INT 2A - NETWORK - BEGIN DOS CRITICAL SECTION
        AH = 80h
        AL = critical section number (00h-0Fh)
            01h DOS kernel, SHARE.EXE
                apparently for maintaining the integrity of DOS/SHARE/NET
                  data structures
            02h DOS kernel
                ensures that no multitasking occurs while DOS is calling an
                  installable device driver
            05h DOS 4.x only IFSFUNC
            06h DOS 4.x only IFSFUNC
            08h ASSIGN.COM
            0Ah MSCDEX
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 through 3.31 contain a zero-terminated list of
          words beginning at offset 02C3h in the IBMDOS segment; each word
          contains the offset within the IBMDOS segment of a byte which must
          be changed from C3h to 50h to enable use of critical sections.
SeeAlso: AH=82h,AH=87h,INT 21/AX=5D06h,INT 21/AX=5D0Bh