A virtual device driver is a 32-bit EXE file which runs in protected mode and supports the flat memory model. Figure "Virtual COM and Physical COM Device Drivers" shows the structure of a virtual device driver and the interfaces to a physical device driver.
Nearly all virtual device drivers provided in OS/2 V2.0 are written in a high-level language ("C"), although several have portions that were developed using assembler language. Since software interrupts and hooked I/O port operations cause a trap to privilege level 0, time critical code for these operations should be written in assembler language to achieve the maximum possible performance.
A virtual device driver is a 32-bit EXE file that can contain some, all, or none of the following types of objects:
A virtual device driver must have at least one object of the following types:
A virtual device driver should contain resident objects for code and data which must be accessed at physical hardware interrupt time, that is, when a physical device driver calls the virtual device driver. A virtual device driver which does not interact with a physical device driver needs no resident objects. Examples of such device drivers are VEMM and VXMS.
A virtual device driver locates its instance data above the 1MB + 64KB line, but below 4MB. The instance data is therefore outside the VDM's V86 mode address space. This linear address range is the same for all VDMs; that is, all VDMs have the instance data for a particular virtual device driver at the same linear address. The offset from the VDM's linear base address to the virtual device driver's instance data is returned by the OS/2 kernel when the device driver is initialized.
A virtual device driver may need to access its instance data area at physical hardware interrupt time. This may be required even when the VDM is not currently executing in the foreground. Since the instance data is system data located above the V86 addressing range, the virtual device driver may address the per-VDM buffer regardless of which process is currently running. VDM instance data is accessed by adding the VDM's handle + instance data area offset + data offset within the instance data area.
Note that memory objects created by a virtual device driver for passing to a physical device driver must not exceed 64KB in size. This limitation results from the fact that many physical device drivers are still written using the 16:16 segmented memory model, and cannot therefore support memory objects greater than 64KB in size.