SNAP combines four modules: SNAP.ASM, COMMON.ASM, HANDLERS.ASM, and INSTALL.ASM. Source files are located on one of your distribution disks. Each module stores temporary code and data in the segments INSTALLCODE and INSTALLDATA. These segments apply only to SNAP's installation phase; DOS recovers the memory they occupy when the program exits through the terminate-and-stay-resident function. The following briefly describes each module:
SNAP.ASM contains the TSR's main code and data.
COMMON.ASM contains procedures used by other example programs.
HANDLERS.ASM contains interrupt handler routines for Interrupts 08, 09, 10h, 13h, 15h, 28h, and 2Fh. It also provides simple error-trapping handlers for Interrupts 1Bh, 23h, and 24h. Additional routines set up and deinstall the handlers.
INSTALL.ASM contains an exit routine that calls the terminate-and-stay-resident function and a deinstallation routine that removes the program from memory. The module includes error-checking services and a command-line parser.
This building-block approach allows you to create other TSRs by replacing SNAP.ASM and linking with the HANDLERS and INSTALL object modules. The library of routines accommodates both keyboard-activated and time-activated TSRs. A time-activated TSR is a program that activates at a predetermined time of day, similar to the example program ALARM introduced in Section 19.3. The header comments for the Install procedure in HANDLERS.ASM explain how to install a time-activated TSR.
You can write new TSRs in assembly language or any high-level language that conforms to the Microsoft conventions for ordering segments. Regardless of the language, the new code must not invoke a DOS function that sets up the I/O stack (see Section 19.4.3). Code in Microsoft C, for example, must not call getche or kbhit, since these functions in turn call DOS Functions 01 and 0Bh.
Code written in a high-level language must not check for stack overflows. Compiler-generated stack probes do not recognize the new stack setup when the TSR executes, and therefore must be disabled. The example program BELL.C, included on disk with the TSR library routines, demonstrates how to disable stack checking in Microsoft C using the check_stack pragma.