I'm writing a program to reverse a string, and I can't seem to find the error in my program. It doesn't seem to want to output the reverse string. Here's what I've got: .model small .stack 100h .data prompt db "Please enter a message: ",0 OutputID db "The reverse of that is: ",0 string db 0 strlen equ 32 string2 db strlen dup(0) .code extrn Writechar:proc,Writestring:proc,Readstring:proc, Clrscr:proc,Crlf:proc main proc mov ax,@data mov ds,ax call Clrscr mov dx,offset prompt call Writestring mov dx,offset string call Readstring mov dx,offset OutputID call Writestring lea si,string+strlen-1 lea di,string2 mov cx,ax L1: mov ah,[si] mov [di],ah dec si inc di loop L1 mov dx,offset string2 call Writestring mov ax,4c00h int 21h main endp end main Thanks for any help you can provide!

output? What's your return value look like? How do you figure strlen?

Hi, Well, my asm is rusty. Your code seemed to be in perfect logic. Have you test it under a debugger? The first thing come to mind is the code : mov ah, [si] mov [di], ah If I'm not mistaken (well, if it is, sorry) (E)SI regs defaulted to use DS reg as segment reg, while (E)DI defaulted to ES. At the start of your code, you only set DS reg. ES reg could point anywhere (or is it point to the PSP... hmm I wonder...) not necessarily to your data seg. If my suspicion is right, you might be writing the reversed string somewhere, and the writestring routine (I assume) will assume that you want to print the string pointed by DS:DX, which may or maynot contain the reversed string. If this is the problem, the answer is either a. init ES with the data segment at the start of your code, i.e. mov ax, @data mov ds, ax mov es, ax b. use les instead of lea for string2 before L1 loop, i.e. lea si, string+strlen-1 les di, string2 c. use segment prefix when moving the data, i.e. mov ah, [si] mov [ds:di], ah You only need to take only one of the above options. An improvement is to use string instructions inside the loop, in this case, stosb. A slightly modified code : lea si, string2 les di, string+strlen-1 L1: mov ah, [di] stosb dec di loop L1 The loop is one instruction less, which can be significant for very large strings. HTH

why do u have to use mov ax,@data mov ds,ax why we cannot do mov ds,@data