N_Asm Assembly system calls (sol)
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System calls allow programs to interface with the kernel and perform tasks like input/output. In assembly, system calls are made by putting the system call number in the EAX register, arguments in registers like EBX and ECX, and calling the interrupt 80h. Common system calls include sys_write to output to the screen and sys_exit to terminate a program. The example code demonstrates using sys_write to prompt for user input, sys_read to get input, and sys_write again to display it before exiting with sys_exit.
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N_Asm Assembly system calls (sol)
- 1. http://www.tutorialspoint.com/assembly_prog ramming /assembly_system_calls.htm Copyright © tutorialspoint.com ASSEMBLY - SYSTEM CALLS Systemcalls are APIs for the interface betweenuser space and kernelspace. We have already used the system calls sys_write and sys_exit for writing into the screenand exiting fromthe program, respectively. Linux System Calls Youcanmake use of Linux systemcalls inyour assembly programs. Youneed to take the following steps for using Linux systemcalls inyour program: Put the systemcallnumber inthe EAX register. Store the arguments to the systemcallinthe registers EBX, ECX, etc. Callthe relevant interrupt (80h). The result is usually returned inthe EAX register. There are six registers that store the arguments of the systemcallused. These are the EBX, ECX, EDX, ESI, EDI, and EBP. These registers take the consecutive arguments, starting withthe EBX register. If there are more thansix arguments, thenthe memory locationof the first argument is stored inthe EBX register. The following code snippet shows the use of the systemcallsys_exit: mov eax,1 ; system call number (sys_exit) int 0x80 ; call kernel The following code snippet shows the use of the systemcallsys_write: mov edx,4 ; message length mov ecx,msg ; message to write mov ebx,1 ; file descriptor (stdout) mov eax,4 ; system call number (sys_write) int 0x80 ; call kernel Allthe syscalls are listed in/usr/include/asm/unistd.h, together withtheir numbers (the value to put inEAX before youcallint 80h). The following table shows some of the systemcalls used inthis tutorial: %eax Name %ebx %ecx %edx %esx %edi 1 sys_exit int - - - - 2 sys_fork struct pt_regs - - - - 3 sys_read unsigned int char * size_t - - 4 sys_write unsigned int const char * size_t - - 5 sys_open const char * int int - - 6 sys_close unsigned int - - - - Example The following example reads a number fromthe keyboard and displays it onthe screen: section .data ;Data segment
- 2. userMsg db 'Please enter a number: ' ;Ask the user to enter a number lenUserMsg equ $-userMsg ;The length of the message dispMsg db 'You have entered: ' lenDispMsg equ $-dispMsg section .bss ;Uninitialized data num resb 5 section .text ;Code Segment global _start _start: ;User prompt mov eax, 4 mov ebx, 1 mov ecx, userMsg mov edx, lenUserMsg int 80h ;Read and store the user input mov eax, 3 mov ebx, 2 mov ecx, num mov edx, 5 ;5 bytes (numeric, 1 for sign) of that information int 80h ;Output the message 'The entered number is: ' mov eax, 4 mov ebx, 1 mov ecx, dispMsg mov edx, lenDispMsg int 80h ;Output the number entered mov eax, 4 mov ebx, 1 mov ecx, num mov edx, 5 int 80h ; Exit code mov eax, 1 mov ebx, 0 int 80h Whenthe above code is compiled and executed, it produces the following result: Please enter a number: 1234 You have entered:1234