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

   AH = 80h
   AL = critical section number (00h-0Fh) (see #2243)

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 #1373 at 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 (PUSH AX) under DOS 3.x or from 00h to a nonzero value under
     DOS 4.0+ to enable use of critical sections.  For DOS 4.0+, all
     words in this list point at the byte at offset 0D0Ch.
   MS Windows patches the DOS kernel's calls to INT 2A/AH=80h-81h into
     far calls to its own handler, and does not reflect the calls back
     to INT 2A unless SYSTEM.INI contains ReflectDOSInt2A=1 or
     ModifyDOSInt2A=0 in the [386Enh] section
   Novell NETX does not issue INT 2A/AH=80h and INT 2A/AH=81h calls when
     it intercepts INT 21 calls and processes them itself

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

(Table 2243)
Values for DOS critical section number:
 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, CORELCDX
 0Fh   IBM PC LAN server (while intercepting INT 10/AH=06h,07h,0Eh)