Q. What do you mean by Byte?
Without question, the most vital data structure used by the 80x86 microprocessor is the byte this is true since the ASCII code is a 7-bit non-weighted binary code that is used on the byte boundary in most computers and A byte consists of eight bits and is the smallest addressable datum (data item) in the microprocessor.
I/O addresses and Main memory in the PC are all byte addresses this means that the smallest item that can be individually accessed by an 80x86 program is an 8-bit value. To access anything smaller needs that you read the byte containing the data and mask out the unwanted bits.
The bits in the byte are numbered from bit zero (b0) through seven (b7) as follows:
A Bit 0 is the low order bit or least significant bit, bit 7 is the high order bit or most important bit of the byte. We'll refer to all other bits by their number.
A byte as well contains exactly two nibbles and Bits b0 through b3 comprise the low order nibble, and bits b4 through b7 form the high order nibble. Ever since a byte contains exactly two nibbles, byte values require two hexadecimal digits.
Ever since a byte contains eight bits, it can represent 2^8, or 256, different values. Generally, we'll use a byte to represent:
- unsigned numeric values in the range 0 => 255
- signed numbers in the range -128 => +127
- ASCII character codes
- Other special data types requiring not more than 256 different values. Many data types have less than 256 items so eight bits is usually sufficient.
Ever since the PC is a byte addressable machine, it turns out to be more efficient to manipulate a whole byte than an individual bit or nibble. Therefore, most programmers use a whole byte to represent data types that require no more than 256 items, even if fewer than eight bits would suffice. For illustration, we'll often represent the boolean values true and false by 00000001 and 00000000 (respectively).
Most likely the most important use for a byte is holding a character code. Characters typed at the keyboard displayed on the screen, and printed on the printer all have numeric values. To permit it to communicate with the rest of the world, the IBM PC uses a variant of the ASCII character set as there are 128 defined codes in the ASCII character set. IBM PC uses the remaining 128 possible values for extended character codes including European characters, Greek letters, graphic symbols, and math symbols.