[TUHS] Unix with TCP/IP for small PDP-11s

Paul Ruizendaal pnr at planet.nl
Mon May 22 19:28:19 AEST 2017

I had somehow convinced myself that Ultrix-11 needed split I/D, but indeed it does not:

# file unix
unix:   (0450) pure overlay executable not stripped
# size unix
14784+(8192,8000,8064,8000,8064,8128,8000,7808,7936,7936,7680,7360,1344)+3524+13500 = 31808b = 076100b (111296 total text)

With only 16KB of permanent kernel there will be a lot of overlay switching. I'm not entirely sure why bss could not be 1KB smaller, enabling 8KB more of permanent kernel. The loss of performance from 2 disk buffers less really outweighed less overlay switching?

If I understand correctly, the network code continuously switches around segment 5 to access the right mbuf.

According to the notes in the TUHS archive (http://www.tuhs.org/Archive/Distributions/DEC/Ultrix-11/Fred-Ultrix3/setup-3.1.txt), running Ultrix-11 with networking on a 11/40 class machine is borderline workable:

"I have personally tested it on a 23+, 53 and 83.  I know it runs
fine on the 73.  The smaller machines (34, 40 etc) should work
akin to the 23, meaning using overlays and be very tight on RAM
for the drivers.  TCP/IP is a biiiiig load for those systems!"

There are two other routes to TCP/IP on a PDP11 without split I/D:
- 3COM's TCP/IP package (initially an overlay over V7, soon after also over 2BSD); I believe the source to this is lost.
- DCEC's adaptation of the Wingfield TCP/IP library, designed to work with V6. It is mostly a user space daemon, but requires some kernel enhancements. The Wingfield code is in the TUHS archive, but that version has a modified V6 kernel that also supports NCP networking and requires split I/D. If used with a minimally enhanced V6 kernel, it would easily fit in 64KB, without overlays.

Note that these last two options have very different API's and would not be so easy to work with.


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