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Introduction

  • The lecture discusses processes in computer science, specifically focusing on program execution and bootstrapping.

Instruction Execution

  • The execution of instructions occurs line by line.
  • The program counter is a critical component that regulates the next instruction to be executed.
  • Once the program counter is initialized with the first instruction, it increments to the next instruction continuously.

Key Components of Program Execution

  • Program Counter: It holds the address of the next instruction.
    • Initiates the execution process.
    • Increments automatically to fetch the subsequent instruction.

Bootstrapping

  • Definition: The process of starting the operating system when a computer is powered on or reset.
  • When the reset button is pressed, the operating system begins to execute.
    • The operating system is the main program that allows the execution of other applications.
  • The challenge lies in identifying the first instruction of the operating system and loading it into the CPU for execution.

Reset Signal

  • Activates the bootstrapping procedure.
  • Example of a common processor: 6800 Processor.
  • The reset signal directs the CPU to a preset memory address where the bootstrap program is located.

Vectors

  • Definition: A vector is an address held in memory that points to the start of the bootstrap program.
  • Examples of vector addresses: FFFE and FFFF.
  • Upon a reset signal (e.g., a 5V signal), the processor halts its current process, if any, and proceeds to execute instructions from these preset locations.

Execution of Bootstrap Program

  1. The CPU accesses the vector addresses where the bootstrap program is stored (FFFE and FFFF).
  2. Information contained in these memory locations is loaded into the program counter.
  3. The program counter is now set with the starting address of the operating system (e.g., 300F2) and the execution process begins.

Memory Retention

  • The memory space where the bootstrap program is stored must retain its data permanently even when the computer is switched off.
  • Typical RAM does not retain data; hence, this requires a special type of memory that is non-volatile.

Interrupts and Other Vectors

  • Beyond bootstrapping, computers have to handle various interrupts, such as:
    • Non-maskable interrupts
    • Interrupt requests (IRQ)
  • These interrupts prompt the processor to stop executing the current instructions and execute service routines placed at specific memory locations (e.g., FMSE and FFFD).

Flow Control in Programming

  • Once the bootstrap program is executing, program execution continues until it encounters specific instructions.
  • Key terms:
    • SWI Instruction: Software interrupt
    • WAI Instruction: Wait for a signal
  • The need to manage subroutines within programming is discussed, emphasizing how multiple calls to a subroutine can be handled.

Subroutines

  • Subroutines allow code reuse by encapsulating code that can be called from multiple points in the program.
  • When a subroutine is called, the program counter must remember where to return after the subroutine has completed its task.

Stack Mechanism

  • Definition: A stack is a First In First Out (FIFO) data structure that helps manage function calls and returns.
  • The stack pointer indicates the current top of the stack in memory.
  • Operations:
    • Push: Adds an element to the stack and moves the stack pointer downwards (e.g., 600 to 599).
    • Pull: Removes an element from the stack and moves the stack pointer upwards.
  • Last In First Out (LIFO): The last value added is the first to be removed.

Implementation in 6800 Processor

  • Every time a subroutine is called, the current address after the call (return address) is pushed to the stack.
  • The stack pointer then updates its position to reflect the new top of the stack.
  • When the subroutine completes, the return value is pulled from the stack and placed in the program counter to continue execution where it left off.

Illustrative Example of Usage

  • Example of control flow:
    • Main program calls subroutine (e.g., GCD).
    • When invoking GCD, the instruction to branch to the subroutine (BSR) also pushes the return address onto the stack.
    • Upon completion of the GCD subroutine, the RTS (Return from Subroutine) instruction pulls the top of the stack back into the program counter and resumes execution.
    • This mechanism enables multiple nested subroutine calls while keeping track of each call's return address.

Summary

  • Program counter and bootstrapping are foundational for starting operations in computer systems.
  • Stack structure serves as an effective memory management tool for handling subroutines ensuring the correct sequence of operation.
  • Understanding these mechanisms is crucial in programming and programming language implementations, especially in systems programming with specific processors like the 6800.