info-vax@ucbvax.ARPA (05/02/85)
From: decwrl!ios!richa@BERKELEY I have a question on Vax hardware operation. I am having trouble understanding the correct operation of disk drivers in handling asynchronous Attention Status interrupts. I have looked at the 4.1/4.2 up.c (Emulex Unibus SC21), hp.c (Emulex Massbus SC750), and Sys V hm.c (Massbus RM05). In each case, on occurance of an interrupt, the driver merely checks whether it had last started a transfer operation, if not then process Attention Status, else transfer is complete and check error bits. My problem is I can see no assurance that since a transfer was started, that the interrupt occuring must be due to transfer complete. It would be required that a Unibus controller which is issuing Bus Request to try to cause an interrupt (e.g. AS, due to Seek complete on another drive) but is not being Granted due to driver interrupt masking, would cancel or defer that interrupt request when it received a command (transfer). Do all Unibus controllers behave this way? This means the device drops Bus Request without having received a Bus Grant. It is also implied here that Bus Request/Grant must propagate all the way to the processor to be resolved against processor IPL. The Adapter may not do its own priority arbitration and queueing of vector(s) to be forwarded to the processor. This seems to cause interesting effects such as the processor executing a MTPR to mask interrupts possibly having to pause execution for a bus timeout period if a Grant had just been issued (the arbiter must wait for the incoming vector or timeout). Otherwise there is a possibility of accepting a vector when the processor will not take the interrupt (violating the sequence assumption above). The reason I am wondering if things really work this way is the whole structure seems rather delicate. Especially when I don't find it described anywhere. Even the excellent Emulex manuals don't say exactly when an asynchronous interrupt might be issued. Thanks for any help. Please respond directly to me by mail. {wjvax, decwrl, oliveb, qubix, durango, idi, apple, pesnta, sun}!ios!richa Rich Altmaier, Integrated Office Systems 2111 Grant Rd Los Altos, CA 94022