The VMM provides the Install_IO_Handler service to install a callback procedure to handle protection faults for a given port. When a virtual device calls Install_IO_Handler, the VMM sets the appropriate bit in the I/O permission map (IOPM) and registers the procedure. When a virtual machine executes an instruction that reads or writes data from an I/O port, the 80386 looks up the port number in the I/O permission map. If the corresponding bit in the IOPM is set, then the instruction will cause a protection fault that results in calling the registered procedure.
There are the following I/O services:
| Service | Description |
|---|---|
| Disable_Global_Trapping | Disables global I/O trapping (I/O works directly with hardware and is not intercepted). |
| Disable_Local_Trapping | Disables local I/O trapping (I/O works directly with hardware and is not intercepted). |
| Enable_Global_Trapping | Enables global I/O trapping (I/O requests are trapped instead of working directly with hardware). Enabled by default at system startup. |
| Enable_Local_Trapping | Enables local I/O trapping (I/O requests are trapped instead of working directly with hardware). |
| Install_IO_Handler | Install an I/O callback procedure. |
| Install_Mult_IO_Handlers | Install I/O handlers for multiple ports. |
| Remove_IO_Handler | Removes the I/O handler for the specified port and disables trapping for that port. |
| Remove_Mult_IO_Handlers | Repeatedly calls the Remove_IO_Handler service for each entry in the VxD_IO_Table, removing the specified IO handlers. |
| Simulate_IO | Reduces complex I/O instructions to simpler I/O operations. |
| _Simulate_VM_IO | Simulate ring 3 port I/O from the current vm (inside a ring 0 VxD). |