8/17/2019 COA(NOTES)

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4 LOVELY PROFESSIONAL UNIVERSITY

Computer Organization and Architecture/Introduction to Computer Organization and Architecture

Notes 7.  Error   Correcting Codes: These codes not only detect errors in data, but also correct themsignificantly. Error correction codes are a method by which a set of symbols can be representedsuch that even if any 1 bit of the representation gets accidentally flipped, we can still clearlyidentify the earlier symbol. Error correcting codes depend mainly on the notations and results

of linear algebra. Error correction can be done using many methods like parity checking,Hamming codes, Single-bit Error Correction Double-bit Error Correction (SECDED), and soon.

 Notes  Error correcting codes are used in memories, networking, CDROM, and so on.

Example: Error correction using parity checking is as follows:

In parity check, an extra bit is added to the binary number to make all the digitsin the binary number to sum up to an even or odd value. When the number addsup to an even number, we call it even parity and when the number sums up to anodd number, we call it odd parity. Consider the following two binary numbers:

1011010

1101011

Now, if we want to use even parity, we can add a parity bit to these numbers toobtain an even number as shown below:

01011010 4

11101011 6

If we want to use odd parity, we can add a parity bit to the number as follows:

11011010 5

01101011 5

Most of the modern applications use even parity. Let us consider even parity inour example.

The two binary numbers that need to be transmitted are:

01011010……………….The even parity

11101011……………….The even parity

Suppose during transmission the bits get changed as follows:

01111010 5

10101011 5

We can observe that the digits in the number sum up to odd numbers. Since weare using even parity, the computer knows that there is an error in thetransmission.

8.  Alphanumeric Codes: These are codes that consist of both numbers and alphabets. The mostcommonly used alphanumeric codes are ASCII and EBCDIC.

(a)  EBCDIC Code: EBCDIC (Extended Binary Coded Decimal Interchange) is mainly used  with large computer systems like mainframe computers. It is an 8-bit code which  accommodates up to 256 characters.