Topic: packed data

topics > computer science > data > data type > Group: derived data types

access to components of a data object
boolean values, binary numbers, and bit strings
compressed code
compressed data
data alignment
data record


Storage requirements can often be reduced by packing data. For instance three 5-bit enumerated values can be packed into a 16-bit word along with a 1-bit boolean value. Packing is dependent on data representation. Some packing adds little or no access cost-- for instance a boolean value in a high or low-order bit; while others increase access cost significantly-- for instance a short-integer value embedded in the middle of a word. Inefficient packed access can often be improved by packing or unpacking all component values at once. Space requirements for packed data access may negate possible space improvements. (cbb 5/80)
Subtopic: packed data up

Quote: a packed type defines the bit length of a value, without gaps between fields; an implementation may disallow any length [»cardL_1991]
QuoteRef: cbb_1973 ;;Jovial packing structures both in series and in parallel eg 123123 or 112233 where "123" is the unit
QuoteRef: wirtN1_1971 ;;58 pack (array, index, structure ?? and unpack

Subtopic: bit fields up

Quote: define bit fields by endian convention, width, low and high bit; generate shifts and masks automatically [»ramsN5_1997]
Quote: bit fields are better than bit strings for binary representation: order independent, check for overlaps and gaps, specified length
Quote: fast addition, subtraction, and compare of masked integers; i.e., a subset of bits in a computer word [»adamMD5_2006]

Subtopic: instruction encoding up

Quote: define machine instructions with fields, tokens, patterns, and constructors; for a wide variety of processors
Quote: an instruction stream consists of tokens in several classes; a token consists of bit fields [»ramsN5_1997]
Quote: a SLED pattern constrains the values of fields in one or more tokens
Quote: a SLED constructor produces abstract, binary, and assembly-language instructions from a operand list

Subtopic: bit array up

Quote: a Modula-3 set is represented by a packed bit array

Related Topics up

Topic: access to components of a data object (4 items)
Topic: boolean values, binary numbers, and bit strings (44 items)
Topic: compressed code (17 items)
Topic: compressed data (16 items)
Topic: data alignment (2 items)
Topic: data record
(57 items)

Updated barberCB 12/05
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