General Architecture of Central Processing Unit (Microprocessor)

In our previous article we have already talked about Pin configuration of Intel 8085 Microprocessor, which has made you understand more about circuitry of Microprocessor. So now we will know how does microprocessor operates, with its each units.

The microprocessor is a clock driven semiconductor device consisting of electronic logic circuits manufactured by using either a large scale integration (LSI) or very-large scale integration (VLSI) technique. The microprocessor is capable of preforming various computing functions and making decisions to change the sequence of the program execution. In large computers, a CPU implemented on one or more circuit boards performs these computing functions. The microprocessor can be divided into three segments for the sake of clarity, as shown in above figure Arithmetic Logic Unit (ALU), register array and control unit which is further connected with I/O devices and Memory unit through system bus. The figure will show the general architecture of central processing unit, microprocessor.

general architecture of central processing unit

Architecture of Central Processing Unit, How it talks to other units?

Arithmetic and Logic Unit (ALU) 

This is the area of the microprocessor where various computing functions are performed on data. The ALU unit performs such arithmetic operations as addition and subtraction, a logic operations such as AND, OR and exclusive OR.

Register Array

This area of the microprocessor consists of various registers identified for holding the instructions and data during the program execution time. Registers are also the memory blocks made by flip-flop and are volatile in nature.

See also: Designing 555 Timer IC as an Astable Multivibrator

There are also few registers with microprocessor to hold the memory or I/O address where data are stored. Some common registers are:

a)      Program Counter Register

b)      Memory address Register

c)       General Purpose Register

d)      Instruction Register

e)      Status Register

Control Unit 

Control unit controls all the operation of the system through its timing and control unit. It controls the flow of data between the microprocessor and memory and peripherals. The unit synchronizes all the microprocessor’s operation with the clock and generates the control signal necessary for communication between the microprocessor and peripherals.

control unit interrelating with ALU and Register array

Control unit, the only captain here to handle ALU and Register Array

In microprocessor, major function of control unit is the exception of the program so that the command and data handling devices are fully monitored and controlled by it. Hence, execution of program is done in specific sequences.

Memory Units

Memory units are used for holding the instructions and data of specified size depending on the system requirement of microprocessor based system carries two types of memory.

         i.            Primary Memory – It holds primary data of the program or any function RAM and ROM are its example. RAM is volatile and ROM is non-volatile in nature. Due to their distinct properties, ROM is used for storing frequently used to store instantaneous instruction and data.

       ii.            Secondary Memory – It is the mass storage devices and is used to store the program and data which may require in future. Direct operation on secondary memory is impossible so before operating on the programs of this memory they are retrieved within the primary memory (RAM).

Input Devices

These are also called as peripherals devices. The input devices are used for giving the commands and data to the system. Various examples of input devices in microprocessor based on system are –

  • DIP (Dual Inline Packet) switch
  • Keyboard
  • Mouse
  • Web cameras, etc.

Output Devices

These devices are used to give the output information to the users. Various output devices are –

  • LED (Light Emitting Diode)
  • Seven Segment Display
  • Video Display
  • Tape Recorder
  • LCD Unit

System Bus

System bus is a communication path between the microprocessor and peripherals; it is nothing but a group of wires to carry bits. All peripherals (and memory) share the same bus; however, the microprocessor communicates with only one peripheral at a time. The timing is provided by the control unit of the microprocessor.

See also: 555 Timer IC as Monostable Multivibrator

a)      Address Bus – The address bus is a group of 16 lines generally identified as A0 to A15. The address bus is unidirectional, i.e. bits follows in one direction from microprocessor to peripheral devices. The microprocessor uses the address to perform the function of identifying or a memory location.
In a computer system, each peripheral or memory location is identified by a binary number, called an address, and the address bus is used to carry a 16 – bit address. The number of address lines of the microprocessor determines its capacity to identify different memory locations (or peripherals). The 8085 microprocessor with its 16 address lines is capable of addressing 2^16 = 65,536 memory locations.

b)      Data Bus – The term Data refers to any binary information that may include an instruction, an address, or a number. The data bus is a group of eight lines used for data flow. These lines are bidirectional, i.e. data flows in both directions between the microprocessor and memory and peripheral devices. The microprocessor uses the data bus to perform the function of transferring binary information.

c)       Control Bus – The control bus is comprised of various single lines that carry synchronized signals. The microprocessor uses such lines to perform the function of providing timing signals.
The term bus, in relation to the control signals, is somewhat confusing. These are not groups of lines like address or data buses, but individual lines that provide a pulse indicate a microprocessor operation. The microprocessor generates specific control signals for every operation (such as Memory Read or I/O Write) it performs. These signals are used to identify a device type with which the microprocessor intends to communicate.