The following compatibility's requirements should be considered before running an application in multi-processor mode:

• An application or associated subsystem must not use the 'INC' instruction as a semaphore without prepending a 'LOCK' prefix. On a UniProcessor (UP) system this instruction can be used as a high-performance semaphore without calling any other operating system service if the semaphore is free and when the semaphore is clear and there are no waiters for the semaphore. Because the INC instruction cannot be interrupted once started, and because the results would be stored in the flags register which are per thread, then it could be used safely as a semaphore.

  In an OS/2 for SMP environment, this technique will not work because it is possible that two or more threads could be executing the same INC instruction, receiving the same results in each processor's or thread's flag register and thinking that they have the semaphore.

• A 486 or greater instruction such as CMPXCHG has the same problem as above if a 'LOCK' prefix is not prepended before the instruction.

• An application or associated subsystem which relies on priorities to guarantee execution of it's threads within a process will not work in OS/2 for SMP. For example, an application may have a time-critical and an idle thread, and may assume that while the time-critical thread is executing the idle thread will not get any execution time unless the time-critical thread explicitly yields the CPU. In an OS/2 for SMP environment it is possible that both the time-critical and the idle threads are executing simultaneously on different processors.

The above compatibility requirements apply only to multi-threaded applications, and therefore do no apply to DOS and WINOS2 applications. However, you are strongly encouraged to write 32-bit multi-threaded applications for better performance and portability on OS/2 for SMP.

Given the possibility that some set of applications may use one of these techniques, OS/2 for SMP Version 3 provides a mechanism whereby these multi-threaded applications can execute in UP mode. Only one thread of that process would be allowed to execute at any given time. That thread could execute on any one of the processors. MARKEXE can be used to mark the EXE file as uniprocessor only. OS/2 forces the process to run in the uniprocessor mode when the loader detects that the EXE file has been marked as uniprocessor only.

Note: OS/2 Warp Version 3 (non-SMP) still ships the EXECMODE utility.