Brian Hechinger wrote: On Mon, Oct 19, 2009 at 12:31:41PM -0400, John Wilson wrote: its behalf instead of assuming the CPU card is The Decider. I'm trying to picture where per-CPU peripherals go in that case so the I/O page is still private but DMA isn't ... ow my head! If you are talking multiple CPUs on a single QBus is there much benefit to assigning peripherals to CPUs other than CPU0? That wouldn't disallow you to do DMA from another CPU to that peripherals I wouldn't think. You don't do DMA from CPU to a device. Remember what DMA stands for? :-) The benefit of assigning different devices to different CPUs are that you can balance the CPU load of the different CPUs. For each I/O operation, the CPU have to run through the driver, which takes a non-zero amount of time. And the same is true when the I/O completes. Hmm, I realized one thing, though. With a common Qbus, all interrupts will probably go to the bus master arbiter, which means that all devices actually must be attached to CPA, except such devices that are local to each CPU, such as the console terminal. And in RSX speak, the CPUs are called CPA, CPB, CPC and CPD. Here is how it looks on MIM:: .con dis full att for sys SYS SYS Online,Accpath PDP-11/74mP, EIS,UNIBUS_Map,D-Space,SWR,Cache,FPP, Clock=KW11-L, $TKPS=50., $TTPRM=000002 .con dis full att for cp CPA CPA Online,Accpath Cache_control=000001, Timer=Off, Alarm=Off CPB CPB Offline,Accpath Cache_control=000001, Timer=Off, Alarm=Off CPC CPC Online,Accpath Cache_control=000001, Timer=Off, Alarm=Off CPD CPD Offline,Accpath Cache_control=000001, Timer=Off, Alarm=Off .con dis full att for du DUA CPA Online,Accpath,Driver Csr=172150, Vector=000154, Pri=000005, Urm=000001 DU0: CPA DUA0: Online,Accpath,Context,Driver,Type=RZ29 DU1: CPA DUA1: Online,Accpath,Driver,Type=RD53 DU2: CPA DUA2: Online,Accpath,Driver,Type=RD52 DU3: CPA DUA3: Offline,Accpath,Driver DU4: Online,Accpath,Context,Driver,Type=VRA82 . Notice that CPA and CPC are online. And DUA is attached to CPA, and so on... Johnny