Ethernet on PDP 11/70s

[jbn at wdl1.UUCP]

     We have had full IP/TCP via Ethernet on a PDP 11/70 since the beginning
of this year, but we run System V with our own version of 3COM's UNET
package using a 3COM Ethernet board.  It works quite well for us (Internet
host FORD-WDL2.ARPA is such a machine) but there are a few things to watch
for if you are bringing up something using that board.

    1.  You have to modify the PDP 11/70's MMU board with a soldering iron!
	3COM provides instructions on how to do this.  Still, many sites
	may not want to.  The problem is that the Ethernet board provides
	memory on the UNIBUS, a concept foreign to the PDP 11/70, and so
	one must change some jumpers on the MMU board to allow the 
	CPU to address addresses below the top 4K of the UNIBUS address
	space.

    2.  You need a good copy routine to copy the data to and from the
	Ethernet board's memory, out in high physical address space,
	and there is a nice convenient assembler routine in UNIX system
	V for doing exactly that, which has a nice little hole where it
	momentarily releases interrupt lockout while ineptly manipulating
	the PS.  This has been reported to AT&T, which proposes to fix it
	in a future release, perhaps.  The routine that does this is new
	in system V, and it follows the UNIX convention of not commenting
	the assembly code, but the people coding in the kernel nowadays 
	don't seem to be good enough to get away with that.  This little gem 
	cost us three months work to find.

    3.  You have to modify the kernel not to use all the Unibus mapping
	registers so that it doesn't try to use the space you took away
	to allow the Ethernet controller memory to occupy UNIBUS address
	space.

    There isn't a problem with UNIBUS contention with this card, since it
is shared-memory, not DMA, and the processor requests are of course preempted
by the NPR requests.  But the Interlan card probably integrates more easily
into UNIX, being a DMA card, which both UNIX and the PDP 11/70 understand
better.  On VAXen, both ways work fine, by the way.

We still need a PTY handler for system V, preferably a 4.1BSD conversion.
Anybody have one?