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POhow to debug an NASM assembly program requiring user input?
 

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  1. POhow to debug an NASM assembly program requiring user input?
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    <p>I'm going through jeff's amazing book <a href="http://rads.stackoverflow.com/amzn/click/0470497025" rel="nofollow">assembly step by step</a>, and I am on chapter 8 where he shows an example of an assembly program that takes a file from the user this way:</p> <pre><code>SECTION .bss ; Section containing uninitialized data BUFFLEN equ 1024 ; Length of buffer Buff: resb BUFFLEN ; Text buffer itself </code></pre> <p>it reads the file text into <code>Buff</code>, and outputs a version of that text in ALL CAPS to a different file. </p> <p>I want to run this program in debug mode to step through it to analyze what is happening with all the registers.</p> <p>I am running this on ubuntu with INSIGHT. </p> <p>I am a complete beginner. I know how to use Insight to step through, but the way the user needs to run this program is:</p> <pre><code>myProgram &gt; outputfile.txt &lt; inputfile.txt </code></pre> <p><strong>How do I mimic this in a debugger?</strong></p> <p>here's the full source:</p> <pre><code>; Executable name : uppercaser2 ; Version : 1.0 ; Created date : 3/25/2009 ; Last update : 3/25/2009 ; Author : Jeff Duntemann ; Description : A simple program in assembly for Linux, using NASM 2.05, ; demonstrating simple text file I/O (through redirection) for reading an ; input file to a buffer in blocks, forcing lowercase characters to ; uppercase, and writing the modified buffer to an output file. ; ; Run it this way: ; uppercaser2 &gt; (output file) &lt; (input file) ; ; Build using these commands: ; nasm -f elf -g -F stabs uppercaser2.asm ; ld -o uppercaser2 uppercaser2.o ; SECTION .bss ; Section containing uninitialized data BUFFLEN equ 1024 ; Length of buffer Buff: resb BUFFLEN ; Text buffer itself SECTION .data ; Section containing initialised data SECTION .text ; Section containing code global _start ; Linker needs this to find the entry point! _start: nop ; This no-op keeps gdb happy... ; Read a buffer full of text from stdin: read: mov eax,3 ; Specify sys_read call mov ebx,0 ; Specify File Descriptor 0: Standard Input mov ecx,Buff ; Pass offset of the buffer to read to mov edx,BUFFLEN ; Pass number of bytes to read at one pass int 80h ; Call sys_read to fill the buffer mov esi,eax ; Copy sys_read return value for safekeeping cmp eax,0 ; If eax=0, sys_read reached EOF on stdin je Done ; Jump If Equal (to 0, from compare) ; Set up the registers for the process buffer step: mov ecx,esi ; Place the number of bytes read into ecx mov ebp,Buff ; Place address of buffer into ebp dec ebp ; Adjust count to offset ; Go through the buffer and convert lowercase to uppercase characters: Scan: cmp byte [ebp+ecx],61h ; Test input char against lowercase 'a' jb Next ; If below 'a' in ASCII, not lowercase cmp byte [ebp+ecx],7Ah ; Test input char against lowercase 'z' ja Next ; If above 'z' in ASCII, not lowercase ; At this point, we have a lowercase char sub byte [ebp+ecx],20h ; Subtract 20h to give uppercase... Next: dec ecx ; Decrement counter jnz Scan ; If characters remain, loop back ; Write the buffer full of processed text to stdout: Write: mov eax,4 ; Specify sys_write call mov ebx,1 ; Specify File Descriptor 1: Standard output mov ecx,Buff ; Pass offset of the buffer mov edx,esi ; Pass the # of bytes of data in the buffer int 80h ; Make kernel call jmp read ; Loop back and load another buffer full ; All done! Let's end this party: Done: mov eax,1 ; Code for Exit Syscall mov ebx,0 ; Return a code of zero int 80H ; Make kernel call </code></pre>
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      1. POhow to debug an NASM assembly program requiring user input?
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      1. USFlávio Toribio
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    1. COEasiest thing is probably to not redirect stdin/stdout - just type in some text to convert...
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      1. USFrank Kotler
    2. COConsidering that Insight is an old unsupported GDB frontend, I suggest you learn good old console GDB. It's the most powerful you'll get.
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      1. USLinuxios
    3. CO@Linuxios thank you, can you suggest how this can be done in gdb? ive never used it
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