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In tor-Office l\Iemoranclum
To CSL, SSL Date May ILl, 1975
From P. Deutsch Location Palo Alto
Subject Status report on Alto Lisp Organization PAnC/CSL
)(EROX
File: (LPD)alispreport.momo (Bravo format)
Keywords: Alto, Lisp
There has been some recent confusion about the status of the
Alto Lisp project. This memo attempts to dispel that confusion by
presenting a brief history of the project, a summary of its present state,
and our current plans "for further work.
The project began in the Fall of 1872 as an outgrowth of some
preliminary work on the design of an instruction set which would be
tailored to compact representation and rapid execution of Lisp programs:
this work resulted from, and the results encouraged, a long-standing
interest of mine in Lisp systems for small machines. We decided to
proceed by first constructing a system with only an S-expression
interpreter (i.e. not using a new instruction set) by simply making a copy
of the PDP-I0 Interlisp implementation, converting it from tlfACRO-lO to
BCPL and running it on a Nova until Altos became avai lable; then adding
the ability to run code which had been compiled into the new instruction
set, but emulating the instruction set with a BCPL program; then writing
microcode for the Alto to perform the emulation at high speed. The first
instruction set design was completed in the Spring of 1973 and presented
at the 3IJCAI conference at Stanford in August 1973.
In early 1975 we successfully completed the last stage of
microcode implementation. It had become clear, meanwhile, that core
space would be at a great premium, and that the best way ~o gain space
for paging buffers would be to reduce the amount of BCPL code.
Especially in view of the fact that Lisp code using the specialized
instruction set and a microcoded emulator executes at speeds comparable
to BCPL code, we decided to embark on a major program of simplifying the
BCPL implementation code and replacing it by Lisp code. In this process
we W 0 u I d a p pro a chill U c h m0 r e c los ely the ide a 0 f a Lis p mac h i ne" \'t" h i c h
II
was entirely self-$ufficient (i.e. did not depend on a Nova/BCPL
emulator), gain flexibility which might allow us to incorporate some of the
interesting ideas from Smalltalk and Mesa, and make the system more
easily maintainable. We are currently pursuing this effort. Our first
major step, to replace the original S-expression interpreter (EVAL and
APPLY) by compiled Lisp code, is nearing completion.
Throughout the course of the Alto Lisp project there has been
conflict between the desire to produce a working system for the
(relatively) large and demanding Pare Lisp user community, on the one
hand, and the desire to experiment with system designs, on the other. It
has been my opInion that the project's chief justification has beon in the
latter activity, and consequently experimentation and redesign has
generally taken precedence over concerted efforts to produce a working
system. Recently, however, three factors have combined to give more
weight to producing a usable system: the arrival of more Altos and RAMs;
the increasing load on hlaxc, especially by the Understander project
(heavy Lisp users); the availability of a third person to work on the Lisp
project. Consequently, starting in June 1975 we will be able to devote
more resources to this goal without compromising our design
investigations.
The project to date has had research results in four areas.
(1) Instruction set design: we have experimented with two
significantly different instruction set designs for Lisp, and taken
extensive statistics on t11eir worth for encoding the present corpus of Lisp
programs.
(2) Processor design: having completed the Lisp microcode, we.
have had several useful comments to make on the design of the successor
to the Alto.
(3) Data encodings: we have collected numrirous interesting
statistics on the entropy of Lisp list structures and designed an efficient
encoding for these structures.
" (4) Storage management: we have developed an interesting new
class of storage reclamation (garbage collection) techniques.
We have publ ished papers on topics (1), (3), and (4).
The current status of the system may best be described as
"demonstrable" but not "usable". It is possible to sit down at an Alto
equipped with a RAM and certain minor ROM changes (of which there are
only two at present), insert an appropriately initialized disk pack, and
interact with a system that looks very much like Tenex Interlisp. (Willie
Sue or I will gladly give short demos.) However, there are some basic
elements missing (interrupt characters being the most noticeable), none of
Warren's assistance features are available (editor, prettyprint, file
package, break/trace, DWIM, CLISP, etc.), the system is 5 to 7 times
slower than a completely unloaded hlaxc (i.e. comparable to hlaxc with
load average of 5), and there is no garbage collector, so it is quite
possibl"e to run entirely out of space in the course of a few hours' work.
Our current work attacks all of these problems. Larry Masinter
will be working on bringing over Warren's code this summer: since it is
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