Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
InDos Flag Interrupt
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     In DOS versions 3.1 and higher, DOS stores the Critical Error
     flag in the byte preceding the InDos flag. A call to function 34h
     thus effectively returns the addresses of both flags.
 
     TSR programs that make calls to DOS must monitor the InDos flag to
     ensure they do not interrupt a non-reentrant DOS function.
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