Int 2A Fn 80  CU - Network - Begin DOS Critical Section                    [D]

   AH = 80h
   AL = critical section number (00h-0Fh)
       01h DOS kernel, SHARE.EXE, DOSMGR
       apparently for maintaining the integrity of DOS/SHARE/NET
         data structures
       02h DOS kernel, DOSMGR
       ensures that no multitasking occurs while DOS is calling an
         installable device driver
       05h network redirector
       06h DOS 4.x only IFSFUNC
       08h ASSIGN.COM
       0Ah MSCDEX
       0Fh IBM PC LAN server (while intercepting INT 10/AH=06h,07h,0Eh)

Notes: normally hooked to avoid interrupting a critical section, rather than
     called
   the handler should ensure that none of the critical sections are
     reentered, usually by suspending a task which attempts to reenter
     an active critical section
   the DOS kernel does not invoke critical sections 01h and 02h unless it
     is patched.  DOS 3.1+ contains a zero-terminated list of words
     beginning at offset -11 from the Swappable Data Area
     (see INT 21/AX=5D06h); each word contains the offset within the
     DOS data segment of a byte which must be changed from C3h (RET)
     to 50h (POP AX) under DOS 3.x or from 00h to a nonzero value under
     DOS 4+ to enable use of critical sections.  For DOS 4+, all words in
     this list point at the byte at offset 0D0Ch.

See Also: AH=81h,AH=82h,AX=8700h,INT 21/AX=5D06h,INT 21/AX=5D0Bh