Computer Architecture: Stacks, Instructions, and Cycles

Posted on Dec 1, 2024 in Computers

Stack Organization in Computer Architecture

Stack organization refers to a structure where instructions and data are processed using a last-in, first-out (LIFO) principle. It’s crucial for managing data during subroutine execution and complex operations in the CPU. Here’s a breakdown:

What is a Stack?

A stack is a collection of elements with two primary operations:

  • Push: Adds an element to the top.
  • Pop: Removes the top element.

The top of the stack is where the latest data is stored and retrieved first.

Stack Pointer (SP)

The Stack Pointer (SP) is a special register tracking the top element’s address.

  • When a value is pushed, SP is decremented (downward-growing stack) or incremented (upward-growing stack).
  • When a value is popped, SP adjusts to point to the new top element.

Types of Stack Organization

  • Register Stack: Uses a small set of CPU registers.
  • Memory Stack: Uses a section of main memory for larger capacity but slower access.

Applications of Stack Organization

  • Function Calls: Stores return addresses and local variables.
  • Expression Evaluation: Supports postfix expressions and recursive functions.
  • Interrupt Handling: Saves the CPU state to be restored after handling the interrupt.

Stack Instructions

  • PUSH: Stores data on the top.
  • POP: Removes data from the top.
  • CALL: Pushes the return address for function calls.
  • RET: Pops the return address to return from a function.

Advantages of Stack Organization

  • Simplifies memory management for function calls and interrupts.
  • Enables easy recursion implementation.
  • Efficient for evaluating expressions, especially in postfix notation.

Programming Language Levels

  • Machine Language:

    • Binary code (0s and 1s), directly executed by the CPU.
    • Difficult to read; hardware-specific; fastest execution.
    • Manual memory management; used for hardware control.

  • Assembly Language:

    • Human-readable mnemonics (e.g., MOV, ADD).
    • More readable than machine language; processor-specific.
    • Faster than high-level languages; manual memory management.
    • Used for system programming and device drivers.

  • High-Level Language:

    • English-like syntax (e.g., Python, Java).
    • Readable and portable; slower execution due to abstraction.
    • Automatic memory management; used for applications and web development.

Instruction Types

Memory Reference Instructions

These instructions involve memory locations for read/write operations.

Common operations:

  • Load: Transfers data from memory to a register.
  • Store: Transfers data from a register to memory.
  • Add (Memory Operand): Adds a memory value to a register.
  • Subtract (Memory Operand): Subtracts a memory value from a register.
  • Compare: Compares a memory value with a register value.

Example:

  • LOAD R1, 1000 – Load from address 1000 into register R1.
  • STORE R1, 2000 – Store R1 into address 2000.

Register Reference Instructions

These instructions operate only on CPU registers.

Common operations:

  • MOV: Move data between registers.
  • ADD, SUB: Arithmetic operations.
  • AND, OR, XOR: Logical operations.
  • Increment (INC): Increment register value.
  • Decrement (DEC): Decrement register value.
  • NOP: No operation.

Example:

  • MOV R1, R2 – Move R2 to R1.
  • ADD R1, R2 – Add R2 to R1.

Input/Output Instructions

These instructions interact with external devices.

Common operations:

  • IN: Read data from an input device into a register.
  • OUT: Write data from a register to an output device.
  • Halt: Stops execution and may trigger I/O completion.

Example:

  • IN R1 – Input data into R1.
  • OUT R1 – Output data from R1.

Execution Cycles

Instruction Cycle

The instruction cycle is the process of fetching, decoding, and executing instructions:

  1. Fetch: Retrieve the instruction.
  2. Decode: Understand the operation.
  3. Execute: Perform the operation.

This cycle repeats for each instruction.

Interrupt Cycle

The interrupt cycle handles interrupts:

  1. Interrupt Request: An event sends an interrupt signal.
  2. Interrupt Acknowledgment: The CPU acknowledges.
  3. Save Context: The CPU state is saved.
  4. Execute Interrupt Service Routine (ISR): The ISR handles the interrupt.
  5. Restore Context: The saved state is restored.
  6. Return from Interrupt: Normal processing resumes.

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