system unit

System unit

The main part, processing unit and devices, of a microcomputer is a system unit. A system unit includes a board called a motherboard that holds a microprocessor chip (or a CPU), memory chips, and expansion slots. Electronic circuitry is printed on the board and it connects between two main parts of a microcomputer, the microprocessor and primary storage and other parts. The system unit is housed within the system cabinet. A system unit includes the following parts:

• Motherboard
• Microprocessor
• Memory Chips
• System Clock
• Buses
• Ports
• Expansion Slots and Cards

Motherboard

A Motherboard or system board is the main printed, flat circuit board in an electronic device such as microcomputers. The board contains expansion slots (sockets) that accept additional boards (expansion Cards). In a microcomputer, the motherboard contains the microprocessor, the primary storage chips (or main memory cards), the buses, and all the chips used for controlling the peripherals.

Microprocessor

A microprocessor is a processor whose elements are miniaturized into one or a few integrated circuits contained in a single silicon microchip. It executes instructions. In a microcomputer, the central processing unit (CPU) is held on a single microprocessor. In order to function as a processor, it requires a system clock, primary storage, and power supply.
Several important lines of microcomputers use some families of microprocessor chips. Intel and Motorola are the major companies that produce important microprocessors for IBM compatible and Macintosh computers.

Central Processing Unit (CPU)

The central processing unit (CPU) is the computing part of the computer that interprets and executes program instructions. It is also known as the processor. In a microcomputer, the CPU is contained on a single microprocessor chip within the system unit. The CPU has two parts: the control unit and the arithmetic-logic unit.

Additional storage units called registers within control unit and ALU help make processing more efficient. 

•   Control Unit: A control unit is the circuitry that locates, retrieves, interprets and executes each instruction in the central processing unit. The control unit directs electronic signals between primary storage and the ALU, and between the CPU and input/output devices.

• Atithmetic-Logic Unit (ALU): ALU is a high-speed circuit part in the CPU. The arithmetic-logic unit (ALU) performs arithmetic (math) operations, logic (comparison) operations and related operations. The ALU retrieves alphanumeric data from memory and then does actual calculating and comparing. It sends the results of the operation back to memory again.

Memory Chips

A memory chip is a chip that holds programs and data either temporarily or permanently. The major categories of memory chips are RAMs and ROMs.

• RAM Chips  - RAM stands for random-access memory. Random- access memory holds the data or instructions that the CPU is presently processing. The type of primary storage is RAM. That is, a collection of RAM chips builds primary storage. Whenever a CPU writes data or instructions to RAM, it wipes out the previous contents of RAM, and when a CPU read data or instructions from RAM, it keeps their contents.

• ROM Chips - ROM stands for read-only memory. A ROM chip is a memory chip that stores instructions and data permanently. Its contents are placed into the ROM chip at the time of manufacture and cannot be modified by the user. A CPU can read and retrieve the instructions and data from the ROM chip, but it cannot change the contents in ROM .ROM chips usually contain special instructions for computer operations such as ROM BIOS. The variations on the ROM chip are the following:

•  PROM (Programmable Read-Only Memory): A permanent storage device that becomes a read-only memory after it is written once by the customer rather than by the chip manufacturer. For example, a software producer can write instructions onto the PROM using special equipment.

• EPROM (Erasable Programmable Read-Only Memory): EPROM is a reusable PROM-chip that can be erased by a special ultraviolet light. EPROM holds its content until erased and new instructions can be written on it.

• EEPROM (Electrically Erasable Programmable Read-Only Memory): EEPROM-chip can be erased, either within a computer or externally, by electric power. The process usually requires more voltage than the common +5 volts used in logic circuits.

Primary Storage (Memory)

Primary storage (internal storage, main memory or memory) is the computer's working storage space that holds data, instructions for processing, and processed data (information) waiting to be sent to secondary storage. Physically, primary storage is a collection of RAM chip.

The contents are held in primary storage only temporarily. Capacity varies with different computers. Data or instructions are stored in primary storage locations called addresses.

Ports

A port is an external connecting socket on the outside the computer. This is a pathway into and out of the computer. A port lets users plug in outside peripherals, such as monitors, scanners and printers.

• Serial Ports

Serial ports are external I/O connectors used to attach modems, scanners or other serial interface devices to the computer. The typical serial ports use a 9-pin DB-9 or a 25-pin DB-25 connector. Serial ports transmit bits one after another on a single communications line. Serial lines frequently are used to link equipment that is not located close by.

• Parallel Ports

Parallel ports are external I/O connectors on a computer used to hook up printers or other parallel interface devices. The parallel port uses a DB-25 connector. This port transmits several bits simultaneously. Parallel lines move information faster than serial lines do.