Access Linux System Call in User Space

The operating system is responsible for

• Process Management (starting, running, stopping processes)
• File Management(creating, opening, closing, reading, writing, renaming files)
• Memory Management (allocating, deallocating memory)
• Other stuff (timing, scheduling, network management)

An application program makes a system call to get the operating system to perform a service for it, like reading from a file.

One nice thing about syscalls is that you don't have to link with a C library, so your executables can be much smaller.

# -----------------------------------------------------------------------------
# A 32-bit Linux standalone program that writes "Hello, World" to the console
# using system calls only.  The program does not need to link with any external
# libraries at all.
#
# System calls used:
#   4: write(fileid, bufferAddress, numberOfBytes)
#   1: exit(returnCode)
#
# Assemble:
#     gcc -c hello.s
# Link:
#     ld hello.o (to produce a.out)
#     (or) ld -o hello hello.o (to produce hello)
#
# Or, you can assemble and link in one step:
#     gcc -nostdlib hello.s
#
# The symbol _start is the default entry point for ld.
# -----------------------------------------------------------------------------

        .global _start

        .text
_start:
        # write(1, message, 13)
        mov     $4, %eax                # system call 4 is write
        mov     $1, %ebx                # file handle 1 is stdout
        mov     $message, %ecx          # address of string to output
        mov     $13, %edx               # number of bytes to write
        int     $0x80                   # invoke operating system code

        # exit(0)
        mov     $1, %eax                # system call 1 is exit
        xor     %ebx, %ebx              # we want return code 0
        int     $0x80                   # invoke operating system code
message:
        .ascii  "Hello, World\n"

.section .data
msg:
        .ascii "Hello world!\n"
len = . - msg

.section .text
.global _start

_start:
        /* syscall write(int fd, const void *buf, size_t count) */
        mov r0, #1
        ldr r1, =msg
        mov r2, #len
        mov r7, #4
        swi #0

        /* syscall exit(int status) */
        mov r0, #0
        mov r7, #1
        swi #0