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Decompression Routine

IP.com Disclosure Number: IPCOM000086424D
Original Publication Date: 1976-Sep-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 64K

Publishing Venue

IBM

Related People

Gilbert, AM: AUTHOR [+3]

Abstract

Data compression enables data to be transmitted or stored more efficiently by eliminating duplicate character strings. A generalized decompression routine is presented here.

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Decompression Routine

Data compression enables data to be transmitted or stored more efficiently by eliminating duplicate character strings. A generalized decompression routine is presented here.

This routine decompresses data in which escape characters or sequences may be embedded. It requires no intermediate storage and makes only one scan of the data.

The uncompressed data may contain two kinds of escape characters, a character or group of characters that mark the end of a logical record (EOR), and a character that signals the receiver of the data to treat a group of bytes as data, even if escape characters are present in the data. This second character (TRN, transparent) is immediately followed by a byte that gives the length of the data to be skipped over before resuming the scan for escape characters. Further, the data has a maximum logical record length associated with it. If this maximum is reached before an EOR is found, the logical record is complete. If the next byte is an EOR, it is considered part of the just-completed record. Otherwise, it is the first byte of the next record.

The compressed data is described by control bytes (CBs). A CB contains two bits that identify the data being described and six bits which give the length of that data. There are three kinds of CBs. A CB that describes a series of bytes in its original (uncompressed) form is followed immediately by the data. A CB that describes a series of blanks replaces the series. The compressed data appears as a block of any number of logical records. There is no relationship between logical record boundaries and the bou...