TITLE: Writing Device Drivers - Where to Start?
IBM Developer Connection News Volume 2
by Steve Mastrianni

I get a lot of questions from developers just starting to write 
device drivers. One of the most common questions is "How do I 
get started writing OS/2 device drivers?"  Well, that depends on 
your background. 

Because device drivers interact with the OS/2 kernel, you should 
have a good understanding of the basic functions provided by 
OS/2, such as multithreading, priorities, memory allocation, and 

A majority of the questions I get involve a misunderstanding of 
how various types of addresses work.  Driver writers must be 
able to work with virtual, linear, physical, and real addresses.
Since the device driver interacts with the processor at the 
machine level, a good understanding of the processor 
architecture is also invaluable.  Failures in a device driver 
usually hang the system, and tracking them down can be tedious 
without knowing where to look. 

If you're writing your device drivers in C, you'll need a 
16-bit C Compiler such as Microsoft C Version 5.1 or Version 
6.0.  You'll also need an assembler, such as the Microsoft 
Version 6.0 Macro Assembler. Previous versions (such as 
Version 5.1) will also work. 

If you're writing Virtual Device Drivers (VDDs), you'll need 
a 32-bit C compiler, such as IBM C Set/2 or C Set++ 
(recommended) or the special 32-bit compiler, CL386, included 
in the Device Driver Source Kit (DDK).  The DDK also includes 
the kernel debugger and ASDT32, which you will need to debug 
your device drivers.  The Periscope Debugger is available 

You also should get the OS/2 Technical Library, a 50-pound 
collection of developer reference books, which includes the 
OS/2 Physical Device Driver Reference, Virtual Device Driver 
Reference, and Presentation Driver Reference. The library is 
also available as part of the OS/2 Online Book Collection 

You can get the book, Writing OS/2 2.1 Device Drivers in C, 
2nd edition. It's the only tutorial on writing OS/2 2.x 
device drivers.  Call 1-800-842-3636 to order. 

Support for device driver writers is free via IBM's DUDE 
(Dynamic Upload and Download Environment).  Periodically, 
the DUDE team archives the questions into a file (removing 
names), which you can download.  See the Directory of this 
Newsletter on how to access the DUDE. 

You can download the file CDISK.ZIP from the Libraries 
section, Device Drivers, of OS2DF1 on CompuServe.  CDISK.ZIP 
includes several sample device drivers, including a VDD sample, 
all written in C. 

I've always written my drivers in C; IBM has historically 
written them in assembler.  This is evidenced by the code in 
the PDD reference, which contains MASM examples of DevHlp 
calls.  With the advent of Workplace OS, IBM has begun to 
document C-language interfaces to the DevHlps.  Future 
releases of the DDK and driver references will include these. 

Another question I get asked frequently is "How can I 
initialize a memory mapped adapter during initialization 
(Init time)?"  Very often, adapters must have their memory 
loaded with a program or initialized.  Many device driver 
writers experience their first Trap 13 (General Protection 
Fault) when attempting to perform this operation during 

The most common cause is that driver writers sometimes 
forget that Init is run as a ring 3 thread of the system.
Mapping a physical address to a virtual address with 
PhysToVirt yields a GDT-based pointer, which is not usable 
from a ring 3 thread.  Ring 3 threads do not have GDT 
access; only LDT access. 

The solution to this problem is to map the physical 
address of the adapter to a virtual address that's mapped 
into the application's LDT.  This is done with the PhysToUVirt 
DevHlp call, rather than the PhysToVirt call. 

If you have a lot of data to download to the adapter, keep it 
in a disk file, and use the standard DosOpen, DosRead, and 
DosClose APIs.  This is possible because Init is running as a 
ring 3 thread, which is the same ring level at which most 
applications run. 

"How can device drivers can transfer data at interrupt time?" 

This is a little tricky; but easy once it's been explained.  
Remember that Init runs as a ring 3 thread with access to the 
application program's LDT.  The rest of the time, your device 
driver operates in the lowest ring, ring 0. While at ring 0, 
the device driver has full access to the GDT and for the most 
part, the entire system. 

The problem is that when an interrupt occurs, your program 
might not be the program that is currently running.  For 
example, your program might be blocked waiting for I/O or 
waiting on a semaphore.  Because of this, the context, or 
current environment at that instant, might not be known. 

Trying to map an application's buffer address at interrupt 
time will not work. To maintain addressability in any 
context, the application's buffer address must be mapped to 
a GDT selector.  The selector, however, must be allocated 
during Init, and then mapped to the GDT selector for later 
use during interrupt time. Remember that even though you 
map the selector during Init, you can't use it during Init. 

Steve Mastrianni is an industry consultant specializing in 
device drivers and real-time applications for OS/2.  The 
author of "Writing OS/2 Device Drivers in C", Steve is 
regarded as one of the industry's leading experts in OS/2 
and OS/2 device drivers. 

o To access a GDT selector during Init, start a timer handler that 
  will be called within 32ms at ring 0; then perform the access. 

o If you need to post a 32-bit shared even semaphore at interrupt 
  time, which normally can't be done, allocate and arm a context 
  hook.  This hook will be called at task time and therefore will 
  be able to post the semaphore.