Hardware Categories


Digital Codes
Computers and digital circuits processes information in the binary format. Each character is assigned 7 or 8 bit binary code to indicate its character which may be numeric, alphabet or special symbol. Example  Binary number 1000001 represents 65(decimal) in straight binary code, alphabet A in ASCII code and 41(decimal) in BCD code.
Types of codes
BCD (BinaryCoded Decimal) code :
 Fourbit code that represents one of the ten decimal digits from 0 to 9.
 Example  (37)_{10} is represented as 0011 0111 using BCD code, rather than (100101)_{2} in straight binary code.
 Thus BCD code requires more bits than straight binary code.
 Still it is suitable for input and output operations in digital systems.
Note: 1010, 1011, 1100, 1101, 1110, and 1111 are INVALID CODE in BCD code.
ASCII (American Standard Code Information Interchange) code :
 It is 7bit or 8bit alphanumeric code.
 7bit code is standard ASCII supports 127 characters.
 Standard ASCII series starts from 00h to 7Fh, where 00h1Fh are used as control characters and 20h7Fh as graphics symbols.
 8bit code is extended ASCII supports 256 symbols where special graphics and math's symbols are added.
 Extended ASCII series starts from 80h to FFh.
EBCDIC (Extended Binary Coded Decimal Interchange Code) code
 8bit alphanumeric code developed by IBM, supports 256 symbols.
 It was mainly used in IBM mainframe computers.
Gray code
 Differs from leading and following number by a single bit.
 Gray code for 2 is 0011 and for 3 is 0010.
 No weights are assigned to the bit positions.
 Extensively used in shaft encoders.
Excess3 code
 4bit code is obtained by adding binary 0011 to the natural BCD code of the digit.
 Example  decimal 2 is coded as 0010 + 0011 = 0101 as Excess3 code.
 It not weighted code.
 Its selfcomplimenting code, means 1's complement of the coded number yields 9's complement of the number itself.
 Used in digital system for performing substraction operations.


Tip Box
Error dectecting and correcting schemes
 Repetition codes
 Parity bits
 Checksums
 Cyclic redundancy checks (CRCs)
 Cryptographic hash functions
 Errorcorrecting codes
 Error correction
 Automatic repeat request
 Errorcorrecting code
 Hybrid schemes

