![]() Hence the numerical range for one unpacked BCD byte is zero through nine inclusive, whereas the range for one packed BCD byte is zero through ninety-nine inclusive. In packed BCD, the same number would fit into a single byte: Īs an example, encoding the decimal number 91 using unpacked BCD results in the following binary pattern of two bytes: Packed: Two decimal digits are encoded into a single byte, with one digit in the least significant nibble ( bits 0 through 3) and the other numeral in the most significant nibble (bits 4 through 7).Unpacked: Each decimal digit is encoded into one byte, with four bits representing the number and the remaining bits having no significance.Both ASCII and EBCDIC character codes for the digits, which are examples of zoned BCD, are also shown.Īs most computers deal with data in 8-bit bytes, it is possible to use one of the following methods to encode a BCD number: In the fifth column ("BCD 8 4 −2 −1"), two of the weights are negative. In the headers, the " 8 4 2 1" indicates the weight of each bit. The following table represents decimal digits from 0 to 9 in various BCD encoding systems. For example, the BCD digit 6, 0110'b in 8421 notation, is 1100'b in 4221 (two encodings are possible), 0110'b in 7421, while in Excess-3 it is 1001'b ( 6 + 3 = 9 ). Others include the so-called "4221" and "7421" encoding – named after the weighting used for the bits – and " Excess-3". This scheme can also be referred to as Simple Binary-Coded Decimal ( SBCD) or BCD 8421, and is the most common encoding. The most obvious way of encoding digits is Natural BCD (NBCD), where each decimal digit is represented by its corresponding four-bit binary value, as shown in the following table. 8.2 Telephony binary-coded decimal (TBCD)īCD takes advantage of the fact that any one decimal numeral can be represented by a four-bit pattern.3.1 EBCDIC zoned decimal conversion table.However, decimal fixed-point and decimal floating-point formats are still important and continue to be used in financial, commercial, and industrial computing, where the subtle conversion and fractional rounding errors that are inherent in binary floating point formats cannot be tolerated. BCD per se is not as widely used as in the past, and is unavailable or limited in newer instruction sets (e.g., ARM x86 in long mode). #C STRING TO BCD SERIES#Its principal drawbacks are a slight increase in the complexity of the circuits needed to implement basic arithmetic as well as slightly less dense storage.īCD was used in many early decimal computers, and is implemented in the instruction set of machines such as the IBM System/360 series and its descendants, Digital Equipment Corporation's VAX, the Burroughs B1700, and the Motorola 68000-series processors. īCD's main virtue, in comparison to binary positional systems, is its more accurate representation and rounding of decimal quantities, as well as its ease of conversion into conventional human-readable representations. The ten states representing a BCD digit are sometimes called tetrades (for the nibble typically needed to hold them is also known as a tetrade) while the unused, don't care-states are named pseudo-tetrad(e)s, pseudo-decimals or pseudo-decimal digits. ![]() The precise 4-bit encoding, however, may vary for technical reasons (e.g. ![]() #C STRING TO BCD FULL#most modern computers), the term unpacked BCD usually implies a full byte for each digit (often including a sign), whereas packed BCD typically encodes two digits within a single byte by taking advantage of the fact that four bits are enough to represent the range 0 to 9. Sometimes, special bit patterns are used for a sign or other indications (e.g. In computing and an electronic systems, binary-coded decimal ( BCD) is a class of binary encodings of decimal numbers where each digit is represented by a fixed number of bits, usually four or eight. In this clock, each column of LEDs shows a binary-coded decimal numeral of the traditional sexagesimal time. A binary clock might use LEDs to express binary values. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |