Introduction to Operating Systems

Operating System Overview

• An operating system (OS) is software that simplifies and abstracts the complexities of computer hardware for user programs.
• It manages hardware resources such as CPU, memory, and I/O devices effectively.

Kernel Mode vs. User Mode

• Kernel Mode:
  • OS operates in this mode with full access to hardware and can execute any machine instruction.
• User Mode:
  • Regular applications run here with restricted access to hardware and limited machine instructions.
  • User programs must issue system calls (TRAP) to request services from the OS.

OS as an Extended Machine

• Hardware complexity necessitates the use of abstractions, such as disk drivers, which offer simpler interfaces for tasks like reading and writing data.

OS as a Resource Manager

• The OS organizes and allocates resources (CPUs, memory, I/O devices) to various programs.
• Multiplexing:
  • Time Multiplexing: Processes take turns using resources (e.g., CPU usage).
  • Space Multiplexing: Resources are divided among users/processes (e.g., memory allocation).

Generations of Operating Systems

• 1st Generation (1945-1955):
  • Utilized vacuum tubes and plug boards; slow and large without an OS.
  • Notable systems: Colossus (Alan Turing), ENIAC (William Mauchley).
• 2nd Generation (1955-1965):
  • Shift to transistors and batch systems; first OS developed.
  • Key advancements: faster machines, core memory, use of magnetic tapes and disks.
• 3rd Generation (1965-1980):
  • Introduction of integrated circuits, multiprogramming, and spooling.
  • Examples include IBM System/360.
• 3rd Generation Continued (1965-1971):
  • Emphasized low power consumption and the usage of high-level languages.
• 3rd Generation Continued (1971-1980):
  • Advances with microprocessors, portable computing, and RAID storage technology.
• 4th Generation (1980-1990):
  • Birth of personal computers using LSI circuits; first microcomputer featured Intel 8080 CPU.
  • CP/M became the first disk-based OS, followed by IBM PC's development in the 1980s.
• 5th Generation (1990-Present):
  • Focus on mobile computers, reflecting modern computing trends.

Computer Hardware Relationships

• An OS is closely tied to the computer's hardware components, managing resources including processors, memory, disks, I/O devices, and buses.

CPU Operations

• The basic cycle of a CPU includes:
  • Fetch Instruction: Read next instruction.
  • Decode Instruction: Interpret the instruction and its operands.
  • Calculate Operands: Determine addresses of source operands.
  • Fetch Operands: Retrieve operands from memory as necessary.
  • Execute Instruction: Perform the specified operation.
  • Write Operand: Store the result back in memory.
• Registers within CPU:
  • General Registers: Hold temporary variables.
  • Program Counter (PC): Indicates next instruction location.
  • Stack Pointer: Points to the current stack in memory.
  • Program Status Word (PSW): Contains flags indicating the state of the processor.