[TUHS] Groups origins

[Norman Wilson]

  Groups appeared surprisingly late (given how familiar
  they seem now): they don't show up in the manual
  until the Sixth Edition.

Mea culpa; read too hastily.  The change actually
came with the Fourth Edition, at the same time as
several other landmark system changes:
-- Time changing from a 32-bit count of 60Hz clock
ticks (which ran out so quickly that its epoch kept
moving) to the modern 32 bits of whole seconds, based
at 1970-01-01T00:00:00 GMT (which takes much longer
to run out, though the horizon is now visible).
-- The modern super-block.  In 4/e, the super-block
began at block 0, not 1 (so bootstrapping was rather
more complicated); the free list was a bitmap rather
than the later list of blocks containing lists of
free block numbers.
-- The super-block contained a bitmap of free
i-numbers too.  All told, the free block and free
i-node map made up a 1024-byte super-block.
-- I-numbers 1-40 were device files; the root
directory was i-number 41.  The only file-type
indication in the mode word was a single bit to
denote directory.

It was already clear that the lifetime of the
bitmaps was running out: the BUGS section says
two blocks isn't enough for the bitmaps for a
20-megabyte RP02.

Norman Wilson
Toronto ON