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Category: DOS kernel
Flags: callout or callback (usually hooked rather than called), Undocumented function


	AH = 80h
	AL = critical section number (00h-0Fh) (see #02555)
Notes:	normally hooked to avoid interrupting a critical section, rather than
	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 #01687 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
SeeAlso: AH=81h,AH=82h,AX=8700h,INT 21/AX=5D06h,INT 21/AX=5D0Bh

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  Copyright 2000   by Ralf Brown     Updated Jul 2000