AH = 34h
Return: ES:BX points to DOS "Critical Section Flag"
Notes:
   When byte pointed to is zero, DOS is supposed to be
   safe to interrupt. NOT RELIABLE according to Chris Dunford.

   Examination of DOS 2.10 code in this area indicates that the
   byte immediately FOLLOWING this "Critical Section Flag" must
   be 00 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, and for COMPAQ DOS 3.0,
   the byte 01AAh before it must be zero.
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