The Legos systems in the CS machine rooms were in what we referred to as the
North and South clusters.  Each had a "mother" Interdata 70 and about four
"baby" Interdata 74s.  Each baby had a teletype (Olivetti) which was
switchable, connecting either to the baby or to the mother. We prepared our
programming exercises by editing them on the mother (with its disk file
system), assembling them there too, and then downloading them into the baby to
run.

In the mothers, the editor was permanently resident, together with ISYS
itself, so that everybody could edit at the same time, and there was still
enough memory left to run the assembler, which was not resident, but loaded
on demand, as were a number of other utilities such as the disk copying program
for backups, and "compac" the de-fragmenter.  Only one person could use the
assembler at a time.  Often there was a queue, in which case it was considered
rude to generate a printer listing directly to the (slow) printer instead of
into a file which could be printed later without hogging memory. 

On the PDP9/15 the editor used all the spare memory as its editing buffer, so
you could hold as much as possible of a large file in memory.  When the
buffer got too full, the top part would get irretrievably expelled to the
output file, after which you could not "move back" beyond that point into the
expelled bit (you therefore had to plan your edits if you wanted to avoid
having to close and re-open).  The ISYS editor, on the other hand, confined
its working area to a much smaller window, basically by shuffling a bunch of
disk buffers up and down the output file as the user moved around, thus
exploiting the luxury of random access.

When PSR's compiler came along later, it too ran in the mother, each subsequent
pass overlaying the previous. The control program which called the passes one
after each other used, if I remember right, a trick whereby it relocated itself
into a disk buffer, and then ran there, freeing up the main memory resource
before loading each pass.


See also http://www.gtoal/history.dcs.ed.ac.uk/archive/os/interdata74-basic/