Arithmetic Logic Unit (ALU)

It is one of the main components of the Central Processing Unit. This is where all arithmetic and logic operations are done. Data received from an input device is stored in primary memory before being passed on to ALU for processing.
Memory
Memory is the location where data and instructions are stores. They can be retrieved from memory whenever required. Memory is used to:
1. Hold the data received from input device temporarily and ready it for processing.
2. Hold data that has been processed and the intermediate results generated within.
3. Hold the finished results of processed data, until released to output devices.
4. Hold the system software and application software in use.

Data is stored in memory as bytes. A byte is made up of eight bits. A bit is the smallest unit of memory. Other units of memory are Kilobytes(KB), Megabytes(MB), Gigabytes(GB) and Terra bytes (TB) where:

1 KB = 1024 bytes
1 MB = 1024 KB = 1024 * 1024 bytes
1 GB = 1024 MB
1 TB = 1024 GB

Memory can be classified into two groups:
1. Main Memory
2. Secondary Memory

Main Memory
Every computer comes with a certain amount of physical memory, usually referred to as the main memory of RAM( Random Access Memory)
There are different kinds of main memory:
RAM (Read/write or R/W). this type of memory is also known as random access memory. This is volatile, which means, when the power is turned off, all the contents are destroyed. This memory is again of two types: Static RAM and Dynamic RAM.
ROM (Read Only Memory). The ROM is non-volatile, which means, it retains the stored information even if the power is turned off.
ROM is again of four types:
Masked ROM, PROM (Programmable ROM), EPROM( Erasable Programmable ROM) and EEPROM(Electrically Erasable Programmable ROM).
Secondary Memory:
Secondary Memory includes storage devices like cassette tape, magnetic tape, floppy disk, hard dist, Winchester disk, etc.