ICT - Notes - Operating System, Lecture notes of Information and Communications Technology (ICT)

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Operating System Basics

Introduction to Operating Systems

Definition

• (^) An operating system is an intermediary between a computer user and the hardware.
• (^) Make the hardware convenient to use.
• (^) Manages system resources.
• (^) Use the hardware in an efficient manner.

Types of Systems

• (^) Batch
  • (^) submit large number of jobs at one time
  • (^) system decides what to run and when
• (^) Time Sharing
  • (^) multiple users connected to single machine
  • (^) few processors, many terminals
• (^) Single User Interactive
  • (^) one user, one machine
  • (^) traditional personal computer

Types of Systems

• (^) Parrallel
  • (^) traditional multiprocessor system
  • (^) higher throughput and better fault tolerance
• (^) Distributed
  • (^) networked computers
• (^) Real Time
  • (^) very strict response time requirements
  • (^) hardware or software

Multitasking System

• (^) Very complex
• (^) Serious security issues
  • (^) how to protect one program from another sharing the same memory
• (^) Much higher utilization of system resources
• (^) Example: Unix, Windows NT

Hardware Basics

• (^) OS and hardware closely tied together
• (^) Many useful hardware features have been invented to compliment the OS
• (^) Basic hardware resources
  • (^) CPU
  • (^) Memory
  • (^) Disk
  • (^) I/O

Memory

• (^) Limitted in capacity
  • (^) never enough memory
• (^) Temporary (volatile) storage
• (^) Electronic storage
  • (^) fast, random access
• (^) Any program to run on the CPU must be in memory

Disk

• (^) Virtually infinite capacity
• (^) Permanent storage
• (^) Orders of magnitude slower than memory
  • (^) mechanical device
  • (^) millions of CPU instructions can execute in the time it takes to access a single piece of data on disk
• (^) All data is accessed in blocks
  • (^) usually 512 bytes

Protection and Security

• (^) OS must protect itself from users
  • (^) reserved memory only accessible by OS
  • (^) hardware enforced
• (^) OS may protect users from one another
  • (^) not all systems do this
  • (^) hardware enforced again

Protection and Security

• (^) Dual -Mode Operation
  • (^) user mode
    • (^) limited set of hardware instr and memory available
    • (^) mode all user programs run in
  • (^) supervisory mode
    • (^) all hardware instr and memory are available
    • (^) mode the OS runs in
• (^) Never let user run in supervisory mode

Interrupt Philosophy

• (^) One way to handle interrupts is with one standard program - (^) big case-switch statement that gets executed on any interrupt - (^) inefficient
• (^) Second alternative is to use an interrupt table and special hardware - (^) this is the way modern systems operate

Interrupt Table

• (^) Large array indicating what code to run for a given interrupt
• (^) Each interrupt has a corresponding number associated with it - (^) on Intel processors this is from 0 to 255 - (^) this gives fixed size interrupt table
• (^) Use the interrupt number to index into the array to find out what code to run

System Startup

• (^) On power up
  • (^) Everything in system is in random, unpredictable state
  • (^) Special hardware circuit raises RESET pin of CPU
    • (^) Sets the program counter to 0xfffffff
      • (^) This address is mapped to ROM (Read-Only Memory)
• (^) BIOS (Basic Input/Output Stream)
  • (^) Set of programs stored in ROM
  • (^) Some OS’s use only these programs
    • (^) MS DOS
  • (^) Many modern systems use these programs to load other system programs - (^) Windows, Unix, Linux

BIOS

• (^) General operations performed by BIOS
  1. Find and test hardware devices
     • POST (Power-On Self-Test)
  2. Initialize hardware devices
     • Creates a table of installed devices
  3. Find boot sector
     • May be on floppy, hard drive, or CD-ROM
  4. Load boot sector into memory location 0x00007c
  5. Sets the program counter to 0x00007c
     • Starts executing code at that address