INT 0x21 - 0x34 - Get Address to DOS Critical Flag

INDOS (Undocumented DOS 2.0+)

Request/Call:

reg	value	descr
AH	0x34

reg	descr
ES:BX	address of a byte indicating whether a DOS call is in progress. No DOS calls should be made if set.

AKA the INDOS flag, this critical section flag may be checked from within an interrupt handler before requesting a DOS service. It is a semaphore that is non-zero when DOS is busy, and zero otherwise.
though this flag indicates whether a DOS interrupt is active, it should not be used alone to determine DOS is safe for re-entry; Here's the standard rule for safe DOS entry: if INT 28 is active or this flag and the critical error flag are clear then it is safe to call DOS
this interrupt should be used only during TSR initialization; the returned pointer should be used thereafter
this flag is cleared after a critical error (INT 24)
- ES:BX-1 points to the critical error flag for DOS 3.x+
- ES:BX+1 points to the critical error flag for DOS 2.x
- ES:BX-1AA points to the critical error flag for COMPAQ DOS 3.0

INT 21,5D in DOS 3.x+ can be used to locate the critical error flag