Central Processing Unit

1. Introduction

The Central Processing Unit (CPU) is the brain of the computer, responsible for executing instructions from programs. It processes data and manages the flow of information within the computer.

2. General Register Organization

In a CPU, the general register organization refers to the arrangement and management of registers, which are small storage locations used for holding data temporarily.

2.1 Control Word

The Control Word is a binary code that specifies the operation to be performed by the CPU. It controls the actions of various components during execution.

Example of Control Word:

• A control word might specify an operation like ADD or SUB, along with the source and destination registers.

2.2 Examples of Micro-Operations

Micro-operations are basic operations on the data stored in registers. Examples include:

• Transfer: Moving data from one register to another.
  • Example: R1 <- R2 (Transfer data from register R2 to R1).
• Arithmetic Operations: Performing addition, subtraction, etc.
  • Example: R1 <- R2 + R3 (Add contents of R2 and R3, store in R1).

3. Stack Organization

Stacks are data structures that operate on a Last-In-First-Out (LIFO) principle. In CPU architecture, there are two main types of stacks:

3.1 Register Stack

The Register Stack utilizes CPU registers to store data temporarily. It is faster because it operates within the CPU.

Example:

• Push operation: PUSH R1 adds R1 to the stack.
• Pop operation: POP R1 removes the top element of the stack into R1.

3.2 Memory Stack

The Memory Stack uses memory to hold data. It is slower compared to the register stack but can handle larger data sets.

Example:

• In the memory stack, the operations would be similar but involve memory addresses.

3.3 Reverse Polish Notation

Reverse Polish Notation (RPN) is a mathematical notation in which operators follow their operands. For example:

• The expression ( (3 + 4) ) is written as ( 3 4 + ).

Example of Evaluating RPN:

1. Push 3 onto the stack.
2. Push 4 onto the stack.
3. Pop 3 and 4, perform addition, and push the result 7 onto the stack.

3.4 Evaluation of Arithmetic Expressions

To evaluate expressions using a stack:

1. Push operands onto the stack.
2. Pop the operands when an operator is encountered.
3. Apply the operator and push the result back onto the stack.

4. Instruction Formats

Instruction formats define the structure of instructions in a CPU. Common types include:

4.1 Three-Address Instructions

These instructions specify three operands, usually for complex operations. For example:

ADD R1, R2, R3

This adds the values in registers R2 and R3 and stores the result in R1.

4.2 Two-Address Instructions

These use two operands. The result is stored in one of the operands:

ADD R1, R2

Here, R1 is updated with the sum of R1 and R2.

4.3 One-Address Instructions

These instructions typically have one address and implicitly use a register. For example:

INCR R1

This increments the value in R1.

5. Addressing Modes

Addressing modes determine how the operand of an instruction is accessed. Common modes include:

5.1 Immediate Addressing

The operand is specified directly in the instruction.

MOV R1, #5  ; Load immediate value 5 into R1

5.2 Direct Addressing

The address of the operand is given explicitly.

MOV R1, 1000  ; Move the value at memory address 1000 into R1

5.3 Indirect Addressing

The address of the operand is held in a register or memory location.

MOV R1, (R2)  ; Move the value pointed to by R2 into R1

6. Data Transfer and Manipulation

6.1 Data Transfer Instructions

These instructions move data from one location to another, such as from memory to a register.

MOV R1, R2  ; Transfer data from R2 to R1

6.2 Data Manipulation Instructions

These perform operations on data, including logical and arithmetic operations.

6.3 Arithmetic Instructions

These are used for basic arithmetic operations, such as addition, subtraction, multiplication, and division.

ADD R1, R2  ; Add the contents of R1 and R2

6.4 Logical and Bit Manipulation Instructions

These operations manipulate individual bits within a word or byte, such as AND, OR, NOT, and XOR.

AND R1, R2  ; Logical AND of R1 and R2

6.5 Shift Instructions

Shift instructions move bits left or right within a register, often used for multiplication or division by powers of two.

SHL R1, 1  ; Shift R1 left by 1 (multiply by 2)

7. Program Control

7.1 Status Bit Conditions

The CPU uses status bits (also known as flags) to indicate the status of the operations (e.g., zero, carry, overflow).

7.2 Conditional Branch Instructions

These instructions change the flow of control based on certain conditions. For example, an instruction may jump to a different part of the program if a specific condition is met.

JZ LABEL  ; Jump to LABEL if the zero flag is set

7.3 Program Interrupt

Program interrupts allow the CPU to respond to asynchronous events, such as I/O operations or errors. This mechanism temporarily halts the current process and transfers control to an interrupt handler.

7.4 Types of Interrupts

Interrupts can be categorized into:

• Hardware Interrupts: Generated by hardware devices (e.g., keyboard input).
• Software Interrupts: Triggered by programs (e.g., system calls).