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The CPU also known as the processor is mounted on the system board (mother board) inside the system unit. It’s regarded as the brain of the computer because it does all the processing activities in the computer. The figures below shows the top and bottom view of a the CPU respectively.

Processing devices
Top View of CPU


 Parts of the CPU

The CPU consists of three main elements as shown in the diagram below.

Processing Devices
Parts of the CPU

The CPU Speed, Links, History, Speed and Types

- The speed of the CPU is measured in HERTZ (HZ). i.e., number of operations that can be done per second. Heat is released when CPUs run. High temperature, however, decrease the performance of CPU. You, therefore, need a fan to cool down the CPU. You can change the speed of CPU in BIOS of your computer. Some people change the speed of their CPU to a very high MHz. This can increase the speed of the computer but would make the CPU overload and overheat. Finally, the whole computer may break down. Therefore, it is not recommend doing so.
-Links/ Pathways- the ALU, the main memory and the control unit use electrical pathways/links called buses to communicate to one another.

These buses include:
  • Ø  Control bus- it’s a pathway for all timing and controlling functions sent by the control unit to other parts of the system.
  • Ø  Address bus- it’s a pathway used to locate the storage position in memory where the next instruction data to be processed is held
  • Ø  Data bus- it’s a pathway where the actual data transfer takes place.

History of the CPU

-Types of the CPU
There aren't really different types of CPU, but there are some major differences between CPUs. Like Bus Sizes - we have 32 & 64 bits. But there are different processor architectures namely:

  • Ø  CISC: Complex Instruction Set Computers
  • Ø  RISC: Reduced instruction Set Computers
  • Ø  MISC: Minimal Instruction Set Computers
CISC (Complex Instruction Set Computers)
This is a computer where single instructions can execute several low-level operations. Examples of CISC instruction set architectures are System/360 through z/Architecture, PDP-11, VAX, Motorola 68k, and x86.
RISC (Reduced instruction Set Computers)
This is a CPU design strategy that uses fewer instructions set compared to CISC. Well known RISC families include DEC Alpha, AMD 29k, ARC, ARM, Atmel AVR, Blackfin, MIPS, PA-RISC, Power (including Power-PC), Super H, and SPARC.
MISC (Minimal Instruction Set Computers)
            This is processor architecture with a very small number of basic operations and corresponding op codes.
Probably the most commercially successful MISC was the IN MOS transputer.

Control Unit

Co-ordinate all the processing activities in the CPU as well as input, storage and output operations. The CPU uses a system clock to coordinate these activities. The system clock also determines the speed of the CPU. The higher the number of cycles/pulses per second which is also called frequency the faster the CPU.

Arithmetic and logic unit

this is a unit in the CPU where all the logical and arithmetic operations are carried out. It does this by decoding instructions from the control unit to processes data. Some of the arithmetic operations include: addition, subtraction, division, multiplication etc. logical operations include: less than, equal to, greater than, sorting, filtering etc

Main memory

The main memory is also called primary storage. It stores data that is directly accessible by the CPU. The main memory of a computer can be classified as RAM (random access memory) and ROM (read only memory)


Read only memory (ROM)

This is a class of storage medium used in computers and other electronic devices.
Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty

momoery storage devices
Read Only Memory

It is mainly used to distribute firmware (software that is very closely tied to specific hardware and unlikely to need frequent updates)

 Types of ROM

read only memory
Types of RAM

Random Access Memory (RAM)

memory storage devices
Random Access Memory
RAM (random access memory) is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be quickly reached by the computer's processor. RAM is much faster to read from and write to than the other kinds of storage in a computer, the hard disk, floppy disk, and CD-ROM.
However, the data in RAM stays there only as long as your computer is running. When you turn the computer off, RAM loses its data. When you turn your computer on again, your operating system and other files are once again loaded into RAM, usually from your hard disk

Type of RAM

  •  Static RAM (SRAM) - SRAM (static RAM) is random access memory (RAM) that retains data bits in its memory as long as power is being supplied. Static RAM provides faster access to data and is more expensive than DRAM. SRAM is used for a computer's cache memory and as part of the random access memory digital-to-analog converter on a video card.
  • Dynamic RAM (DRAM) - Dynamic random access memory (DRAM) is the most common kind of random access memory (RAM) for personal computers and workstations. It needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. DRAM stores each bit in a storage cell consisting of a capacitor and a transistor. Capacitors tend to lose their charge rather quickly; thus, the need for recharging.

 Static RAM vs. Dynamic RAM

memory storage devices
Static vs Dynamic RAM


memory storage devices

Special purpose memories

These are types of memories housed in the CPU, system board, input and output devices to enhance performance.

            Types of special purpose memories:

-          Cache memory- A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations. As the microprocessor processes data, it looks first in the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the more time-consuming reading of data from larger memory. Cache memory is sometimes described in levels of closeness and accessibility to the microprocessor. An L1 cache is on the same chip as the microprocessor.

-          Buffers- a buffer is a region of physical memory storage used to temporarily hold data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such as a mouse) or just before it is sent to an output device (such as speakers). However, a buffer may be used when moving data between processes within a computer. Like a cache, a buffer is a "midpoint holding place" but exist not so much to accelerate the speed of an activity as to support the coordination of separate activities.

-          Registers- a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are (typically) addressed by mechanisms other than main memory and can be accessed more quickly. Almost all computers, load-store architecture or not, load data from a larger memory into registers where it is used for arithmetic, manipulated, or tested, by some machine instruction. Manipulated data is then often stored back in main memory, either by the same instruction or a subsequent one.

Types of registers 

special memories
Types of Registers

 Memory capacities

  The SI unit for measuring memory and storage capacity is called bytes.  A byte is a single character which can either be from 0-9, a-z, symbols and spaces.
Memory quantities can be expressed in:

Byte                             approx             8-bits

Kilobyte (KB)             approx             1000 bytes

Megabyte (MB)         approx             106 bytes

Gigabyte (GB)            approx             109 bytes

Terabytes                    approx             1012 bytes

Hierarchy of storage

memory storage devices
Types of computer storage

-          Primary storage (or main memory or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU. Examples include: RAM, ROM, Cache memory, Registers and buffers.

-          Secondary storage (also known as external memory or auxiliary storage), these memory permanently store data for future reference. Secondary storage does not lose the data when the device is powered down—it is non-volatile. Example: Hard disk, Floppy disk.

-          Off- line storage- In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Off-line storage is computer data storage on a medium or a device that is not under the control of a processing unit. The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. Example: CD, DVD, Tapes, Floppy disk, Zip and Jazz disks etc.