System Callback Procedures

The system callback procedures are "hooks" into DOS (in the case of VDMs, into the DOS Emulation kernel) which allow application programs to change the default processing action taken for certain system events. These hooks are specified in the V86 mode interrupt vector table as trap service routines. By default, the vectors reference a single IRET instruction if an application does not register its own hook procedure.

The vectors used to specify the callback routines are:

INT 22h - Terminate Address

The DOS Emulation kernel stores the termination return address at this vector location.

The method used to invoke this callback procedure works the same way as in a DOS 5.0 system.

Hard errors are normally generated from within the OS/2 kernel. When such an error is detected, the file system checks to see if the requesting task is a VDM. If so, the error indication is returned to the protected mode portion of DOS Emulation, which determines if the INT 24h vector has been changed.

If it has not been changed, the normal OS/2 hard error monitor is called to display the hard error information and to prompt for a reply. If it has been changed, the specified critical error handler is invoked.

The method used to call the INT 28h callback routine is similar to that used in DOS 5.0, but takes into account the fact that the DOS session is running in a multitasking environment.

The OS/2 scheduler maintains a flag in the VDM address space which indicates if another process in the system is ready to run. While in the idle loop, the DOS Emulation kernel repeatedly examines this flag. If no other OS/2 tasks are ready, the loop proceeds as normal. If the flag indicates that other tasks are waiting, DOS Emulation yields control to the operating system, which dispatches the waiting task.

In all other respects, callback procedures operate under MVDM in an identical manner to that experienced under DOS 5.0.