A computer consists of a Central Processing Unit(CPU), memory and
various devices which can be categorised as Input/Output Devices.
Information is communicated between these separate units by the Systems
Bus.
The Central Processing Unit (CPU) consists of a Control Unit and an
Arithmetic/Logic Unit (ALU). The control unit controls the operation
of the peripheral devices and the transfer of information between the
units that make up the computer. The Arithmetic/Logic Unit performs
calculation.
The memory of the computer is split into main memory and external
memory.
Main memory is fast and limited in capacity. The CPU can only directly access
information in main memory. Main memory cannot retain information when
the computer is switched of. Main memory consists of a series of
numbered locations called bytes, each byte being eight
bits. The number associated with a byte is the address
of the byte.
Secondary memory is slow and virtually unlimited in capacity. It retains
information when the computer is switched off. Information on external
memory can only be accessed by the CPU if it is first transferred to main
memory.
The internal representation of information in the computer and on
external memory is in terms of the Binary system using only the basic
symbols 0 and 1.
Programs to be executed by the computer are placed in main memory and
the CPU fetches each instruction in turn from memory and executes it.
Each type of CPU has its own machine language, the set of instructions
it can obey.
