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

See also: 2A81

2A80 - NETWORK - BEGIN DOS CRITICAL SECTION