The programmer assigns the number in the program.A program communicates with a multiplex handler by calling Interrupt 2Fh with an identity number in the AH register. As each handler in the chain gains control, it compares the value in AH with its own identity number. If the handler finds that it is not the intended recipient of the call, it passes control to the previous handler. The process continues until control reaches the target handler. When the target handler finishes its tasks, it returns via an IRET instruction to terminate the interrupt.
The target handler determines its tasks from the function number in AL. Convention reserves Function 0 as a request for installation status. A multiplex handler must respond to Function 0 by setting AL to 0FFh, to inform the caller of the handler's presence in memory. The handler should also return other information to provide a completely reliable identification. For example, it might return in ES:BX a far pointer to the TSR's copyright notice. This assures the caller it has located the intended TSR and not another TSR that has already claimed the identity number in AH.
Identity numbers range from 192 to 255, since DOS reserves lesser values for its own use. During installation, a TSR must verify the uniqueness of its number. It must not set up a multiplex handler identified by a number already in use. A TSR usually obtains its identity number through one of the following methods:
The programmer assigns the number in the program.
The user chooses the number by entering it as an argument in the command line, placing it into an environment variable, or by altering the contents of an initialization file.
The TSR selects its own number through a process of trial and error.
The last method offers the most flexibility. It finds an identity number not currently in use among the installed multiplex handlers and does not require intervention from the user.
To use this method, a TSR calls Interrupt 2Fh during installation, with AH = 192 and AL = 0. If the call returns AL = 0FFh, the program tests other registers to determine if it has found a prior installation of itself. If the test fails, the program resets AL to zero, increments AH to 193, and again calls Interrupt 2Fh. The process repeats with incrementing values in AH until the TSR locates a prior installation of itself—in which case it should abort with an appropriate message to the user—or until AL returns as zero. The TSR can then use the value in AH as its identity number and proceed with installation.
The SNAP.ASM program in Section 19.8 demonstrates how a TSR can use this trial-and-error method to select a unique identity number. During installation, the program calls Interrupt 2Fh to verify that SNAP is not already installed. When deinstalling, the program again calls Interrupt 2Fh to locate the resident TSR in memory. SNAP's multiplex handler services the call and returns the address of the resident code's program-segment prefix. The calling program can then locate the resident code and deinstall it, as explained in Section 19.7.