Instruction Codes

1. Stored Program Organization

In a Stored Program Organization, both instructions and data are stored in the computer memory. The instructions are fetched from memory and executed sequentially or based on control logic.

• Example: In a program that adds two numbers, the instructions like "load", "add", and "store" are all fetched from memory and executed by the CPU.

2. Indirect Addressing

In Indirect Addressing, the address field of the instruction contains a memory location where the effective address is stored. The CPU fetches the address indirectly through another address.

• Example: If instruction at address A points to another address B, and B contains the actual data, then data is fetched indirectly.

3. Computer Registers

3.1 Common Bus System

The Common Bus is used for transferring data between registers. All registers are connected to this common bus, and the control unit directs the data flow.

• Example: Data from Register A is transferred to Register C via the common bus.

4. Computer Instructions

4.1 Instruction Set Completeness

An instruction set is considered complete if it can perform operations on data in any required manner. It includes data transfer, arithmetic, logical, and control instructions.

5. Timing and Control

The Timing and Control Unit generates signals for the proper timing of fetching and executing instructions. It ensures that the right operations are performed at the correct time.

6. Instruction Cycle

The Instruction Cycle is the process by which a computer retrieves an instruction from memory, decodes it, and executes it. This cycle repeats until the program is completed.

6.1 Fetch and Decode

• Fetch: The CPU fetches the next instruction from memory.
• Decode: The instruction is decoded to determine the operation to be performed.

6.2 Types of Instructions

• Data Transfer Instructions: Move data between registers or between memory and registers.
• Arithmetic Instructions: Perform arithmetic operations such as addition, subtraction.
• Logical Instructions: Perform bitwise logical operations.
• Control Instructions: Control the flow of the program (e.g., jump, call, return).

7. Register-Reference Instructions

Register-Reference Instructions operate directly on registers. These instructions typically perform operations like clearing, complementing, or shifting the contents of registers.

• Example: CLA (Clear Accumulator) instruction clears the content of the accumulator register.

8. Microprogrammed Control

8.1 Control Memory

In Microprogrammed Control, a sequence of micro-operations is stored in control memory. The control memory holds the micro-instructions that are fetched and executed to carry out the machine instruction.

8.2 Address Sequencing

Address Sequencing involves determining the sequence of addresses for fetching instructions during the execution cycle. The sequence can vary based on the type of instruction and control signals.

9. Addressing Modes

9.1 Conditional Branching

Conditional branching allows the program to branch to a different memory location if a specified condition is met.

9.2 Subroutine Call

A Subroutine is a block of code that performs a specific task and can be called by other parts of the program.

10. Mapping of Instructions

The Mapping of Instructions refers to converting a high-level instruction into a corresponding set of machine instructions that can be executed by the computer.

• Example: A high-level if statement could be mapped to a conditional branch instruction in machine code.