Processing
A computer is a general-purpose machine that processes data according to a specific set of instructions. The instructions the computer uses are either stored permanently (in read-only memory, or 'ROM') or temporarily (in random-access memory, or 'RAM'). The computer and the equipment attached to it ('peripherals') are called hardware. The instructions the computer receives are called software. A set of instructions grouped together to perform a certain task is called a program.

Input and Output
The computer receives input through the keyboard or another input device. This input is transferred into memory (RAM), and then processed by calculating, comparing, or copying it. The computer outputs the results of the processing, usually on the screen, onto a disk, or over a communications channel.

Computer Architecture
Abstractly, the essential elements of a computer comprise:-
• a central processing unit (CPU);
• memory;
• input and output devices.
Almost all personal computers, workstations, minicomputers, and mainframes are based on the von Neumann design principle:
1. a single central processor;
2. a single path between the central processor and memory;
3. program is stored in memory; and
4. central processor fetches, decodes, and executes the stored instructions of the program sequentially.

A program is input into memory, and the CPU executes the instructions in the program, inputting data as needed, and outputting the results.
The CPU is the key part of a computer because its operational speed predominantly determines the speed of the computer as a whole. The CPU and fast memories are implemented with transistor circuits.
The memory stores programs and the initial, intermediate, and final computational data.
Programs are often activated and operated in a graphical user interface using the mouse to click on buttons or pull down menus.

Data Representation and Memory
Digital computers operate on data represented as binary numbers, such as 0, 1, 10, 11, 100, 101, 110, 111, ... (0, 1, 2, 3, 4, 5, 6, 7, ...). All data, whether in the form of magnitudes, text, pictures, sound, symbols, or calculus (such as integration and differentiation), are represented as binary numbers in a digital computer. Any type of data can be processed by addition, multiplication, comparison, or other simple operations by the ALU.
A "1" in binary is evaluated as an ON switch in the electronics. A "0" in binary is evaluated as OFF switch in the electronics. This value can also represent another meaning: a "1" in binary is TRUE and "0" is FALSE. Through these values you get binary logic, which allows a computer to actually "compute".
In a binary number with p positions, each position is occupied by either 1 or 0. 1 in the rightmost position represents 1 (i.e., 20), 1 in the second position from the right represents 2 (i.e., 21), 1 in the third position represents 4 (i.e., 22), and so on, and 1 in the pth position means 2p - 1. Thus, the binary number 101 is 1 x 22 + 0 x 21 + 1 x 20 --that is, decimal number 5.

Data are stored in a computer as binary digits, or bits. If a binary number consists of n positions, it is said to be an n-bit number. Eight consecutive bits is called a byte. Therefore, a binary number of 16 bits has 2 bytes, a binary number of 32 bits has 4 bytes, and so on.
One byte is enough information to store one alphanumeric character (e.g. letter or decimal digit). A kilobyte is 1024 bytes, a megabyte is 1024 kilobytes (1,048,576 bytes), and a gigabyte is 1024 megabytes (1,073,741,824 bytes).

Each location in a memory is assigned a unique numeric address, by which the location is accessed. Data stored in each memory location consists of a fixed number of bits. This number is usually a power of 2 -- i.e., 4, 8, 16, 32, or 64 bits); such a binary sequence is called a memory word, or simply word.
The physical memory of a computer is either random access memory (RAM), which can be read or changed by the user or computer, or read-only memory ( ROM), which can be read but not altered. Computer chips hold memory, as do floppy disks, hard disks, and CD-ROMs (compact discs).