Central Processing Unit(s) (CPU)

The Central Processing Unit (CPU) is often called “the brain” of the computer. It executes instructions fetched from memory. And while

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Core Components

1. Control Unit (CU): The conductor. It directs the flow of data and tells the ALU what to do.
2. Arithmetic Logic Unit (ALU): The calculator. Performs Math (ADD, SUB) and Logic (AND, XOR) operations. (See Digital Logic).
3. Registers: Fast, on-chip memory locations.
   • Program Counter (PC): Holds the memory address of the next instruction.
   • Instruction Register (IR): Holds the current instruction being executed.
   • Accumulator / General Purpose: Stores data for ALU operations.
   • Stack Pointer (SP): Points to the top of the Call Stack(s)

Context Switching

The CPU can only run one process at a time (per core). To multitask, the OS saves the current state of all registers to RAM (the Process Control Block) and loads the registers of the next process. This is a Context Switch.

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The Machine Cycle (Fetch-Decode-Execute)

1. Fetch: The CPU asks RAM for the instruction at the address in the PC.
2. Decode: The Control Unit figures out what the bytes mean (e.g., 0101 might mean ADD).
3. Execute: The signal is routed to the ALU or I/O to perform the action.
4. Store: Results are written back to a Register or RAM.

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Instruction Set Architecture (ISA)

The ISA is the interface between Hardware and Software. It defines the set of machine instructions a CPU understands.

• CISC (Complex Instruction Set): x86 Assembly (Intel/AMD). Many complex instructions.
• RISC (Reduced Instruction Set): ARM (Mobile/Mac), RISC-V. Fewer, simpler instructions.

From C to CPU: $\text{C Code}\stackrel{\text{Compiler (gcc)}}{\to }\text{Assembly (ISA)}\stackrel{\text{Assembler}}{\to }\text{Machine Code (Binary)}$

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