User Program and System Call

Terminologies : Kernel and Utility

1. Kernel : Operating System의 핵심적인 기능을 수행하는 코드 (Software)
   • Memory resident part of OS
   • Majority written in C / Rest in assembler language (HW dependent, for speed)
   • Consists of a set of functions
     • Provide fundamental mechanisms (process management, synchronization, CPU scheduling, memory management, device management, interrupt handling)
     • Provide services via a system call
2. Utility (Command) : Disk resident programs (loaded on demand)
3. Shell : A special utility for command control

 

Resource Protection

Dual-Mode Operation

• Need HW help to "prevent" illegal action.
• Provide hardware support to differentiate between at least two modes of operations

• Privileged instructions
  • I/O instructions
  • Instructions that manipulate memory management state
  • Setting of special mode bits
  • Halt instruction
• Mode transition by Interrupt / Trap / System Call
  • Mode bit : Kernel (0) or User (1) in PSW

 

CPU Protection

• CPU protection mechanism : prevent monopolizing CPU, supporting a timer (HW)
• Programmable Interval Timer (PIT)
  • Relative Time
  • A hardware device to drive an interrupt at a periodic rate
  • Almost kernel functions are time-driven
• Real Time Clock (RTC)
  • Absolute Time
  • A nonvolatile device to keep time when OS is not running
  • Managing the current time of day
• System Timer (Privileged)
  • Called PIT
  • Used in time sharing systems
  • Used to update the wall time
• Timer Interrupt Handler in Linux
  1. Update the jiffies (Relative Time; tics)
  2. Update the wall time, which is stored in xtime (Absolute Time)
  3. Update the process time
     • Update resource usages for the currently running process
     • Update time slice : check for quantum (time slice) expiration

 

Memory Protection

Memory Management Unit (MMU)

• Support the translation of logical (virtual) addresses to physical addresses
• Memory protection, cache control, bus arbitration

 

I/O Protection

• All I/O instructions are privileged instructions
• User cannot be allowed direct access to I/O

 

System Call

System Call

• Programming interface to OS services
• Allows applications to communicate with OS
• Purpose of system calls
  1. To protect system resources
  2. For reliability : buggy user program
  3. For security : malicious user program
• Important design principle
  • Policy : What will be done?
  • Mechanism : How to do it?

 

System Call Implementation

• User Programs
  1. Calls system call function in library
  2. Places arguments in registers and/or pushes tem onto user stack
  3. Places syscall number in a dedicated register
  4. Executes syscall machine instruction
• Kernel
  1. Executes syscall interrupt handler
  2. Executes dedicated kernel function in system call interface
  3. Place result in dedicated register
  4. Executes RETURN_FROM_INTERRUPT
• System Call Interface (Table)
  • A number is associated with each system call
  • A table indexed according to these numbers
  • Invokes intended system call

System Call Parameter Passing

• Pass the parameters in registers
• Pass the parameters stored in a block in memory (address of block)
• Pass the parameters pushed onto the stack by the program

 

Type of System Calls

• Process
• Scheduling
• Interprocess communication (IPC)
• File system
• Socket (networking)
• Miscellaneous (기타 등등)