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Carries out instructions and tells the rest of the computer system what to do. This is done by the
What does the CPU do?
Control Unit of the CPU which sends command signals to the other components of the system.
Performs arithmetic calculations and data manipulation, eg. comparisons, sorting, combining, etc. The computer's calculator is a part of the CPU known as the Arithmetic Logic Unit.
Holds data and instructions which are in current use. These are kept in the Main Store or Memory.
To understand how the whole system works, consider the diagram shown below. This diagram shows the basic components of a generalised CPU. An actual CPU may have these components, or other with different names that provide the same functions.
The control unit directs the entire computer system to carry out stored program instructions. The control unit must communicate with both the arithmetic logic unit and main memory.
The control unit uses the instruction contained in the Instruction Register to decide which circuits need to be activated.
The control unit co-ordinates the activities of the other two units as well as all peripheral and auxiliary storage devices linked to the computer.
The control unit instructs the arithmetic logic unit which arithmetic operations or logical operation is to be performed.
The control unit is literally in control.
Arithmetic Logic Unit
The arithmetic logic unit executes arithmetic and logical operations.
Arithmetic operations include addition, subtration, multiplication and division.
Logical operations compare numbers, letters and special characters.
Comparison operations test for three conditions:
equal-to condition in which two values are the same less-than condition in which one value is smaller than the other greater-than condition in which one value is larger than the other
Relational operations (=, <, >) are used to describe the comparison operations used by the arithmetic logic unit.
The arithmetic logic unit performs logic functions such as AND, OR and NOT.
If the Central Processing Unit needs to store the results of calculations it can store them in RAM.
Random Access Memory can have instructions READ from it by the CPU and also it can have numbers or other computer data WRITTEN to it by the CPU.
The more RAM in your computer, the larger the programs you can run.
When we switch a computer off, whatever is stored in the RAM gets erased.
The following is a photo of a common RAM chip.
The following is a diagram showing the relationship between the
ROM
ROM stands for Read Only Memory.
The CPU can only fetch or read instructions from Read Only Memory (or ROM). ROM comes with instructions permanently stored inside and these instructions cannot be over- written by the computer's CPU.
ROM memory is used for storing special sets of instructions which the computer needs when it starts up.
When we switch the computer off, the contents of the ROM does not become erased but remains stored permanently. Therefore it is non-volatile.
Central Processing Unit and the Main Memory (RAM and ROM).
Other Types of ROM.
PROM (acronym for Programmable Read-Only Memory) In computing, a memory device in the form of an integrated circuits (chip) that can be programmed after manufacture to hold information permanently. PROM chips are empty of information when manufactured, unlike ROM (read-only memory) chips, which has information built into them. Other memory devices are EPROM (erasable programmable read-only memory) and RAM (random-access memory).
Cache Memory
EPROM (acronym for Erasable Programmable Read-Only Memory) Computer memory device in the form of an integrated circuit (chip) that can record data and retain it indefinitely. The data can be erased by exposure to ultraviolet light, and new data recorded. Other kinds of computer memory chips are ROM (read-only memory), PROM (programmable read-only memory), and RAM (random-access memory).
In computing, a reserved area of the immediate access memory used to increase the running speed of a computer program.
The cache memory may be constructed from SRAM , which is faster but more expensive than the normal DRAM. Most programs access the same instructions or data repeatedly. If these frequently used instructions and data are stored in a fast-access SRAM memory cache, the program will run more quickly. In other cases, the memory cache is normal DRAM, but is used to store frequently used instructions and data that would normally be accessed from backing storage. Access to DRAM is faster than access to backing storage so, again, the program runs more quickly. This type of cache memory is often called a disk cache.
