instruction format

Understanding Program Execution in C

Overview

  • Objective: To understand how a simple addition program in C is executed from a computer organization perspective.

  • Key Components: Code writing, compilation, machine execution, interaction of processor, memory, and hardware.

Program Code Example

#include <stdio.h> 
int main() { 
 int a=5; 
 int b=3; 
 int sum = a + b; 
 printf("Sum: %d\n", sum); 
 return 0; 
}

Step-by-Step Execution

Writing and Compiling the Code

  • Source Code: Written in C, which is human-readable.

  • Compilation:

    • The C compiler (e.g., GCC) translates the code into assembly code, and then into machine code, generating an executable file (e.g., program.exe).

Linking

  • Library Linking: Necessary libraries (e.g., standard I/O) are linked, resulting in an executable binary for the CPU.

Loading the Program into Memory

  • OS Load: The OS loads the compiled program from storage into main memory (RAM).

  • Memory Allocation: Memory is allocated for program code, data (e.g., variables a, b, sum), and stack space.

  • Setting Program Counter (PC): The OS sets the PC to the starting point of the program (main function).

Fetching the First Instruction

  1. Instruction Fetch: CPU reads the first instruction from memory using the PC.

  2. Instruction Decode: CPU decodes instruction to determine the operation.

  3. Increment PC: PC is incremented to point to the next instruction.

Initializing Variables

Variable Initialization

  • Load Immediate: CPU fetches and decodes instructions to initialize a and b with values 5 and 3, loading them into registers (e.g., R1 for a, R2 for b).

  • Using MOV/LOAD: Instructions store these values into specific registers or memory addresses.

Performing the Addition Operation

  • Addition Instruction: Fetch and decode instruction for sum = a + b.

  • Execution of Addition:

    • The ALU adds the values in R1 (a) and R2 (b), storing the result in another register (e.g., R3) or memory.

Storing the Result in Memory

  • Store Operation: The result (stored in R3) is written to the memory allocated for variable sum, typically using a STORE instruction.

Executing the printf Function Call

Function Call Preparation

  • Setting Arguments: CPU prepares necessary arguments for printf, including format string "Sum: %d
    " and value of sum.

  • Stack Usage: Arguments are pushed onto the stack along with the return address.

Library Function Execution

  • Jump to printf: The CPU goes to the memory address of printf, retrieves arguments from the stack, performs output operation.

  • Return from Function: After completion, CPU uses the return address from the stack to continue execution in main.

Returning from main and Program Termination

  • Return Value: Prepares return value (0) for the OS, indicating successful termination.

  • Cleanup: The OS deallocates memory used by the program and releases resources.

  • End of Execution: CPU finishes executing the program.

Breakdown of Key Components Involved

  • CPU: Executes instructions, performs arithmetic operations, manages registers.

  • ALU (Arithmetic Logic Unit): Performs addition and calculations.

  • Registers: Hold variables (a, b, sum) and intermediate results.

  • Memory (RAM): Stores program code, variables, stack for function calls.

  • Stack: Stores function arguments and return addresses.

Summary of Execution Steps

  • Multiple stages involved: compilation, loading, execution, and termination.

  • Coordination among CPU, memory, and OS to complete the task.

Micro-Operations for Execution

Program Loading

  1. Fetch Instruction: PC holds address of the first instruction.

  2. Increment PC: After fetching, PC is incremented.

Initializing Variables a and b

  • Each assignment involves micro-operations:

    • Fetch instruction to load immediate values 5 and 3 into respective locations (a and b).

    • Execute: Load values into temporary registers, move values to respective memory locations.

Performing Addition: int sum = a + b;

  1. Fetch Instruction: Fetch the ADD instruction.

  2. Decode: Identifies operands (a, b) and destination (sum).

  3. Execute: Load values from registers, perform addition in ALU, store result.

Printing the Result

  1. Setup printf Arguments: Push format string and sum onto the stack.

  2. Function Call: Save return address, update PC to printf's address.

  3. Inside printf: Manage arguments, format output, interface with output device.

Cleanup After Execution

  • Clear function arguments from the stack upon return.

  • Deallocate stack frame for main and return control to the OS.

Instruction Formats

Types of Instruction Formats

  1. Zero-Address: Stack operations with implicit operands.

  2. One-Address: Accumulator-based architecture.

  3. Two-Address: General-purpose processors.

  4. Three-Address: Used in RISC and CISC architectures.

  5. Four-Address: Rarely used, allows complex operations.

Key Components of Instruction Formats

  • Opcode: Specifies operation to be performed (e.g., ADD).

  • Addressing Mode Field: Determines interpretation of address fields.

  • Register Fields: Specifies involved registers.

  • Mode/Flag Bits: Modifies instruction behavior.

Practice and Questions

Questions on Instruction Formats

  1. Evaluate given expressions using various address instruction formats (zero, one, two, three).

  2. Convert high-level expressions to respective instruction formats.

  3. Discuss advantages and challenges of various instruction formats.