I am supposed to write a small assembly "program" that simply reads a file using a loop and outputs the contents to STDOUT.
Now, I made a program that can read the file and unfortunately only outputs the first line. I also can't figure out how to use the loop.
This is my very first time working with assembly and I do it because I have to, not because I want to.
Here's my code:
section .text
global _start ;must be declared for using gcc
_start: ;tell linker entry point
;open the file for reading
mov eax, 5
mov ebx, file_name
mov ecx, 0 ;for read only access
mov edx, 0777 ;read, write and execute by all
int 0x80
mov [fd_in], eax
;read from file
mov eax, 3
mov ebx, [fd_in]
mov ecx, info
mov edx, 26
int 0x80
; close the file
mov eax, 6
mov ebx, [fd_in]
; print the info
mov eax, 4
mov ebx, 1
mov ecx, info
mov edx, 26
int 0x80
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
file_name db './passwd'
section .bss
fd_out resb 1
fd_in resb 1
info resb 26
Related
I am new to NASM and i have a project from school.
For start i need to know how to read char by char from .txt
after a long research this is my code:
section .text
global _start
_start:
;open the file
mov eax, 5 ;sys call open
mov ebx, file_name ;file name
mov ecx, 0 ;read only
mov edx, 0777 ; exe by all
int 0x80
mov [fd_in], eax
;read
loop:
mov eax, 3 ;sys call read
mov ebx, [fd_in] ;file descriptor
mov ecx, buff
mov edx, 1
int 0x80
cmp eax, 0 ;cmp EOF
je eof
;print
mov eax, 4 ;sys call write
mov ebx, 1 ;std out
mov ecx, buff
mov edx, 1
int 0x80
jmp loop
eof:
mov eax, 1
int 0x80
section .data
file_name db 'hey.txt'
section .bss
fd_in resb 1
buff resb 1
but it only print endlessly to fist char in my .txt file.
BTW in the .txt file it says "Hey my name is blah".
i would love any help or suggestions.
thanks.
This question already has answers here:
In NASM labels next to each other in memory are printing both strings instead of first one
(1 answer)
How does $ work in NASM, exactly?
(2 answers)
Closed 4 years ago.
running the code below generates a file with Welcome to jj Shashwat as content. what i didn't get is why does it writes Shashwat at the end of the file, Shashwat is in a totally different variable. Any idea why does this happen?
section .text
global _start ;must be declared for using gcc
_start:
;create the file
mov eax, 8
mov ebx, file_name
mov ecx, 0777 ;read, write and execute by all
int 0x80 ;call kernel
mov [fd_out], eax
; close the file
mov eax, 6
mov ebx, [fd_out]
;open the file for reading
mov eax, 5
mov ebx, file_name
mov ecx, 2 ;for read/write access
mov edx, 0777 ;read, write and execute by all
int 0x80
mov [fd_out], eax
; write into the file
mov edx,len ;number of bytes
mov ecx, msg ;message to write
mov ebx, [fd_out] ;file descriptor
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
; close the file
mov eax, 6
mov ebx, [fd_out]
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
section .data
file_name db 'myfile.txt', 0
msg db 'Welcome to jj', 0
mgafter db ' Shashwat', 0
lntwo equ $-mgafter
len equ $-msg
section .bss
fd_out resb 1
fd_in resb 1
info resb 26
That's because you said len equ $-msg after defining both msg and msgafter, so len is set to the length of both msg and msgafter, making your write call write both strings. This is because len equ $-msg means “set len to be the difference between the current location ($) and the location of msg.”
To fix this, move the len equ $-msg line right after the definition of msg.
I'm writing an assembly program that would print even numbers between 0-9 using a loop. I encountered this problem, segmentation fault while running the code. I check other answers on the site but couldn't find an answer that satisfies my issue.
I suspect that the function nwLine might be the source of the problem.
;;this program prints even numbers from 0-8 using loop function
section .text
global _start
cr db 10
_start: ;tell linker entry point
mov ecx, 5
mov eax, '0'
evenLoop:
mov [evnum], eax ;add eax to evnum
mov eax, 4
mov ebx, 1
push ecx
mov ecx, evnum
mov edx, 1
int 80h
call nwLine
mov eax, [evnum]
sub eax, '1'
inc eax
add eax, '2'
pop ecx
loop evenLoop
nwLine: ;function to move pointer to next line
mov eax,4 ; System call number(sys_write)
mov ebx,1 ; File descriptor 1 - standard output
mov ecx, cr
mov edx, 1
int 80h ; Call the kernel
ret
mov eax,1 ;system call number (sys_exit)
int 80h ;call kernel
section .bss
evnum resb 1
if anyone knows how to solve the problem with the nwLine function, please tell me.
I try write follow code, use int 0x80 to print number 5 but it don't print anything.
segment .bss
num1 resb 1
section .text
global _start
_start:
mov dword [num1],5
add [num1],byte '0'
mov ecx, dword [num1]
mov eax, 4
mov ebx, 1
mov edx, 1
int 0x80
mov eax, 1
int 0x80
System call 4 expects ecx to contain a pointer to a NUL-terminated string.
So instead of mov ecx, dword [num1] you should use mov ecx, num1.
I have this piece of code in NASM (for linux) that supposed to open an existing file, read it and print the content on the screen, but does not work, can someone tell me what am I doing wrong?(hello.txt is the name of the file)
section .data
file db "./hello.txt", 0
len equ 1024
section .bss
buffer: resb 1024
section .text
global _start
_start:
mov ebx, [file] ; name of the file
mov eax, 5
mov ecx, 0
int 80h
mov eax, 3
mov ebx, eax
mov ecx, buffer
mov edx, len
int 80h
mov eax, 4
mov ebx, 1
mov ecx, buffer
mov edx, len
int 80h
mov eax, 6
int 80h
mov eax, 1
mov ebx, 0
int 80h
mov ebx, [file] ; name of the file
mov eax, 5
mov ecx, 0
int 80h
Is wrong. Loose the square brackets around file. You are passing the file name instead of a pointer to the filename.
mov ebx, file ; const char *filename
mov eax, 5
mov ecx, 0
int 80h
I see here a lot of mistakes, in order:
mov ebx, [file] ; name of the file
mov eax, 5
mov ecx, 0
int 80h
Here, as said, u must lose square brackets (because the function needs a pointer, not a value)
mov eax, 3
mov ebx, eax
mov ecx, buffer
mov edx, len
int 80h
Here, u must save the file descriptor from eax, before writing there value 3, else u just loose it
mov eax, 4
mov ebx, 1
mov ecx, buffer
mov edx, len
int 80h
Well. Here u using ebx register, so better way is to save file descriptor in memory. And for display, you take 1024 bytes from buffer, which is not correct. After reading from the file, the eax register will contain the number of characters read, so after reading from the file it will be better to store the value from the eax register in edx
mov eax, 6
int 80h
Again. U close the file, but ebx contains dirt, although it must contain a file descriptor
Correct code must look like this:
section .data
file db "text.txt",0 ;filename ends with '\0' byte
section .bss
descriptor resb 4 ;memory for storing descriptor
buffer resb 1024
len equ 1024
section .start
global _start
_start:
mov eax,5 ;open
mov ebx,file ;filename
mov ecx,0 ;read only
int 80h ;open filename for read only
mov [descriptor],eax ;storing the descriptor
mov eax,3 ;read from file
mov ebx,[descriptor] ;your file descriptor
mov ecx,buffer ;read to buffer
mov edx,len ;read len bytes
int 80h ;read len bytes to buffer from file
mov edx,eax ;storing count of readed bytes to edx
mov eax,4 ;write to file
mov ebx,1 ;terminal
mov ecx,buffer ;from buffer
int 80h ;write to terminal all readed bytes from buffer
mov eax,6 ;close file
mov ebx,[descriptor] ;your file descriptor
int 80h ;close your file
mov eax,1
mov ebx,0
int 80h
This is not a perfect code, but it should work