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OSdata.com: assembly language 

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Assembly Language

binary coded decimal arithmetic

summary

    This web page examines binary coded decimal (BCD) instructions in assembly language. Specific examples of instructions from various processors are used to illustrate the general nature of assembly language.

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binary coded decimals

    Binary coded decimal (BCD) is a method for implementing lossless decimal arithmetic (including decimal fractions) on a binary computer. The most obvious uses involve money amounts where round-off error from using binary approximations is unacceptable. Some early computers used BCD exclusively.

    Decimal digits (0-9) can be encoded in a nibble (half a byte), with some left over bit patterns (hexadecimal A-F). In BCD operations, the processor performs ordinary binary computations, then adjusts the result to conform to BCD. For example, if you add the binary number 5 (bit pattern 0101) to binary number 6 (bit pattern 0110), you get the binary result of 11 (bit pattern 1011, or hexadecimal B). With BCD arithmetic, the processor would adjust the result to make it into a valid BCD result (which in this case would be bit pattern 0001 0001).

    BCD arithmetic includes BCD addition, BCD subtraction, BCD multiplication, BCD division, and BCD negate.

    The Intel 80x86 series uses a two step approach for BCD arithmetic. Instead of having separate BCD instructions, the normal binary addition and subtraction instructions are used, then hardware instructions are used to adjust the results to correct BCD results. There are instuctions for both packed and unpacked adjustments. The advantage of this approach is greater flexibility (more addressing modes and choices of arithmetic operations because of the use of regular binary integer instructions in the first step). The disadvantage of this approach is that it is slower and takes more memory.

    Pack (Motorola 680x0) converts byte encoded numeric data (such as ASCII or EBCDIC characters) into binary coded decimals. Unpack (Motorola 680x0) converts binary coded decimals into byte encoded numeric data (such as ASCII or EBCDIC characters). The ASCII adjustment field is $3030; the EBCDIC adjustment field is $F0F0.

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free music player coding example

    Programming example: I am making heavily documented and explained open source PHP/MySQL code for a method to play music for free — almost any song, no subscription fees, no download costs, no advertisements, all completely legal. This is done by building a front-end to YouTube (which checks the copyright permissions for you).

    View music player in action: www.musicinpublic.com/.

    Create your own copy from the original source code/ (presented for learning programming). Includes how to run this from your own computer if you don’t have a web site.


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    Names and logos of various OSs are trademarks of their respective owners.

    Copyright © 2000, 2001 Milo

    Created: February 21, 2001 (from machcode.htm)

    Last Updated: September 16, 2001


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