## Virtual Device Driver Reference for OS/2

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### DOS Session Creation and VDD Per-DOS Session Initialization

When the user starts a DOS application, the DOS Session Manager notifies the virtual device drivers that have registered creation hooks, and gives them an opportunity to do per-DOS session initialization. Memory is allocated and mapped into the V86-mode address space, and the DOS Emulation code is initialized. Control is passed to the DOS Emulation kernel to load the shell specified by DOS_SHELL (usually COMMAND.COM) and the application selected by the user.

Virtual device drivers perform the following operations (as appropriate) at DOS session creation time:

Initialize the virtual device state
Initialize the virtual ROM BIOS state
Map linear regions (reserved at virtual device driver initialization time) to physical or linear memory
Hook the I/O ports
Enable or disable the I/O port trapping
Hook the software interrupts
Allocate per-DOS session memory