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In article <PAS.90Mar1120125@saffron.lcs.mit.edu> pas@lcs.mit.edu (Paul A. Selkirk) writes:
>Does anyone know of a bulletproof way for a TSR to tell whether DOS (assume
>2.x or 3.x) is idling in the command interpreter, or executing a program?

>I have determined, through much pain and disassembly, that DOS uses service
>0A (buffered string input) to get the next command, but other programs use that
>as well.  I can get by on this knowledge, but I would prefer something a little
>more certain.

>For the record, I am writing a screen-saver TSR that will only trigger when DOS
>is idling.  There are dozens of screen-savers, but I need one that won't kill
>a lengthy graphics program.  (Ever try generating the Mandelbrot set on a 4.77 
>MHz XT with no FPU?  I thought so.)

>Any leads will be duly appreciated.

Quoting from chapter 16 of a preliminary version of "A Textbook on the
8086 Family" by William B. Giles, Copyright 1989, to be published by
MacMillan Publishing Company in October, 1990:  (I reviewed it for
MacMillan and am using it in a class!  Any typo's are probably mine.)

  "DOS keeps a one-byte flag, which is usually called the InDOS flag
   or the DOS Active byte.  The value of this flag at any instant
   equals the number of calls to INT 21h functions which are currently
   in process of execution.  In particular, if the value is zero no
   DOS function is presently active and so DOS functions can be safely
   called.  Finding the value of the InDOS Flag is easy: a call to
   function 34h of INT 21h returns the address of the flag in es:bx."

Furthermore (a paragraph or two later):

  "... But even when DOS is busy, it may not be executing a non-
   interruptible section of code; during such "safe" periods, DOS
   makes regular calls to INT 28H."
   
  "The sole reason for the existence of INT 21h is to provide
   programmers with another method of determining whether it is safe
   to execute DOS functions.  If INT 28h is active, DOS is in a safe
   condition."
   
  "A number of the traditional DOS functions (01h through 0Ch) must
   wait for user input and, during such waiting periods, they call
   INT 28h repeatedly.  In particular, while the DOS prompt is being
   displayed and DOS is calling function 01h, the INT 28h method can
   be used to determine when it is safe to execute DOS function calls."

To actually use the above requires a "trick" almost like that of
writing device drivers.  When your TSR determines it needs to do
something, usually it should just set an internal flag to remind
itself of that fact.  Then it can make periodic tests (by taking
over the timer interrupt) and/or take-over INT 28h to determine
when it is safe to actually do the desired work.

Hope this helps.
--Myron.
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