Lecture 2: Computer Startup and Interrupts

1. Computer Startup

• Bootstrap Program: A small piece of code loaded at power-up or reboot.
  • Storage: Typically stored in ROM (Read-Only Memory) or Firmware (because it is non-volatile).
  • Function: Initializes all aspects of the system, loads the Operating System Kernel, and starts its execution.

2. Computer System Organization

• Components: One or more CPUs and Device Controllers connected through a common System Bus to access shared memory.
• Device Controllers: Each is in charge of a particular device type and has a Local Buffer for temporary data transfer.
• Concurrent Execution: CPUs and I/O devices operate simultaneously for maximum efficiency.

3. Interrupts

• Definition: A signal that temporarily halts the CPU's current operation so that an event (e.g., I/O completion) can be handled.
• OS is Interrupt-Driven: The OS reacts to events as they occur.
• Interrupt Handling Steps:
  1. CPU detects the interrupt signal.
  2. The OS preserves the state of the CPU (saving registers and the program counter).
  3. Control transfers to the Interrupt Service Routine (ISR) using the Interrupt Vector.
  4. ISR executes.
  5. CPU restores the saved state and resumes the interrupted process.
• Trap or Exception: A software-generated interrupt caused by an error (e.g., division by zero) or a user's explicit request (e.g., a system call).
• Interrupt Vector: A table containing the addresses of all Interrupt Service Routines (ISRs).

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