5 Apr
2009
5 Apr
'09
2:38 p.m.
>> How does magtape avoid the stream-of-bytes
issue?
> Magtape has blocks.
What exactly is a block?
Is it defined as a sequence of bits or as a sequence
of bytes?
It depends on the tape technology. The most classical one in my
(admittedly limited) experience has blocks that are variable-sized
strings of bytes. That is, it would be accurate for these purposes to
model the tape as a stream of symbols from a 258-symbol alphabet: 256
bytes plus "block boundary" and "tape mark", where block boundaries
delimit blocks and tape marks delimit files. I'm not sure this model
is totally accurate - for example, I'm not sure a zero-length block is
possible - and it's definitely not a close match to the implementation.
But it's probably close enough to get the flavour.
More recent technologies are a bit different. Some (all?) QIC tapes,
for example, are not streams of variable-sized blocks but streams of
512-octet blocks.
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