INT 2A,80 - Begin DOS Critical Section

	AH = 80h
	AL = critical section identifier (00-0F)
	   = 01  DOS kernel, SHARE.EXE and network data structure integrity
	   = 02  DOS kernel, device driver exclusivity
	   = 05  IFSFUNC (Installable File System, IFS, DOS 4.0+)
	   = 06  IFSFUNC (Installable File System, IFS, DOS 4.0+)
	   = 08  ASSIGN.COM
	returns nothing

	- indicates that the region of code being entered should not be
	  interrupted and identifies the critical region
	- this function is not normally called by user applications but is
	  often hooked to monitor if DOS is in a critical section
	- this information is necessary to reenter DOS using the swappable
	  data area returned by INT 21,5D subfunction 0B
	- see  Bibliography reference to "Undocumented DOS"