[HECnet] PDXVAX - 40MB Memory - VAXstation 4000/VLC?

[Johnny Billquist]

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