! ! compress ! 1 COMPRESS The COMPRESS command invokes a utility to copy a file, generating a file with (usually) fewer bytes. Files compressed by COMPRESS are recovered by DECOMPRESS. COMPRESS Input-file-spec Output-file-spec 2 Parameter Input-file-spec Specifies the name of the file to be compressed. If you have specified /EXPORT=VMS mode, the file must be stored on a disk. Output-file-spec Specifies the name of the file created by COMPRESS. 2 Qualifiers Indicate special actions to be performed by the COMPRESS utility or special properties of either the input or output files. Qualifiers apply to the entire process. The following list shows all the qualifiers available with the COMPRESS command: o /BITS=value o /EXPORT=(option,...) o /METHOD=option o /MODE=(option,...) o /SHOW=(option,...) 2 /BITS /BITS=value This specifies the maximum number of bits to be used in the compression. It implicitly controls both the "quality" of the compression (more bits means more compression) and the amount of memory needed for both compression and decompression (more bits requires more memory). If the compressed file is to be read by a computer with limited memory (such as a PDP-11), choose /BITS=12, else leave BITS at its default of 16. The minimum value is 9 and the maximum value is 16. 2 /EXPORT /EXPORT=(option, [,...]) Export controls the format of the output file. You can select the following: VMS (D) Write a file that can only be read by VMS COMPRESS. UNIX Write a format that can be read by programs compatible with the Unix compress utility. [NO]ENDMARKER Write a special file endmarker after the data if specified. [NO]BLOCK Monitor compression and reinitialize if the quality decreases if specified. [NO]HEADER Write a file header with information for DECOMPRESS if specified. In general, use /EXPORT=VMS for compression where the result will be decompressed on a VMS system and /EXPORT=UNIX where the result will be decompressed on a Unix, RSX-11M, RSTS/E, or other non-VMS system. If /EXPORT=UNIX is specified, BLOCK, HEADER, or ENDMARKER may be negated to further qualify the output file format. 3 VMS Specifies output in VMS ("private") mode. In addition to the contents of the file itself, the "File definition block" is also compressed. The decompression utility can thus recreate the file exactly (including ISAM indexes). This is the default, and the recommended mode for most uses. 3 UNIX Specifies an output format compatible with Unix compress v3.0. This allows transmitting sequential files to non-VMS systems that support a compress-compatible utility. If you have specified /EXPORT=UNIX, the utility can be configured for variants of Unix compress by negating BLOCK, HEADER, and/or ENDMARKER as needed. Notice that file attributes are not preserved by /EXPORT=UNIX. 3 BLOCK Selects an algorithm whereby COMPRESS evaluates its performance and re-initializes the compression tables whenever performance degrades. Older versions of Unix compress do not support this capability. If negated, ENDMARKER must also be negated. 3 HEADER If negated, COMPRESS does not write a header record. This is for compatiblity with very old versions of Unix compress. If negated, BLOCK and ENDMARKER must also be negated. 3 ENDMARKER If specified, a special "endmark" is written after the end of the file. This is necessary if the file is to be decompressed on RT11 or other systems that require the last block of a file to fill the last block. On the other hand, some versions of Unix compress cannot understand the "extra" endmarker. If you guess wrong, a few bytes of garbage may be appended to the decompressed file. A version of Unix compress that handles endmarkers correctly is available. 2 /METHOD /METHOD=(option) This specifies the particular compression algorithm. Currently, only /METHOD=LZW is supported. 3 LZW Use the Lempel-Ziv-Welch compression algorithm. 2 /MODE /MODE=(option) This allows specification of variations on the compression method. 3 BINARY This opens the file in "binary" mode, rather than "text" mode. It is ignored if /EXPORT=VMS is chosen. 3 DELTA Compress the difference between successive bytes, rather than the bytes themselves. For certain file formats, such as bit-mapped graphics, this may yield a 10-15% improvement in compressibility. This is not compatible with some implementations of Unix compress. 2 /SHOW /SHOW=(option, [,...]) Display information about the compression. If omitted, COMPRESS operates silently (except for error messages). 3 ALL Equivalent to /SHOW=(PROGRESS,STATISTICS,FDL) 3 PROGRESS Print status messages at intervals, showing the operation of the program. The report shows the current compression ratio (the ratio of input to output bytes). If this decreases, COMPRESS decides that the characteristics of the file have changed, and resets its internal parameters. The "gap" is the number of input codes used to compute the ratio. 3 STATISTICS Print a report at the end of the process. Note that COMPRESS reports the number of bytes it compresses, which includes the file as well as the information that COMPRESS records about the file (the File Definition Language block and some internal codes), and will therefore be several hundred characters greater than the actual size of the file. 3 FDL Dump the File Definition Language block that describes a VMS input file. 3 DEBUG Print internal debugging information. 3 DEBUG_SERIOUS Print more internal debugging information. 3 DEBUG_IO Dump the output file, too. 2 LZW_Overview LZW stands for a compression method described in "A technique for High Performance Data Compression." Terry A. Welch. IEEE Computer, Vol 17, No. 6 (June 1984) pp. 8-19. This section is abstracted from Terry Welch's article referenced below. The algorithm builds a string translation table that maps substrings in the input into fixed-length codes. The compress algorithm may be described as follows: 1. Initialize table to contain single-character strings. 2. Read the first character. Set (the prefix string) to that character. 3. (step): Read next input character, K. 4. If at end of file, output code(); exit. 5. If K is in the string table: Set to K; goto step 3. 6. Else K is not in the string table. Output code(); Put K into the string table; Set to K; Goto step 3. "At each execution of the basic step an acceptable input string has been parsed off. The next character K is read and the extended string K is tested to see if it exists in the string table. If it is there, then the extended string becomes the parsed string and the step is repeated. If K is not in the string table, then it is entered, the code for the successfully parsed string is put out as compressed data, the character K becomes the beginning of the next string, and the step is repeated." The decompression algorithm translates each received code into a prefix string and extension [suffix] character. The extension character is stored (in a push-down stack), and the prefix translated again, until the prefix is a single character, which completes decompression of this code. The entire code is then output by popping the stack. I.e., the last code put into the stack was the first code in the original file. "An update to the string table is made for each code received (except the first one). When a code has been translated, its final character is used as the extension character, combined with the prior string, to add a new string to the string table. This new string is assigned a unique code value, which is the same code that the compressor assigned to that string. In this way, the decompressor incrementally reconstructs the same string table that the decompressor used.... Unfortunately ... [the algorithm] does not work for an abnormal case. The abnormal case occurs whenever an input character string contains the sequence KKK, where K already appears in the compressor string table." The decompression algorithm, augmented to handle the abnormal case, is as follows: 1. Read first input code; Store in CODE and OLDcode; With CODE = code(K), output(K); FINchar = K; 2. Read next code to CODE; INcode = CODE; If at end of file, exit; 3. If CODE not in string table (special case) then Output(FINchar); CODE = OLDcode; INcode = code(OLDcode, FINchar); 4. If CODE == code(K) then Push K onto the stack; CODE == code(); Goto 4. 5. If CODE == code(K) then Output K; FINchar = K; 6. While stack not empty Output top of stack; Pop stack; 7. Put OLDcode,K into the string table. OLDcode = INcode; Goto 2. The algorithm as implemented here introduces two additional complications. The actual codes are transmitted using a variable-length encoding. The lowest-level routines increase the number of bits in the code when the largest possible code is transmitted. Periodically, the algorithm checks that compression is still increasing. If the ratio of input bytes to output bytes decreases, the entire process is reset. This can happen if the characteristics of the input file change. (This can be supressed by /EXPORT=(UNIX, NOBLOCK)). 2 Unix Is a trademark of AT&T Bell Laboratories. ! ! decompress ! 1 DECOMPRESS The DECOMPRESS command invokes a utility to restore copy a file that had been compressed by COMPRESS. DECOMPRESS Input-file-spec [Output-file-spec] 2 Command_Parameters Input-file-spec Specifies the name of the compressed input file. Output-file-spec Specifies the name of the file created by COMPRESS. If the file was compressed by /EXPORT=VMS, the original file name will be used if the Output file spec. is omitted. If the file was compressed by /EXPORT=UNIX and no Output file spec. is provided, the file will be written to SYS$OUTPUT: 2 Command_Qualifiers Indicate special actions to be performed by the COMPRESS utility or special properties of either the input or output files. Qualifiers apply to the entire process. The following list shows all the qualifiers available with the DECOMPRESS command: o /BITS=value o /EXPORT=(option,...) o /METHOD=option o /MODE=(option,...) o /SHOW=(option,...) 2 /BITS /BITS=value If a header was not provided, this specifies the maximum number of bits that were used in the compression. This parameter is ignored if the compressed file contains a header. 2 /EXPORT /EXPORT=(option, [,...]) Export describes the format of the input file. You can select the following: VMS (D) The file was created by VMS COMPRESS. UNIX The file was created by Unix compress or a compatible program. compatible with the Unix compress utility. [NO]ENDMARKER A special file endmarker follows the data. [NO]BLOCK The compress program may have reinitialized compression. [NO]HEADER The compress program wrote its parameters into a file header. In general, the program can determine the proper value of these flags by reading the first few bytes of the file. If valid, the file header overrides the command line specification. Generally, this option is needed only if you are trying to read a file generated by a version of Unix compress that did not write a header. See the description of COMPRESS for details. 2 /METHOD /METHOD=(option) This specifies the particular compression algorithm. Currently, only /METHOD=LZW is supported. 3 LZW Use the Lempel-Ziv-Welch compression algorithm. 2 /MODE /MODE=(option) This allows specification of variations on the output file format. These values will be taken from the source file description if /EXPORT=VMS is chosen. 3 BINARY This creates the file in "binary" mode, rather than "text" mode. It is ignored if COMPRESS created the file in /EXPORT=VMS mode. The output file will be created in RMS "Stream-LF" format. 3 DELTA Compress used the difference between successive bytes, rather than the bytes themselves. For certain file formats, such as bit-mapped graphics, this may yield a 10-15% improvement in compressibility. This is not compatible with some implementations of Unix compress. This value is normally read from the file header, and generally need not be specified by DECOMPRESS. 3 FIXED Create the file in "fixed-block, 512-byte record" format. This is probably the best format to use for decompressing binary files (such as tar archives) created on Unix. 3 TEXT Create the file in "variable-length carriage-control" format. This is appropriate for decompressing readable text files created by Unix compress. 2 /SHOW /SHOW=(option, [,...]) Display information about the compression. If omitted, DECOMPRESS operates silently (except for error messages). 3 ALL Equivalent to /SHOW=(PROGRESS,STATISTICS,FDL) 3 PROGRESS Print status messages at intervals, showing the operation of the program. 3 STATISTICS Print a report at the end of the process. 3 FDL Dump the File Definition Language block that describes the output file. 3 DEBUG Print internal debugging information. 3 DEBUG_SERIOUS Print more internal debugging information. 3 DEBUG_IO Dump the input file, too. 2 Unix Is a trademark of AT&T Bell Laboratories.