The MCP LABEL recognition routine READALABEL recognizes a large class of installation-defined tape labels in addition to standard tape labels. Refer to the I/O Subsystem Programming Guide for a discussion of standard tape LABEL formats.
To invoke the LABEL recognition routine, a description of the desired nonstandard labels must be written and passed through a table-building program called RLTABLEGEN. RLTABLEGEN places the information concerning nonstandard tape labels into a value array and then compiles the value array into the present MCP called SYSTEM/MCP. The new MCP is called SYSTEM/NEWMCP. The installation then changes the MCP to SYSTEM/NEWMCP through the CM (Change MCP) system command. Refer to the System Commands Operations Reference Manual for a description of this command.
Installation-defined tape labels can be used in the same manner as standard tape labels. However, the following restrictions apply:
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The LABEL records must be a contiguous group of EBCDIC records at the beginning of the tape, and must be separated from the data by one tape mark.
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The LABEL type must be determined by examination of the first record only.
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The LABEL record size must not exceed 229 characters.
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The MCP does not recognize any user labels—for example, users' trailer label (UTL) and users' header label (UHL)—on these LABEL types.
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No up tape labels are recognized. That is, once the file is opened and the tape is positioned to the beginning of the data, the tape is treated as an unlabeled tape—that is, LABEL=OMITTEDEOF.
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Because READALABEL checks for the appearance of installation-defined labels before checking for standard labels, the recognition sequence must be sufficiently complete so that it does not interfere with standard LABEL recognition.
