I have just started to learn assembly (for a hobby) and I have made this small program that gets a number from the user and prints it out:
section .data:
message: db "please enter a number: " ;define message to "please enter a number"
message_length equ $-message ;define message_length to $-message the lenght of the message
message_done: db "you have entered the number " ;define message_done to "you have entered the number "
message_done_length equ $-message ;define message_done_length to the $-message_done the length of message_done
section .bss:
num resb 5 ;num will store the input the user will give
section .text:
global _start
_start:
mov eax, 4 ;set the next syscall to write
mov ebx, 1 ;set the fd to stdout
mov ecx, message ;set the output to message
mov edx, message_length ;set edx to the length of the message
int 0x80 ;syscall
mov eax,3 ;set the next syscall to read
mov ebx, 2 ;set the fd to stdout
mov ecx, num ;set the output to be num
mov edx, 5 ;set edx to the length of the num
int 0x80 ;syscall
mov eax, 4 ;set the syscall to write
mov ebx, 1 ;set the fd to stout
mov ecx, message_done ;set the output to the message
mov edx, message_done_length ;set edx to the message_done_length
int 0x80 ;syscall
mov eax, 4 ;set the syscall to write
mov ebx ,1 ;set the fd to stdout
mov ecx, num ;set the output to num
mov edx, 5 ;the length of the message
int 0x80 ;syscall
mov eax, 1 ;set the syscall to exit
mov ebx, 0 ;retrun 0 for sucsess
int 0x80 ; syscall
When I run this and enter a number followed be enter, I get this:
please enter a number: you have entered the number ����
What am I doing wrong?
Three bugs:
section .data:
This assembles into a section called .data: which is different from .data. Drop the colon. A section directive is not a label. The same problem occurs in all your other section directives.
message_done: db "you have entered the number "
message_done_length equ $-message
Should be $-message_done. As it stands you are writing too many bytes. That is the likely cause of the garbage you see after your message.
mov eax,3 ;set the next syscall to read
mov ebx, 2 ;set the fd to stdout
mov ecx, num ;set the output to be num
mov edx, 5 ;set edx to the length of the num
int 0x80 ;syscall
You want the fd to be stdin (0), your comment says stdout, and file descriptor 2 is actually stderr. Make it mov ebx, 0. It will probably work as is when you run the program from a terminal, because then file descriptors 0, 1 and 2 are all open on the terminal and are all read-write, but it will misbehave if you ever use input redirection.