1. The CPU GapFill

The CPU, known also as the  capitalclockcontrolcentral processing unit, or simply the processor, is the component responsible for performing calculations and executing instructions.

Several components are common to any CPU, and each component has a clearly defined role.  The arithmetic logic unit (ALU) is responsible for performing calculations.  These might take the form of  Booleanperformancearithmeticrelational operations, such as addition and subtraction,   Booleanarithmeticprimaryrelational operations such as OR and NOT, and   performancearithmeticstandardrelational operations, such as less than or greater than.

The ALU will perform such calculations only when instructed to do so by the  command unitcentral unitcritical unitcontrol unit, which manages the execution of instructions, as well as the flow of data through the CPU.

In addition to these components, there is also a collection of registers that form part of the CPU.  A register is a small (typically 64   bitsgigabytesnibblesmegabytes for a modern PC) but quick-to-access storage area, each of which can store a single piece of information.  There are   generalisedgeneral-purposespecialisedmultipurpose registers, whose behaviour will vary depending on the nature of the program being run, and special-purpose registers, which each have a clearly defined role.

Special-purpose registers include the  memory address registerprogram countermemory buffer registermemory data register, which at the beginning of each cycle contains the   colourbufferaddresscounter of the next instruction to be accessed, as well as the memory address register, which can point to memory locations of instructions or data.  The   accumulatorsubtotalprogram counteradder is a register that stores intermediate results of calculations, meaning partially calculated results.

Components both within and beyond the CPU are interconnected using buses.  The   address busdata busprimary bussecondary bus is used to transmit memory locations; the   data busaddress bussecondary busprimary bus is used to transmit, among other things, program instructions between memory and the CPU; the   primary buscontrol bussecondary busdata bus is used when the CPU needs to issue a command to memory, such as 'read' or 'write'.

In order for the behaviour of the various CPU components to be synchronised, the  timermultiplexorpulsarclock sends out a pulse on a regular basis – typically billions of times per second.  A task can be commenced only as one of these pulses take place, although multiple pulses may occur before a task is completed.