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Under the hood

This chapter describes some internals of LispMe. You don't need this to work with LispMe, but some people asked me about the implementation, so here you are:


The latest version 3.2 of LispMe was developed with GCC 2.95.2-kgpd, prc-tools 2.0, PilRC v2.5b3, and the PalmOS 4.0 SDK on a PC running Windows XP and tested with Poser 3.2 and the PalmOS5 Simulator.

Altogether LispMe consists of about 26000 lines of highly compact C code. All error handling is done using Pilot's ErrCatch and ErrThrow macros/functions and not a single function from the C library has been used.

The trick I found to create executables >32k code with GCC is fortunately no more necessary prc-tools 2.0, since multi-segment applications are supported now. If you're interested in these topics, have a look here.

The implementation techniques of LispMe (V3.0) have been presented at the Scheme workshop of the PLI 2001 conference in Florence in September 2001. A detailed article is available as DVI (80 kB) or as compressed PostScript (99 kB).

SECD machine model

LispMe is based on the SECD virtual machine described in
Peter Henderson
Functional Programming - Application and Implementation
Prentice Hall International
ISBN 0-13-331579-7
The name stems from the 4 main registers this machine operates on:
Sstackused to hold intermediate values while evaluating an expression
Eenvironmentused to hold the current environment
Ccontrolthe machine-language program being executed
Ddumpa stack the other registers are pushed on when a new function is called

SECD virtual machine

In fact, all registers hold pointers to list structures in the heap, so for example going to the next machine instruction is accomplished by an assignment of the form C = cdr(C);

The virtual machine currently has 93 different instructions (much less than prior versions, since many builtin procedures have been re-implemented as native C calls). The main extensions to Henderson's machine are

The runtime environment is represented as a list of frames, where each frame is a list of values. Variables are always accessed by lexical adresses (a pair indicating the frame and the offset of the variable). This is exactly the form you see when tapping the names button. In fact, the first step in each evaluation is assigning the value list to the E register.

SECD compiler

In contrast to Henderson's approach, the compiler is not bootstrapped, it's implemented in C like the rest of LispMe. The reasons for that are compilation speed and saving heap memory for user programs.

A LispMe expression is compiled by traversing its tree structure and consing up a list of machine instructions backwards to avoid using append. There's a simple peephole optimizer mainly used for removing unnecessary stack operations and for propagating returns through conditionals to provide proper tail recursion elimination.

There's also a compile time environment (represented as a list of lists, mirroring the structure of the runtime environment) to determine lexical adresses for all variables. You can view the SECD code of a closure with disasm.

The compiler is extensible by registering keyword/C-function pairs.

Macros are expanded by calling the VM with the expansion procedure and the original source expression as argument.

Parsing expressions

Parsing is divided into 2 steps:
  1. The scanner groups characters into tokens by using a finite automaton consisting of 28 states. Most of the states deal with real and complex numbers and escape sequences in strings.
  2. The parser is recursive-descent
Scanning and formatting floating point was particularly nasty, as the Pilot ROM routines FlpFToA and FlpAToF are unusable (limited precision, limited exponent range, no error support), so all this stuff had to be written from scratch.

Memory management

Each LispMe session is kept in a separate database bearing the same name as the session. All LispMe session databases use creator ID fbLM so that the total memory used by them is displayed correctly in the memory application.

In each session database, at least 7 records are used:

Starting with version 1.7, LispMe mo more uses the dynamic heap for its memory, instead it works directly on DB blocks, gaining write access via MemSemaphoreReserve. This is a much discussed technique, as accessing memory out of its bounds can overwrite other databases and even require a hard reset. On the other hand, LispMe has been out for some months now and several hundred people have tested it and I never got a report of an "invalid pointer access". Furthermore, there's no way in LispMe to create an invalid pointer by will or accident (like in other Scheme implementations, too). All pointers are created in the memory subsystem which almost certainly is bugfree. So I finally decided to use this technique to make a bigger heap available on all Pilots.

Garbage collection

LispMe uses mark/scan garbage collector for heap cells and floating point cells. The reasons for preferring mark/scan to copying garbage collection are: All in all, a typical garbage collection of 16k heap takes about 0.2 seconds.

Foreign types can register garbage collection hooks for finalization and marking other referenced cells.

Strings, vectors, and bigints are garbage collected without having to move their actual contents. Instead, DmAttachRecord() is used to swap the memoryblock/record number association and unused records are moved to the end of the workspace database where they can be deleted without having to shift intermediate records.