# Components of the CPU ## What is the CPU? - The CPU (Central Processing Unit) is responsible for processing all data within the computer - It is made up of several key **components** working together: - Arithmetic and Logic Unit (ALU) - Control Unit (CU) - Registers

The CPU is the brain of the computer that processes all instructions and data

## Registers - Registers are **temporary storage/memory locations inside the CPU** used for a single specific purpose - They have a **faster access speed** than RAM or secondary storage - Think of registers like the pockets on a chef's apron - small spaces right at hand that hold exactly what's needed for the current task, rather than walking to the pantry (RAM) every time

Registers are the fastest form of memory in a computer, located directly within the CPU

### Key Registers in the CPU There are five main registers you need to know: 1. **Program Counter (PC)** 2. **Accumulator (ACC)** 3. **Memory Address Register (MAR)** 4. **Memory Data Register (MDR)** 5. **Current Instruction Register (CIR)** ### Register Functions | Register | Abbreviation | Function | |----------|--------------|----------| | **Arithmetic and Logic Unit** | ALU | Performs arithmetic calculations (e.g. adding binary numbers) and logic comparisons (using AND, OR, NOT). Made up of several internal components. | | **Control Unit** | CU | Decodes instructions and controls data flow within the CPU and how it moves around | | **Program Counter** | PC | Stores the memory address of the **next instruction** to be fetched | | **Accumulator** | ACC | Stores values temporarily - either after input/loading, or after calculations in the ALU | | **Memory Address Register** | MAR | Stores memory addresses - either where data is being sent to, or where it's being fetched from | | **Memory Data Register** | MDR | Stores data/instructions temporarily - either before sending to memory, or after fetching from memory | | **Current Instruction Register** | CIR | Holds the current instruction after fetching from memory, before splitting into opcode and operand for decoding |

MAR holds WHERE data lives, MDR holds WHAT the data actually is

## Components of the ALU The ALU is not a single unit - it's made up of several internal components: ### Arithmetic Circuit - Carries out arithmetic operations: - Addition - Subtraction - Multiplication - Division ### Logic Circuit - Carries out logical operations: - AND - OR - NOT - XOR ### Additional Registers - Extra registers beyond the five main ones mentioned above - Used to store intermediate data during calculations - For example, when adding two numbers, temporary storage might be needed for carry bits ### Status Flags - Special indicators that provide information about the results of operations - **Overflow flag**: Set when a value is too large to fit in the register - **Zero flag**: Set when the result of an operation equals zero (useful for comparisons and branching) - These flags help the CPU make decisions about what to do next

Overflow occurs when a calculation result exceeds the register's storage capacity

### Internal Buses - Used to transport data around the ALU and to other parts of the CPU - Allow different components within the ALU to communicate efficiently ## System Buses Three buses connect the **CPU** with **main memory (RAM)** to enable communication: 1. **Data Bus** 2. **Address Bus** 3. **Control Bus**

Buses are communication pathways that transfer data between components inside a computer

### Bus Functions and Operations | Bus | Purpose | Operation | |-----|---------|-----------| | **Data Bus** | Carries actual data being sent to/from the CPU and RAM | Read/Write | | **Address Bus** | Carries memory addresses being sent to/from the CPU and RAM | Read/Write | | **Control Bus** | Sends control signals to determine whether other buses are in read or write mode | Sends signals | ### How Buses Work Together - **Data Bus**: Imagine a delivery truck carrying packages (data) between the CPU warehouse and RAM warehouse - **Address Bus**: Like the GPS coordinates telling the truck exactly which house (memory location) to visit - **Control Bus**: The dispatcher giving instructions whether to "pick up" (read) or "drop off" (write) packages ### Additional Internal Buses - There are also buses **within the CPU** that transport data between different internal components - These allow the registers, ALU, and Control Unit to communicate with each other - For instance, data flows from the MDR to the CIR, or from the ACC to the ALU

The three system buses work together - address says WHERE, data says WHAT, control says HOW

## Summary Table: Complete CPU Components | Component | Location | Primary Function | Speed/Characteristics | |-----------|----------|------------------|----------------------| | ALU | Inside CPU | Calculations and logic | Very fast | | Control Unit | Inside CPU | Decode & control | Very fast | | PC | Inside CPU (Register) | Track next instruction | Fastest (register) | | ACC | Inside CPU (Register) | Temporary storage | Fastest (register) | | MAR | Inside CPU (Register) | Address storage | Fastest (register) | | MDR | Inside CPU (Register) | Data storage | Fastest (register) | | CIR | Inside CPU (Register) | Current instruction | Fastest (register) | | Data Bus | Between CPU & RAM | Data transfer | Fast | | Address Bus | Between CPU & RAM | Address transfer | Fast | | Control Bus | Between CPU & RAM | Control signals | Fast |