Applications OS interface¶

System calls¶

Request OS services

Example

Process control
File management
Device management
Information maintenance
Communications

OS interface to run a program
Request to kernel made via Software interrupt

Print hello world to stdout using System call in linux

#include <syscall.h>
#include <unistd.h>

int main() {
    syscall(SYS_write, 1, "hello world\n", 12);
    // Or do
    // write(STDOUT_FILENO, "hello world\n", 12);
    return 0;
}

Passing parameters¶

Methods

Pass parameters in registers

Print hello world using system call in x86 NASM

# Code Listing 2.3:
# An assembly-language “hello world” program with system calls

.global _start  

.text
_start:
# write(1, message, 13)

# write to stdout
mov $1, %rax                # system call 1 is write
mov $1, %rdi                # file handle 1 is stdout


# write message
mov $message, %rsi          # address of string to output

# the length of the message
mov $13, %rdx               # number of bytes

# do the system call
syscall                     # invoke OS to write to stdout
# the OS checks the values in the registers and decides what to do
# -> pass parameters in registers


# exit(0)
mov $60, %rax               # system call 60 is exit
xor %rdi, %rdi              # we want return code 0
syscall                     # invoke OS to exit

.data
message:
.ascii "Hello, world\n"

Store parameters in a table in memory
Push parameters onto the stack of the program, then OS will pop them of

API¶

Users program against API instead of system call
Maybe won't do system calls

Example

C library
Java API for JVM
POSIX API
Win32 API for windows