Create a file on magnetic tape: Difference between revisions
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END-WRITE
STOP RUN.</lang>
=={{header|Fortran}}==
Fortran performs reads and writes to some device, without caring much about the precise nature of the device: card reader/punch, paper tape reader/punch, keyboard, display console, and of course magnetic tape. Sequential input or output is normal, but random access by record number is also possible, though the interface between the Fortran program and the device may not support such attempts, as with a card reader or a display screen. There are also special statements, such as REWIND and BACKSPACE, and ENDFILE. The file OPEN statement will specify such matters as record length and block size and so all is in place for dealing with magnetic tapes...
Of the simplest sort: "Unlabelled". Such a tape consists of a sequence of data blocks, separated by inter-record-gaps, with two in a row at the end to specify the end-of-data on the tape. Although called inter-record, they are actually inter-block: the tape unit reads or writes blocks of say 5,000 characters, not caring that each block might be regarded as containing 50 records of length 100, or 100 records of length 50, etc. The inter-record gap is a sequence of three quarters of an inch of magnetic tape, containing special magnetisation that can be written or sensed at any speed, because these spaces are where the tape is brought to rest between reads or writes. That is, having written the data of a block at full speed, the tape write head writes an IRG as the tape is brought to a stop, and later, when it resumes motion to reach operating speed for the writing of the next block. With adequate data buffering, tape is moved past the head without pause through the IRG as blocks are read or written. If the tape is being read (and written!) with random access the width of the IRG allows for some slop in positioning. Some caution is in order: a "fast forward" action, followed by a few seconds pause to allow the attainment of maximum speed, then a REWIND resulted in a piece of tape being stretched to double length and half width...
Unlabelled tapes are quite inconvenient in a non-user operated computer installation, as mistakes are very easy to make. The wrong tape gets overwritten and the ensuing conversation sours. Computer-readable labels were introduced to supplement human-readable written labels affixed to the tape reel. Co-ordination and standardisation were weak in the 1970s, so there was a lot of variation and partial interoperability between manufacturers and ANSI standards. The first step was of course non-standard labels: some data would be written to the start of the tape that constituted a label, followed by a tape mark, then the actual data file, and so on. Users at an installation would attempt to conform to the local style, and there would be discussions, especially when tapes were exchanged with different systems. A large organisation such as IBM would have its method, and the basic idea was that at the start of the tape would be a volume label and additional information to allow for a data file (or multiple data files) that extended over many reels of tape, each tape identified in turn. This would start with an eighty-character record, beginning "VOL1" - but in EBCDIC, and only later in ASCII. None of this would be visible to the user's programme because it would be handled by the operating system with statements in Job Control Language for the run to identify the file on tape that is to be accessed when the user's programme executes an OPEN statement of a suitable form. And this access would revert to sequential reading or writing with quite ordinary READ and WRITE statements.
This was developed when tape drives used half-inch tapes in lengths up to 2,400 feet and seven or nine-track recording, storing up to about 150MB per reel - much depended on the length lost to IRG usage between blocks, but block sizes beyond 20,000 were considered large. Subsequent tape designs have included 18, 36, and 128 tracks, helical recording and storage capacities in tens of gigabytes. Naturally, the tape labelling protocols have become even more complex, but, these matters are handled by more complex operating systems.
=={{header|Go}}==
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