Q59884: Determining If Your TSR Has Already Been Installed

Article: Q59884
Product(s): See article
Version(s): 4.00 5.00 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc s_quickasm appnote | mspl13_c
Last Modified: 23-MAR-1990

When writing a TSR (terminate-and-stay-resident), it is possible to
utilize the multiplex interrupt (2fh) to determine whether your TSR
has already been loaded into memory. Set up an Interrupt Service
Routine (ISR) for this interrupt, which will compare the AH register
with a predefined TSR number (ID#) and change the AL register to a
nonzero value if the two are equal.

When using the interrupt keyword in Microsoft C, registers are pushed
onto the stack before each function call. To access these register
values from within an ISR, just define the ISR function as accepting
these registers as parameters.

Once this is set up, the installation part of the TSR can make a call
to the multiplex interrupt with the AH register set to the ID# of the
TSR, and the AL register set to 00h. If the handler is currently
installed, it will pick up these values in the AX register and then
change the AL register to 01h and return this "installed" signal to
the calling program. If the ID# in the AL register is not that of the
TSR, the TSR can simply chain the interrupt back to its original
vector.

In summary, to have your TSR check to see if it is already installed,
do the following:

1. Make an int86 call with the following:

      inregs.h.ah=id#(0xc0-0xff)
      inregs.h.al=0x00

2. If outregs.h.al != 0x00, program is in memory, don't re-install.

3. Else, revector INT2fh to your own ISR. This ISR should do the
   following:

   a. Take as parameters the registers pushed on the stack by the
      interrupt keyword (see REGPAK below).

   b. Check the AH register, hibyte of the AX register, with a TSR
      ID#(0xc0-0xff).

   c. If AH=TSR ID#, change al to 0x01.

   d. Else, chain to the old INT2fh vector.

4. Terminate and stay resident.

The following is a simple example of an ISR that would accomplish Step
3 (above) nicely with a TSR ID# of 0xc9. For more information about
the multiplex interrupt and its function, please refer to "The New
Peter Norton Programmer's Guide to The IBM PC & PS/2," Page 303-306.
For an example of a TSR, see the application note "TSR Example
Dirzap.C," which is available from Microsoft Product Support Services
by calling (206) 454-2030.

Sample Program
--------------

void (interrupt far *original_int2fh)(); /*set to original*/
                                         /* int2fh handler*/

#define HIBYTE(x) (((unsigned) (x) >> 8) & 0xff)

#define REGPAK unsigned es, unsigned ds, unsigned di, \
               unsigned si, unsigned bp, unsigned sp, \
               unsigned bx, unsigned dx, unsigned cx, \
               unsigned ax, unsigned ip, unsigned cs, \
               unsigned flags

void interrupt far new_int2fh(REGPAK)
{
     if (HIBYTE(ax)==0xc9) /* check TSR ID# */
          ax=0xc901;       /* set AL to 01  */
     else
          _chain_intr(original_int2fh);
}

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