Function  34h  Return INDOS Flag
 *   Returns ES:BX pointing to Critical Section Flag, byte indicating whether  
     it is safe to interrupt DOS. Used widely by TSRs (see Int\21f\31)
entry   AH      34h 
return  ES:BX   points to DOS "critical section flag" 
note 1) If byte is 0, it is safe to interrupt DOS. This was mentioned in some 
	documentation by Microsoft on a TSR standard, and PC Magazine reports 
	it functions reliably under DOS versions 2.0 through 3.3. Chris  
	Dunford (of CED fame) and a number of anonymous messages on the BBSs 
	indicate it may not be totally reliable. 
     2) The byte at ES:BX+1 is used by the Print program for this same purpose, 
	so it's probably safer to check the WORD at ES:BX. 
     3) Reportedly, examination of DOS 2.10 code in this area indicates that the 
	byte immediately following this "critical section flag" must be 00h to 
	permit the PRINT.COM interrupt to be called. For DOS 3.0 and 3.1 (except 
	Compaq DOS 3.0), the byte before the "critical section flag" must be 
	zero; for Compaq DOS 3.0, the byte 01AAh before it must be zero. 
     4) In DOS 3.10 this reportedly changed to word value, with preceding byte. 
     5) This call is supported in OS/2 1.0's DOS Compatibility Box 
     6) Gordon Letwin of Microsoft discussed this call on ARPAnet in 1984. He 
	stated: 
	a) this is not supported under any version of the DOS 
	b) it usually works under DOS 2, but there may be circumstances 
	   when it doesn't (general disclaimer, don't know of a specific 
	   circumstance) 
	c) it will usually not work under DOS 3 and DOS 3.1; the DOS is 
	   considerably restructured and this flag takes on additional 
	   meanings and uses 
	d) it will fail catastrophically under DOS 4.0 and forward. 
	   Obviously this information is incorrect since the call works fine 
	   through DOS 3.3. Microsoft glasnost? 
 


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