View source for PDDREF:Running OS/2 Version 1.3 16-Bit PDDs on OS/2

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The following items are possible problems that can occur when OS/2 Version 1.3 16-bit physical device drivers are run on OS/2 2.1 or later.

==Use of Physical Addresses - PhysToUVirt==
Physical device drivers that use DevHlp_PhysToUVirt will notice changes in these areas:
* Returned offset (in BX) can no longer equal 0.
* The ''OS/2 I/O Subsystems and Device Support Device Drivers'' (erroneously) stated that the result found in BX, on return from the call to PhysToUVirt, would always be 0.
* In OS/2 2.1, BX is not always equal to 0 on return from PhysToUVirt.
* Mapped areas that are greater than 60KB might not be fully mapped if their beginning offset is greater than 0. For example, a 64KB object that begins at ''n'' bytes offset from a 4KB page boundary results in a segment/descriptor that has a length of 64KB-''n''.

==Direct Call to Physical Device Drivers==
Physical device drivers that set up their own GDT call gate will not work in OS/2 2.1. The solution is to use the DevHlp_DynamicAPI in order to have the system generate a GDT call gate into the physical device driver.

==Direct Writing of GDT Selectors==
Physical device drivers that set up GDT descriptors for private use will not work in OS/2 2.1. The solution is to use one of three Device Helper services - PhysToGDTSelector, PhysToUVirt or PhysToVirt. The decision about which service to use must be based on the context in which the memory is to be accessed.

==Physical Device Drivers that Need Real Mode==
Physical device drivers that need to switch to real mode will fail. OS/2 defines virtual device drivers that coordinate the access that DOS session have to physical devices. Refer to the ''[[Virtual Device Driver Reference for OS/2|Virtual Device Driver Reference]]'' for further information.

==Physical Device Drivers Support of DOS Applications==
OS/2 physical device drivers, that were written to support a single DOS application, can fail in OS/2 if they start supporting several DOS applications at the same time. This concerns physical device drivers that maintain addresses of items within a DOS program. Because OS/2 2.1 supports the running of multiple DOS sessions, an address within a DOS program might not always refer to the same DOS session.

If a physical device driver supports multiple DOS applications and needs to maintain data relative to each, the device driver must be modified to the VDD/PDD model.