Writing Device Drivers - A Brief Look at OS/2 SMP

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by Steve Mastrianni

Included on IBM Developer Connection News Volume 5

A Brief Look at OS/2 SMP

The OS/2 SMP architecture is actually quite simple. Only one copy of OS/2 is ever running at one time no matter how many processors are present, so there's no need to synchronize multiple copies of the operating system. Access to the operating system is sychronized and serialized using processor spinlocks.

A spinlock is nothing more than a small section of code that executes in a tight loop until a variable is cleared. If you've ever had a bug in your OS/2 device driver where your code executed in a loop at ring 0, you know exactly what a spinlock is. You couldn't interrupt that loop with the debug kernel, and you usually had to power off and power on to reboot. OS/2 SMP spinlocks work the same way.

Platform Specific Drivers

OS/2 SMP provides a level of hardware abstraction using the Platform Specific Driver, or PSD. Like a device driver that shields an application from the specifics of a particular device, the PSD isolates the OS/2 kernel from the specific processor hardware. To provide this layer of abstraction, the PSD exports generic functions that the kernel can call. These functions are translated by the PSD into operations that are specific to the hardware platform.

PSDs are special flat-model device drivers, and are actually 32-bit DLLs loaded with the DEVICE= statement in the CONFIG.SYS file. Like OS/2 ADDs, they must conform to the 8.3 naming convention, and the name must not contain any drive or path information.

OS/2 SMP requires a PSD for system initialization. The system will display an error message if a valid PSD for the current platform cannot be installed. If any step does not complete successfully, the system initialization process will stop, and an error message will be displayed.

New OS/2 SMP DevHlps

The following new physical DevHlps were introduced with OS/2 SMP.

DevHlp Function Code Description
CreateSpinLock 0x6f Create a subsystem spinlock
FreeSpinLock 0x70 Free a subsystem spinlock
AcquireSpinLock 0x71 Acquire a spinlock
ReleaseSpinLock 0x72 Release a spin lock
PortIO 0x76 Processor-independent port I/O³
SetIRQMask 0x77 Set/Unset an IRQ mask
GetIRQMask 0x78 Get state of current IRQ mask

An additonal virtual DevHlp, VDHPortIO, was also added.

New OS/2 for SMP V2.11 APIs

OS/2 SMP provides a new set of APIs to enable applications to be optimized for the SMP environment. The following is a list of the APIs and their corresponding function. This information is subject to change.

API Function
DosCreatSpinLock Create a subsystem spinlock
DosFreeSpinLock Free a subsystem spinlock
DosAcquireSpinLock Acquire a subsystem spinlock
DosReleaseSpinLock Release a subsystem spinlock
DosAllocThreadLocalMemory Allocate memory for a thread
DosFreeThreadLocalMemory Free memory allocated for a thread
DosQuerySysInfo (changed) Return system information

More information on writing device drivers for OS/2 SMP can be found on the OS/2 for SMP CD-ROM and in the third edition of "Writing OS/2 2.x Device Drivers in C", scheduled for release later this year.

TIP: Writing an OS/2 device driver?

Get ready for OS/2 for PowerPC by layering your device driver similar to the ADD model, separating the hardware specific layer form the software layer. This will provide you with the least work when moving your drivers to OS/2 for PowerPC.

TIP: Still writing your driver in assembly language?

Try to write them in C from now on. The current direction in operating systems is to make most device drivers hardware-independent and to have them run as an applications rather than kernel tasks. Writing your driver in a high-level language will make the move to systems such as OS/2 for PowerPC much easier.

TIP: Display a device driver header from the kernel debugger.

The device driver header is always at the start of the device driver's first data segment. The device driver's header contains the device name, the code and data segment selectors, the IDC entry point address and a pointer to the next device driver header.


To display the device driver header, enter .d dev ds:0 at the debugger command prompt. The kernel debugger displays the device driver's header with text strings defining each field. An example:

##.d dev ds:0
        DevNext: 06f0:ffff
        DevAttr: 8940
        DevInt: 0000
        DevName: TESTCFG$
        DevProtCS: 06f8
        DevProtDS: 06f0
        DevRealCS: 0000
        DevRealDS: 0000

Reprint Courtesy of International Business Machines Corporation, © International Business Machines Corporation