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Processes & Interrupts

COMP124 Lectures

The time-sharing mechanism depends crucially on the ability to interrupt execution at regular clock intervals.

There are many circumstances in which it is desirable to interrupt the normal flow of a program to react to special events:

  • User interrupt from the keyboard.
  • Attempt to execute an illegal instruction.
  • Completion of an I/O task.
  • Power on or off.

Interrupts

An interrupt is a form of automatic call of a sequence of instructions, brought about (usually) by an event occurring outside normal program execution.

Such a sequence of instructions is known as an interrupt service routine (ISR). Variants are:

  • Interrupt routine
  • Interrupt handler

Unlike an ordinary subroutine call, an interrupt can take place at any point in the execution sequence.

The address of each ISR is usually contained in an interrupt vector.

Servicing Interrupts

Certain interrupts (e.g. clock timeout) will not return control to the interrupted program. Instead, control is handed to another program (context switch).

If the programmer does not know when this will happen, how do we protect the register values of the original program?

  • Answer is for OS to save them all in a special data structure called the process control block (PCB).
    • The OS maintains a table of such descriptors, one for each process.
  • Register values can be reloaded when execution resumes at a later time.

Processes

A program is a representation of an algorithm in some programming language; i.e. it is static.

  • Source program (Java, assembly language, etc.)
  • Object program (binary machine code)

A process refers to the activity performed by a computer when executing a program; i.e. it is dynamic.

A process is created when a program or command is executed. However, the correspondence is not always one to one:

  • A single program may give rise to several processes.
  • Many programs may be executed within a single process.

OS Structure

Often consists of:

  • A central kernel:
    • Resides permanently in memory.
    • Performs low-level, frequently needed activity.
  • A set of processes:
    • May be created by kernel or to service user activity.

Processes interact with kernel via system calls:

  • Create process
  • Run program
  • Open file

The kernel (and some system processes) may operate in privileged mode (also called kernel mode or special state)

System Initialisation

  1. On power-up or reset, an interrupt causes computer to execute bootstrap program held in ROM.
  2. This code initialises system, runs diagnostic checks, sets up basic I/O.
  3. It loads OS kernel from boot partition of specified external disk drive, and passes control to it.
  4. Kernel performs further initialisation and creates various system processes.
  5. Further processes may be created because of user activity.

Command Interpreter

Often referred to as a shell. Accepts and runs commands specified by user and provides the user’s view of OS:

  • May be graphical:
    • Windows
  • May be textual:
    • bash
    • zsh
    • POSIX shell

Some commands built into shell, others loaded from separate executable files. Shell also has sophisticated control structures such as loops, if-statements and procedures.

The shell is just a normal user-level process.

System Calls

User processes are not allowed to inspect or alter kernel code, or to make calls directly to its subroutines.

System calls allow programs to request the kernel’s services:

  • Often implemented as software interrupt.
  • Many system calls are given a ‘wrapper’ to make them look like ordinary subroutines.
  • Together with other useful functions, they form an Application Programmer Interface (API)