CPU

CPU in today’s computers are microprocessors. A CPU:

• performs logical & arithmetic operations
• controls all the other components and subsystems

Components of a CPU

Arithmetic & Logical Unit

Arithmetic unit handles arithmetic operations. Logical unit handles logical operations.

Control Unit

Controls the operation of the CPU and rest of the machine. Handles the decoding of instructions.

Registers

A type of memory that can hold a unit of data. Can be used for both data processing and control functionalities.

Type of registers

Program Counter

Keeps track of memory address of the next instruction to be executed. Directly connected to ALU.

Instruction Register

After an instruction is fetched into the CPU, it is stored in IR for execution. Keeping IR closer to CU (in the scale of micrometers) enables faster execution speed.

Accumulator

Where result of arithmetic or logical operation is stored immediately.

Flag Register

Stores the status of the last operation carried out by ALU.

General Purpose Registers

Can be used to various tasks. Used to store immediate results of the ALU. Number of GPRs vary with the CPU. Usually denoted as “B”, “C” and so on.

Internal Structure

• There is a secondary ALU after PC. That increments the PC after fetching a new instruction.
• ALU accepts maximum 2 inputs. One input should come from accumulator.

Enhancing CPU Performance

Instruction pre-fetching

The process of fetching next instruction while current instruction is still executing. Reduces idle time of CPU.

Instruction pipelining

The process of dividing instruction execution cycle into multiple stages. Those multiple stages will be executed in parallel. Increases thoroughput of the microprocessor.

Hyper Threading

Allows different resources of the CPU to be used at the same time. CPU, BIOS, OS, and chipset have to support HT technology to use this.

Multicore processors

2 or more separate microprocessors, combined onto a single Silicon chip. Higher performance gain compared to HT.