OS Services

OS Services

1. Interaction

   • Users
     • Interact indirectly through collection of system programs that make up OS interface
       • GUI, CMD
   • Processes
     • Interact by making system calls into the OS proper (kernel)
       • Not directly

2. Services

   • Processes
     • Progam execution
     • I/O operations
     • FS Manipulation
     • Interprocess communication
   • OS (internal)
     • Error handling
     • Resource allocation
     • Accounting
     • Protection and security

3. System Calls

   • Programming interface to the services provided by the OS
     • Written in high level language
       • C/C++ (printf)
     • Accessed by programs via high-level API rather than direct system call
       • WHY?
         • Abstracts complexity
         • Need method to switch from user mode to kernel mode
         • System calls execute privileged kernel code
           • Shouldn't let user switch CPU to privileged mode
         • Controlled by the kernel
         • Allows for backwards compabilibilty
   • Trapping to the kernel
     • User process runs trap execution, which switches CPU to privileged mode and jump to kernel pre-defined address of system calls.
     • HOW?
       • User process calls system call wrapper function (printf) from C library
         • Wrapper function issues low-level trap instruction (in assembly) to switch from user mode to kernel mode
           • CPU specific instruction
           • Specific pre-defined location in kernel
           • Kernel checks system call is legal
           • Mode bits alternated
     • Problem that no call can directly be made from user space to specific function in kernel space.
       • Before issuing trap instruction, index is stored in well know location (CPU registers)
         • Once is kernel space, index used to look up desired kernel service function, which is called
           • Function call may have arguments, proceed only if arguments are valid

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