System Calls

• Involves a context switch into kernel mode for execution
• Usually there is a system library entry point
  • Calling the system library is similar to a procedure call
• Process
  1. The user program calls the system library
     • Uses a normal procedure call (which is a normal function call)
  2. Inside the system library routine, the system call is actually made
     • Parameters and system call numbers are stored by the system library routine at specific spaces
  3. Traps to kernel mode
  4. Kernel dispatcher reads the SysCall number
     • Looks up vector table and makes internal call to code that carries out the system call
  5. System returns to library
  6. Library returns to user program