INT 2A CU - NETWORK - BEGIN DOS CRITICAL SECTION
        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.
SeeAlso: AH=81h,AH=82h,AX=8700h,INT 21/AX=5D06h,INT 21/AX=5D0Bh

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