A6- HOW A SOFTWARE INSTRUCTION IS PROCESSED IN THE COMPUTER

 

By  pragyansharma polavarapu 

Let us explain in a very simple way the basic idea. Each “software instruction”  consists of  two  well defined  parts. The two parts are  the OPCODE and the OPERAND.

 (1)  The   OPCODE  means  the particular arithmetic operation . It is the electronic circuit in the ALU  for operations like  (+,  - ,  X  , / ,square,square root etc).

(2) The OPERAND . It is the arithmetic expression /numbers on which the mathematical operation  (add/subtract/multiply/divide etc  )  is to be done.

 A “software instruction” involving  mathematical work looks  as shown below(in an extremely simple example). The first item in the  “software instruction” is OPCODE . The second item is  OPERAND.

Only after  they get translated into 01010101 code (in a millionth of second),required  OPCODE  circuits (+,-, x etc )will be switched on . Then the OPERANDS  will be transmitted  through the current paths  to the ALU in millionth of second. Simple examples are given below.

[add]  [238  ]

 [multiply ]  [15 ]

[subtract]  [  1354  ]

 [divide]  [20 ]

THE ARITHMATIC AND LOGIC UNIT(ALU)

It is the most important part after the CPU.  It is a group of inter-connected REGISTERS  and contains the highly complicated (and once very costly) mathematical circuits.

 The ALU  contains the part called ACCUMULATOR   which stores the  intermediate answers .The ACCUMULATOR  is like a water  drum into which water is poured with a mug and the latest interim answer is  indicated by the total number of mugs of water  which “accumulated” in the drum. As a very simple example let us take following expression with  100 numers  or even a million numbers.

 6+8-5+3……

All calculations in  Computer’s  ACCUMULATOR  starts with a “0” as the starting number for any calculation. So  the progress in mathematical operations is as follows

0+6=6

6+8=14

14-5=9 

9+3=12

and so on until the hundred or thousand or million  numbers are operated upon.The work in the computer is so fast --(done by electron current flowing at speeds near 300,000 kilometers per second)---that such basic mathematical operations though a million in number go on and get completed in fraction of a second. But is really a”leisurly pace” for  the computer!

The final end answer, is  conveyed to the ‘OUTPUT’ i.e computer screen on orders from CPU.  Computer never tires and never commits mistakes. If a wrong program is given,the computer gives an error message and stops working. Only after human operator rectifies mistake it again starts working.

DUTIES OF IMPORTANT  REGISTERS.

Each register is like a highly  trained  specialist clerk. Let us examine duties performed by some registers.

 (a) INSTRUCTION REGISTER (IR) : A SOFTWARE  INSTRUCTION is first detained for a millionth of second in the  IR. After  decoding it is sent to another register. 

( b )MEMORY DATA REGISTER (MDR): Here the mathematical data is temporarlily stored till CPU orders its transfer to next part.   Here  the “interim answer”  is temporarily stored  till orders are received from   CPU for its next destination.  The “FINAL ANSWER” coming from  ALU is also  temporarily stored here until CPU orders it to be sent  to OUTPUT(computer screen). 

 ( c )MEMORY ADDRESS REGISTER (MAR): 

 The  serial numbers (DIGITAL ADDRESS) of all the   SOFTWARE   INSTRUCTIONS  ( similar to the house number) are maintained here. MAR conveys serial number of next instruction to MAIN MEMORY, concerned REGISTERS/ALU etc .This system ensures utmost speed in processing  software instructions one after another without any waiting.

 ( d )  PROGRAM COUNTER (PC)  ---It  strictly ensures that software instructions are released as per their serial number and conveys the information in advance to MAIN MEMORY , other REGISTERS etc .

(e) THE ARITHMATICAL AND LOGIC UNIT –It is the central mathematical section much like the mathematics teacher. It contains several costly mathematical and logic circuits and in importance next only to the CPU . Its functions are simply explained above already.

A SIMPLE EXAMPLE OF WORK IN ALU  FROM A MATHEMATICAL VIEW.

The formula (equal to a software program)  is  “5a+4b=k”

The input data is as follows

Three values of  a          a=24, 79,64

Three values of b          b=56, 26, 49

Answer K1 =(5x24)+(4x56)

 Answer K2 =(5x79)+(4x26)

 Answer K3 =(5x64)+(4x49)

Even a million sets of values of a and b are processed and solved  in a millionth of second in a similar way and in a leisurely most perfect way .

 Under instructions of CPU  the  REGISTERS and ALU work. The final answer is kept temporarily in MAIN MEMORY. On receiving instructions from CPU the final answer is sent to OUTPUT