I did this in the '80s and it worked very well, I used it as a kind of software interrupt in a communications handler that had to handle both input and output at the same time from the same terminal. Unfortunately I don't rememver exactly how I did it, and I no longer have access to the source code. But I did find this in the HP Pascal for OpenVMS User manual, section 5.1.4 on page 5-6: Asynchronous system trap routines (ASTs) and RMS completion routines must have both the ASYNCHRONOUS and UNBOUND attributes. Because they are asynchronous, such routines can access only volatile variables, predeclared routines, and other asynchronous routines. The Language Reference mentions the User manual near the end in conjunction with volatile variables. You might also need to declare the variable p as volatile somewhere. The User manual is here: http://h71000.www7.hp.com/doc/82final/6140/aa-pxsnd-tk.pdf You would probably want to read chapter 5 in there. Hope this helps. /Jonas