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block size depends on the individual computer system. The maximum length of a block is limited only by the availability of memory to hold the block in the computer. To identify a block on Phase Encoded and Croup Coded Recording there is a preamble before the block and a postamble after the block. These are recorded so that the machine knows where a block starts and where a block finishes. Blocks are not numbered nor is there any record of their length. The system works out the length of a block by counting the number of bytes read between the preamble and the postamble.

BOT/​EOT

The beginning and end of the data area of the tape are identified by a pice of reflective foil stuck on the tape. This is seen by photoelectric sensors when the marks pass. They are known as the BOT (Beginning Of Tape) and the EOT (End Of Tape) markers. When the tape is first loaded (on the tape drives) the tape is automatically moved so that the BOF mark is in front of the photoelectric sensor. The first data block is located a short distance after the BOT mark. Blocks are written one after the other separated by gaps. The purpose of the gap is to separate blocks so that the blocks may be used one at a tine.

File Marks or Tape Marks

In order to divide the tape into files a special type of block is recorded – this is called a file mark or tape mack. The file mark does not contain any useful information. It is just a place marker. When a file mark is read a special signal is sent from the tape deck to the host computer. This usually causes reading to cease. The file mark is used to separate files on a tape. A further use of the file mark is at the end of the data on a tape. Usually the end of the recorded data on a tape is well before the reflective EOT marker. To signify that there is no more valid date on the tape two or more file mark blocks are written next to each other. If one file mark is encountered and the system is asked to read the next file and another file mark is immediately found, with no data block being read, the system knows that it has reached the end of valid data. Some systems write several file marks at the end of data for good measure.

Adding Files to a Tape

When a file is to be added to a tape, the tape is positioned to the file mark just after the last file. When recording restarts all of the extra file marks are overwritten. At the end of the new file multiple file marks are written.

Headers, Volume Labels and Trailers

As can be seen from the above, there is no directory system on a tape. It merely consists of files recorded one after the other. This is quite satisfactory for some applications when it is sufficient to stick a paper label on the tape reel to identify the contents. Most scientific and engineering applications find this adequate.

In commercial data processing departments where many more tapes are handled and they are kept in big libraries a more formal scheme is required. A system of Headers and Trailers has been developed so that each file in fact consists of 3 tape files. The first is the header, the second the data file itself, and the third the trailer or “End Of File header”.

Each reel of tape is known as a Volume – just as with books. To identify the volume the first header file on the tape has an additional record in the first block and this is known as the Volume Label or Volume Header.

Another type of label that is encountered is the End of Volume. This is used when it is necessary for s file to continue on to another volume of tape. Instead of an end of file header at the end of data, the end of volume label is used to signify that the file carries on to another tape volume.

The exact content of these headers varies from system to systen. Standards have been produced by ANSI, ECMA, ISO and IBM. These are all similar in principle but differ in detail. It will be found for instance that IBM use the EBCDIC character representation instead of the ASCII used by the other.