A very interesting question (to me, particularly, see below). I don't think any of the Unix papers answer this question? Indeed, and I've seen operating systems (e.g. a real-time PDP-11 OS I worked with a lot called MOS) that did that. I actually did add asynchronous I/O to V6 UNIX, for use with very early Internet networking software being done at MIT (in a user process). Actually, it wasn't just asynchronous, it was _raw_ asynchronous I/O! (The networking device was DMA, and the s/w did DMA directly into the user process' memory.) The code also allowed more than one outstanding I/O request, too. (So the input could be re-enabled on the device ASAP, without having to wake up a process, have it run, do a new read call, etc.) We didn't redo the whole Unix I/O system, to support/use asyn I/O throughout, though; I just kind of warted it onto the side. (IIRC, it notified the user process via a signal that the I/O had completed; the user software then had to do an sgtty() call to get the transfer status, size, etc.) Anyway, back to the original topic: I don't want to speculate (although I could :-); perhaps someone who was around 'back then' can offer some insight? If not, time for speculation! :-) Noel