INT 21,34 - Get Address to DOS Critical Flag INDOS (Undocumented DOS 2.0+)

AH = 34h

on return:

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