I think this is it, unless I'm mistaken. int TestAndSet(int* pTargetAddress, int nValue) { __asm { mov edx, dword ptr [pTargetAddress] mov eax, nValue xchg eax, dword ptr [edx] } }

Problem solved: I did get a very usefull reply from Mr Miller from Manaware. --- NT can do up to 256. The card manufacturer has not set up the registry and the 'DosNames' for the devices correctly. There might be other names for those serial ports, but the "COM" file names need special aliases created in the registry. Some manufacturers fail to add those when they install or initially load the device driver. Use RegEdit and look for 'SerialPort' or 'Serial Port' and you might find their real names. So the answer to your quesitons is "Yes, you are right, and the card manufacturer still has some work to do on their device drivers." --- The problem was solved by using "\\.\COMxx" In stead of opening "COMxx" (where xx is is the number)