A Tiny Lisp for AltOS
I took a bit of a diversion over the last week or so when I wondered
how small a lisp interpreter I could write, and whether I could fit
that into one of the processors that AltOS runs on. It turns out, you
can write a tiny lisp interpreter that fits in about 25kB of ram with
a 3kB heap for dynamic data.
I decided to target our ChaosKey boards; they're tiny, and I've got a
lot of them. That processor offers 28kB of usable flash space (after
the 4kB boot loader) and 6kB of ram with the processor running at a
steaming 48MHz.
I'm not at all sure this is useful, but I always enjoy doing language
implementations, and this one presented some 'interesting' challenges:
Limited RAM. I don't have space to do a classic stop/copy
collector.
Limited stack. A simple lisp implementation uses the C stack for
all recursion in execution and memory collection. I don't have
enough ram for that.
Iterative Compacting Allocator
I'm betting someone has built one of these before, but I couldn't find
one, so I wrote my own.
The basic strategy is to walk the heap to find a subset of the active
objects which are allocated sequentially in memory with only unused
storage between them. These objects are then compacted in-place, and
then the heap is walked again to update all references to the moved
objects. Then, the process is restarted to find another subset and
move them.
By looking for these subsets starting at the bottom of the
heap, and working upwards towards the top, the whole heap can be
compacted into a contiguous chunk at the bottom of memory.
Allocation involves moving a pointer along at the top of active
memory; when it gets to the top of the heap, collect and see if
there's space now.
As always, the hardest part was to make sure all active memory was
tied down. The second hardest part was to make sure that all active
pointers were updated after any allocation, in case a collect moved
the underlying object. That was just bookkeeping, but did consume much
of the development time.
One additional trick was to terminate the recursion during heap
walking by flagging active cons cell locations in a global bitmap and
then walking that separately, iterating until that bitmap is
empty. Nested lambdas form another recursion which should probably get
the same approach, but I haven't done that yet.
An unexpected "benefit" of the tiny heap is that the collector gets
called a lot, so any referencing bugs will have a good chance of being
uncovered in even a short program execution.
ROM-able Lisp
Instead of implementing all of the language in C, I wanted to be able
to implement various pieces in Lisp itself. Because of the complex
nature of the evaluation process, adding things like 'let' or even
'defun' turn out to be dramatically simpler in Lisp. However, I didn't
want to consume bunches of precious RAM to hold these basic functions.
What I did was to create two heaps, one in ROM and the other in
RAM. References are be tagged as to which heap they're in.
16-bit Values
Lisp programs use a pile of references. Using a full 32 bits for each
one would mean having a lot less effective storage. So, instead, I use
an offset from the base of the heap. The top bit of the offset is used
to distinguish between the ROM heap and the RAM heap.
I needed a place to store type information, so I settled on using the
bottom two bits of the references. This allows for four direct type
values. One of these values is used to indicate an indirect type,
where the type is stored in the first byte of the object. The direct
types are:
| Value | Type |
| 0 | Cons cell |
| 1 | 14-bit int |
| 2 | String |
| 3 | Other |
With 2 tag bits, the allocator needs to work in 32-bit units as the
references couldn't point to individual bytes. Finally, I wanted 0 to
be nil, so I add four to the offsets within the heaps.
The result is that the ROM and RAM heaps can each cover up to
32k - 4 bytes.
Note that ints are not stored in the heap; instead they are immediate
values stored in 14 bits, providing a range of -8192 to 8191. One can
imagine wanting more range in ints at some point.
Heap-based Evaluator
A simple lisp implementation uses the fact that eval is re-entrant and
do the operation on the C stack:
val eval(val exprs) {
val vals;
while (exprs) {
vals = append(vals, eval(car(exprs)));
exprs = exprs->cdr;
}
return execute (car(vals), cdr(vals));
}
This makes things really simple and provides for a clean framework for
implementing various bits of lisp, including control flow and
macros. However, it rapidly consumes all of the available memory for a
stack, while also requiring separate bookkeeping for the in-use memory
in each frame.
I replaced this design with one which keeps the lisp stack on the
heap, and then performs eval with a state machine with all state
stored in global variables so that the memory manager can reference
them directly.
Each eval operation is performed in a separate 'stack' context, which
holds the entire eval state except for the current value, which lives
in a separate global variable and is used to pass values out of one
stack frame and into another. When the last stack context is finished,
the evaluation terminates and the value is returned to the caller.
There are nine states in the state machine, each of which is
implemented in a separate function, making the state machine a simple
matter of pulling the current state from the top of the stack and
invoking the associated function:
while (ao_lisp_stack) {
if (!(*evals[ao_lisp_stack->state])() || ao_lisp_exception) {
ao_lisp_stack_clear();
return AO_LISP_NIL;
}
}
return ao_lisp_v;
Because there's no C recursion involved, catching exceptions is a
simple matter of one test at this level.
Primitives like progn, while, cond and eval all take special magic in
the state machine to handle; getting all of that working took several
attempts before I found the simple loop shown above.
Lexical Scoping
The last time I did a lisp interpreter, I implemented dynamic
scoping. Atoms were all global and had values associated directly with
them. Evaluating a lambda started by saving all of the existing global
values for the parameter atoms and then binding the new values. When
finished, the previous values would be restored. This is almost
correct, but provides surprising results for things like:
> (setq baz 1)
> (def foo (lambda (bar) (+ baz bar)))
> (def bletch (lambda (baz) (foo baz)))
> (bletch 2)
4
The value that foo gets for 'baz' is 2 instead of 1 under dynamic
scoping, which most people find surprising. This time, I was
determined to use lexical scoping, and it turned out to be
surprisingly easy.
The first trick was to separate the atoms from their 'value'; each
atom can have a different value in different lexical scopes. So, each
lexical scope gets a 'frame' object, those contain the value for each
atom defined in that scope. There's a global scope which holds all of
the globally defined values (like baz, foo and bletch above). Each
frame points to its enclosing scope, so you can search upwards to find
the right value.
The second trick was to realize that the lexical scope of a lambda is
the scope in which the lambda itself is evaluated, and that the
evaluation of a lambda expression results in a 'function' object,
which contains the lambda and its enclosing scope:
> (def foo (lambda (bar bletch)
((lambda (baz)
(+ baz bar))
bletch)))
> (foo 2 3)
5
In this case, the inner lambda in foo can 'see' the value of bar from
the enclosing lambda. More subtly, even if the inner lambda were
executed multiple times, it would see the same baz, and could even
change it. This can be used to implement all kinds of craziness,
including generators:
> (defun make-inc (add)
((lambda (base)
(lambda ()
(progn
(setq base (+ base add))
base)))
0)
)
> (setq plus2 (make-inc 2))
> (plus2)
2
> (plus2)
4
The current implementation of each frame is a simple array of
atom/value pairs, with a reference to the parent frame to form the
full scope. There are dramatically faster implementations of this same
concept, but the goal here was small and simple.
A Tiny Allocator Optimization
With eval consuming heap space for stacks, frames and argument lists,
the interpreter was spending a lot of time in the collector. As a
simple optimization, I added some free lists for stack frames and cons
cells.
Stack frames are never referenced when they're finished, so they can
always go on the free list. Cons cells used to construct argument
lists for functions are usually free.
Builtin functions have a bit which indicates whether they might hold
on to a reference to the argument list. Interpreted lambdas can't get
the list while nlambdas, lexprs and macros do.
Each lambda execution creates a new frame, and while it would be
possible to discover if that frame 'escapes' the lambda, I decided to
not attempt to cache free ones yet.
Save and Restore
To make the lisp interpreter more useful in tiny computers, I added
the ability to save and restore the entire heap to flash. This
requires leaving enough space in the flash to preserve the heap,
further constraining the amount of flash available for the application.
Code
All of this code is in the 'lisp' branch of my AltOS repository:
AltOS
The lisp interpreter is independent from the rest of AltOS and could
be re-purposed for another embedded operating system. It runs fine on
ChaosKey hardware, and also on the
STM32F042 Nucleo-32 board
There's also a test framework which runs on Linux, and is how I
developed almost all of the code. That's in the src/test directory in
the above repository, and is called 'ao_lisp_test'.
Towers of Hanoi
Here's an implementation of the classic recursive Towers of Hanoi
game; it shows most of the current features of the language.
;
; Towers of Hanoi
;
; Copyright © 2016 Keith Packard <[email protected]>
;
; This program is free software; you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 2 of the License, or
; (at your option) any later version.
;
; This program is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
; General Public License for more details.
;
; ANSI control sequences
(defun move-to (col row)
(patom "\033[" row ";" col "H" nil)
)
(defun clear ()
(patom "\033[2J" nil)
)
(defun display-string (x y str)
(move-to x y)
(patom str)
)
; Here's the pieces to display
(setq stack '("*" "**" "***" "****" "*****" "******" "*******"))
(setq top (+ (length stack) 3))
;
; Here's all of the stacks of pieces
; This is generated when the program is run
;
(setq stacks nil)
; Display one stack, clearing any
; space above it
(defun display-stack (x y clear stack)
(cond ((= 0 clear)
(cond (stack (progn
(display-string x y (car stack))
(display-stack x (1+ y) 0 (cdr stack))
)
)
)
)
(t (progn
(display-string x y " ")
(display-stack x (1+ y) (1- clear) stack)
)
)
)
)
; This should probably be included in the rom image...
(defun length (list)
(cond (list (1+ (length (cdr list))))
(0)
)
)
; Position of the top of the stack on the screen
; Shorter stacks start further down the screen
(defun stack-pos (y stack)
(- y (length stack))
)
; Display all of the stacks, spaced 20 columns apart
(defun display-stacks (x y stacks)
(cond (stacks (progn
(display-stack x 0 (stack-pos y (car stacks)) (car stacks))
(display-stacks (+ x 20) y (cdr stacks)))
)
)
)
; Display all of the stacks, then move the cursor
; out of the way and flush the output
(defun display ()
(display-stacks 0 top stacks)
(move-to 1 21)
(flush)
)
; Reset stacks to the starting state, with
; all of the pieces in the first stack and the
; other two empty
(defun reset-stacks ()
(setq stacks (list stack nil nil))
(length stack)
)
; more functions which could usefully
; be in the rom image
(defun min (a b)
(cond ((< a b) a)
(b)
)
)
(defun nth (list n)
(cond ((= n 0) (car list))
((nth (cdr list) (1- n)))
)
)
; Replace a stack in the list of stacks
; with a new value
(defun replace (list pos member)
(cond ((= pos 0) (cons member (cdr list)))
((cons (car list) (replace (cdr list) (1- pos) member)))
)
)
; Move a piece from the top of one stack
; to the top of another
(defun move-piece (from to)
(let ((from-stack (nth stacks from))
(to-stack (nth stacks to))
(piece (car from-stack)))
(setq from-stack (cdr from-stack))
(setq to-stack (cons piece to-stack))
(setq stacks (replace stacks from from-stack))
(setq stacks (replace stacks to to-stack))
(display)
(delay 100)
)
)
; The implementation of the game
(defun _hanoi (n from to use)
(cond ((= 1 n)
(progn
(move-piece from to)
nil)
)
(t
(progn
(_hanoi (1- n) from use to)
(_hanoi 1 from to use)
(_hanoi (1- n) use to from)
)
)
)
)
; A pretty interface which
; resets the state of the game,
; clears the screen and runs
; the program
(defun hanoi ()
(setq len (reset-stacks))
(clear)
(_hanoi len 0 1 2)
)
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