Introduction to Digital Systems: Modeling, Synthesis, and Simulation Using VHDL

2.4 DATA ORGANIZATION

A binary number is a sequence of bits that may represent an actual binary number, a character, or an instruction. Therefore, microcomputers must use a specific data structure or big groupings to express the various binary representations. As learned earlier, in each binary grouping the rightmost bit is called the least significant bit and the leftmost bit is called the most significant bit. A group of consecutive 4 bits is called a nibble. A nibble is used to represent a BCD or hexadecimal digit. A group of consecutive 8 bits is called a byte, which is the smallest addressable data in memory. A byte is also used to represent an alpha-numeric character. A group of consecutive 16 bits, called a word, can be divided into a high byte and a low byte. For example, in 16-bit general-purpose registers and accumulators, the high and low bytes can be manipulated separately. In general, the size of the microcomputer internal registers determines the size of binary grouping. A 16-bit microcomputer has two bytes, or a 16-bit word size. However, the memory unit is divided into an 8-bit, or byte, word length. For example, to store a 16-bit number, the microcomputer uses two consecutive byte locations in the memory space. High-end (32 and 64-bit) microcomputers use double-word and quad-word data structures. These wide data structures are used mainly in highly pipelined and parallel microcomputers.