>On good old big slow 9-track tapes (watch an old episode of Hawaii
>Five-O that has computers in it if you've never seen one), the
>record size tells where to put the "inter-record gap" (or "end of
>record mark") which really is a gap of some ridiculous size like
>1/4" where no data is recorded. I'm pretty sure the tape drive
>uses this to be able to figure out were it is, kinda like the
>sector marks on your standard (soft-sectored) floppy diskettes.
>The practical outcomes of this are that all I/O to the tape drive
>has to be done a full record at a time and that using a small
>record size wastes a *lot* of space so that you can't fit as much
>on the same length of tape.
>Because helical-scan tapes exhibit these same behaviors, I assume
>that their end of record marks are also real gaps. I think they
>are different from 9-track tapes in that 9-track tapes store data
>as 9 horizontal rows of bits lines up together so that the record
>size has to be an even number of bytes (with a 9th bit for parity)
>while helical-scan tapes use diagonal stripes of bits that require
>the record size to be a multiple of 512 bytes, for example.
>Anyone got a nice picture of a strip of helical-scan tape media
>showing precisely how the bits get lined up in 512-byte groups
>and the shape/size of the inter-record gap?
Helical scan tapes write physical tracks across the tape at an angle.
Each one of these tracks are made up of a number of physical records
plus a fast tape search mark. Each physical record is made up of
header information, data, CRCand ECC error correcting code data.
For instance on an Exabyte 8500 tape, there are 8 physical records
per track, and each physical record contains 14 bytes of header
information, 1K bytes of data, 2 bytes of CRC and 400 bytes of
ECC error correcting code.
Logical records may be be from 0 to 240 KB in length. Logical records
may start anywhere in a physical block and span physical blocks. There
are now IRG (inter-record gaps) like in 9 track tapes. The IRG was a
means of correctly positioning the 9 track tape. The helical scan tapes
have a linearly recorded track on one edge of the tape that is used to
position the tape accurately. There is a special read head that is used
to position the tape. Filemarks do take up space. A short filemark
uses 1KB and a long filemark takes 48KB on 8mm tapes.
The only time that you lose capacity on these tapes is when you are
streaming data to the tape at less then the maximium capacity of the
tape drive. In order to keep the tape moving, null data is sent to
the tape. The reason for doing this, is to keep the tape moving.
Stopping, repositioning and starting tape movement takes 8 seconds on
an 8mm tape. Therefore, it is important to keep the tape moving.
_ __ ./|_ Robert Upton Nothern Telecom
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