Date   : Fri, 10 Jun 1983 08:14:00 EDT
From   : Ronald G. Fowler <RGF@mit-mc>
Subject: File Open BDOS patch

  There is indeed a gotcha: as you read (or write) a file opened in
this manner, eventually you're gonna come to the end of extent #0.
The BDOS automatically closes that extent and opens the next.  The
problem is that the 2nd extent will be opened in the CURRENT user
area; and will not be found (since the file was opened in some
other user area along the search path).  Hence, premature EOF.
  Two additional steps would have to be taken to get around this:

       1) When the file is opened, the user # of the file would have
          to be recorded somewhere; either in a table (not very desirable
          since a fixed-size table implies a limit to the number of open
          files -- although the table could be made arbitrarily large)
          or in the fcb itself (much better insofar as # of open files
          is concerned, but WHERE in the FCB do you store the user #?
          Look at the S1 byte; I'm pretty sure it's unused).

       2) The read-sequential, read-random, write-sequential and
          write-random system calls must be intercepted to allow the
          setup of the user # the REAL system calls get control; the
          intercept routines must get control back immediately after the
          real routines execute, in order to restore the user #.

Note that there is no problem with default drives, since the FCB has
a DR field to record the drive number, and this may be changed
easily.

Both TurboDOS and MP/MII allow opens to default to user #0 if a file
is not found in the current user area; rather than recording the user #,
both set an attribute bit in the in-memory FCB to flag the fact that the
file was opened under user 0 (this technique limits you to only one
public user area, however).

Finally, this change will not impact ZCPR or ZCPR2 at all; it will
think it has found its file in the current user area, and will not
continue the search.  (this will not be true of any programs that do
functions 17/18 searchfirst/next before opening the file, since you
can't apply this mod to them -- such programs simply won't find their
files).

I would like to get a copy if the mod if you proceed with this;
it's something I've thought about doing for some time, and never
had the time.          --Ron Fowler