The CPUThe CPU is the “brain” of the computer, the place where data is manipulated. In large computer systems, such as super computers and mainframes, processing task may be handled by multiple processing chips. In the average microcomputer, the entire CPU is a single unit called a microprocessor. Every CPU has at least two parts: the control units (CU) and the arithmetic and logic unit (ALU).

The Control Unit (CU)All the computer resources are managed from the control unit. The control unit controls the flow of data through the CPU, and to and from other devices. The control unit is the logical hub of the computer.

The Arithmetic and Logic Unit (ALU)Many instructions carried out by the control unit involve simply moving data from one place to another from memory to storage, from memory to the printers and so forth. When the control unit encounters an instruction that involves arithmetic or logic, however, it passes the instruction to the second components of the CPU, the arithmetic and logic unit or ALU. The ALU actually performs the arithmetic operations (+,-,X, ÷) and the logical operations (=,>,<,>=,<=,!= etc). The ALU includes a group of registers- high speed memory locations built directly into the CPU that are used to hold data currently being processed.

Machine CyclesEach time the CPU executes an instruction, it takes a series of steps. The completed series of steps is called a machine cycle. A machine cycle itself can be broken down into two smaller cycles: the instruction cycle and the execute cycle.Instruction cycle takes two steps

1. Fetching: Before the CPU can execute an instruction the control unit must retrieve (or “fetch”) a command or data from the computer’s memory.

2. Decoding: Before a command can be executed, the control must break down ( or decode) the command into instructions that corresponds to those in the CPU’s instruction set.

At this point, the CPU is ready to begin the execution cycle. When the command is executed, the CPU carries out the instruction in order by converting them into microcode. The CPU may be requested to store the results of an instruction in memory

The BUSIn microcomputers, the term bus refers to the path between the components of a computer. There are two main buses in a computer: the internal (or system) bus and the external ( or expansion) bus. The internal bus resides on the motherboard and connects the CPU to other devices that reside on the motherboard. An external bus connects external devices, such as the keyboard, mouse, modem, printer etc to the CPU. Cables from disk drives and some other internal devices may also be plugged into the bus.

Modem: The basic function of a modem is to modulate and demodulate signals. It converts digital signal which are generated by the computer into analog signal which pass through a telephone line. This process is called modulation and the reverse is known as demodulation. Telephone lines are used to transfer the data among computers located at larger distances. However, the telephone lines are designed for voice communication. So these lines are not suited for direct transmission of the computer’s digital data stream. In order to send the data over telephone lines, it must be converted into an audio like signals which can be accommodated with relatively narrow bandwidth required by the telephone system. To convert computer data into audio like signals, a modulator is required and at the other end a demodulator is required to convert audio data into digital data. So we call MOulator/DEModulator system or MODEM.