endl;
	cout << s2 << endl;
	return 0;
}

Retired & Loving It

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30,049 posts since Aug 2005

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Does anyone have a solution to the problem with the above program using the assembly language from Kip Irvine's Book?

Newbie Poster

4 posts since Oct 2005

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Does anyone have a solution to the problem with the above program using the assembly language from Kip Irvine's Book?

Nobody is going to just hand you the code. Use your head for something other than a hat rack and apply the code I posted to your problem. It can't be all that hard to do assuming they are both written with the same assembly language.

Retired & Loving It

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Ancient Dragon,
I never asked anyone to hand code to me. I had already written the program and needed suggestions on what could be causing the program not to display output.

Many textbooks give snippets as examples without writing the program for someone. Thank you for your code and your comment about using my head for something other than a coat rack.

I was successful at creating a working program with only 20-lines of code.

P.S My original question was concerning a program that prompts the user for input, not a string constant. I already had a working program that reverses a hard coded string. Thank You for your help.

Newbie Poster

4 posts since Oct 2005

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I think this is because you also copy null or ambigious bytes trailing the actual input string into the reversed one. For instance "abc" may be last 3 bytes of 80 byte reverse buffer as the first 77 is ambigous (most likely null's).

Using push/pop is more complicated then it needs to be in my opinion. The solution is you need the actual length of the input string, I'd do a simple string length scan with a repne scasb, once null is reached (E)DI-1 can be the starting point to copy backwards into the reversing buffer, as you already have the count.

Ryu

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24 posts since Jul 2006

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Does anyone have a solution to the problem with the above program using the assembly language from Kip Irvine's Book?

Ancient Dragon, I never asked anyone to hand code to me. .

Hummmm. But I'm glad you found the solution on your own. Now, would you mind posting it so we can all learn from it ?

Retired & Loving It

Team Colleague

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Quoted by frantonio:
Does anyone have a solution to the problem with the above program using the assembly language from Kip Irvine's Book?

Quoted by frantonio:
Ancient Dragon,
I never asked anyone to hand code to me. .

I know that it is mid-term election time, but I thought that taking out a few words from what one says and twisting it, belongs to the political arena. This reminds me of a bad political commercial.

The second quote above was taken out of context and the central meaning was distorted. The full context of the sentence is:

I never asked anyone to hand code to me. I had already written the program and needed suggestions on what could be causing the program not to display output.

It is not uncommon for one to seek advice when one is unsure of what could be causing a problem. :)

Newbie Poster

4 posts since Oct 2005

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This is really a common problem of not asking the right question(s). I frequently see people ask "Do you know how to .... ", to which my answer is either "Yes" or "No". Afterall if someone is going to ask a yes-or-no question that is how I will answer. In your case you asked us if we would give you code to do something. That was the wrong question. You should have asked how to improve your code to make it do whatever you want it to do.

>>My original question was concerning a program that prompts the user for input
The algorithm for reversing a string is the same whether the string is from user input or a hard-coded string. Again, you may have asked the wrong question in your original post. Do you want to know how to get a string from the keyboard? Or how to reverse the string. ? Now I am not sure of your intent.

>My original question was concerning a program that prompts the user for input
you are quite right and everyone is encouraged to ask lots of questions. But you also need to ask the right question(s).

Anyway -- good luck with your program.