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Kosheri

Kosheri is a stack-based virtual machine. Here's what it offers:

  • Garbage-collection, tagged tuples, dictionaries, function values, coroutines, lightweight processes, textual and binary serialization, and an assembler and disassembler. See "Features", below, for more information.
  • A really small code footprint. On Ubuntu 10.04 LTS, a minimal VM executor compiles to an executable less than 16K in size.
  • A clean, almost pedantic implementation. See "Implementation", below.
  • Very few build dependencies. See "Build requirements", below.
  • An extremely orthogonal architecture. See "Architecture", below.
  • A reasonably efficient implementation. See "Performance", below.

Features

  • Ability to manipulate values of boolean, 32-bit integer, process reference ("pid"), immutable string ("symbol"), and tagged tuple type.
  • Support for manipulating certain tagged tuple types: dictionaries, function values, and activation records.
  • Support for closures and coroutines via appropriate use of activation records. (ARs retain some state after being called; if this is not cleared, they can be continued.)
  • A simple, mark-and-sweep garbage collector (for tuples and symbols; everything else lives on the stack.)
  • Concurrent operation. Each lightweight process can be a VM process or a native process. Native processes are used to implement interfaces to the rest of the world. Multitasking is pre-emptive for VM processes, and co-operative for native processes. Concurrency is implemented in the VM; system threads and processes are not used. Interprocess communication is done with Erlang-style messaging to processes' mailboxes.

Implementation

The code is generally written in BSD style(9). It compiles under -ansi -pedantic with almost all gcc warnings enabled and treated as errors. The core interfaces are constified. asserts are plentiful. I've tried to limit the amount of typecasting used in the code, but of course there are some places where it is necessary.

Functions are often written in a straightforward, almost pedantic fashion; there are of course some exceptions here too, like the direct threading support.

Build requirements

Only the following tools are required to build the VM and its tools:

  • an ANSI C compiler (default is gcc) and linker (ranlib, ar)
  • GNU Make

I have vague plans to remove even the dependency on GNU Make (the code is so small that rebuilding the whole thing is not a huge burden, so it could be done with a shell script or a custom build tool.)

Even libc is not a strict requirement for building the VM; a few functions from libc that the code uses are implemented independently in the code. The standalone target builds the system with -nostdlib. However, while it builds it currently does not link, as it requires some memory allocator to link to, and it doesn't have one... yet.

Architecture

From inside the virtual machine, it's a truism that "everything's a value, and every value that's not an atomic value is a tagged tuple." The latter is true even for VM code and activation records and dictionaries.

Moreover, this truism largely holds in the C code as well. In many places in the assembler and elsewhere, we don't just use C structs to hold our data, we use struct values.

Every value has defined external representations, in human-readable and compact binary forms, which can be generated and parsed.

The rest of this section is kind of scattered...

Some more notes on how the runtime is written in C...

Every C function in the Kosheri runtime is run in some environment.

We try to assume as little about the surrounding environment (e.g., OS) as possible, and to present the world to these functions in as Kosheri- like a way as possible.

Maybe not every function, but lots of functions. The ones that really matter. Top-level functions especially -- (the ones that would be command-line utilities in a Unix system.)

Every such function is passed a runtime environment. This runtime environment consist of:

-- arguments that were passed to the function in the form of a Kosheri dictionary (value_dict). The function may also have a data structure associated with it (the "argdecl") that facilitates checking these arguments for syntactic correctness and automatically returning an error if they don't meet those checks. (this part's not done yet)

-- environment variables that were made available to the function. (This is possibly not OK. Environment variables have a way of hanging around that arguments don't. Some kind of adapter is called for that translates (relevant) env vars to arguments first.)

-- streams available to the function, in the form of Kosheri processes. The function should not assume the presence of a particular stream, or at least, a function which does assume the presence of a particular stream should not be assumed to always work. The function should also not assume that the streams that are made available to it behave in a particular way (for example, that a stream is directed to a vt100 terminal.) Instead it should look for a stream that abstractly presents the behaviour that it wants.

Performance

Kosheri was designed to be reasonably efficient, for a virtual machine without a JIT compiler.

The design decisions for performance are kind of all over the place, sometimes being extremely optimized, sometimes sacrificing pure performance for flexibility.

  • "Direct threading" is possible with C compilers that support it, such as gcc. This basically optimizes the main instruction- selection switch into a computed goto.

  • The compiled VM is small, really small. This means it can usually fit entirely in the cache, and stay there. This can sometimes result in a significant performance benefit.

On the other hand...

  • The orthogonality of "everything is a tuple" extends far and wide. All values are 8 bytes, including VM instructions. VM code size could be reduced by packing 2 or 4 instructions into that 8 bytes, but we don't do that yet.

  • Input and output are modelled as processes, which means there is some small overhead (to pack and unpack a message) added to I/O; but I/O is I/O-bound anyway, so this is probably not something to worry about.

Tour of the distribution

bin/

Where compiled binaries go.

eg/

Example assembly language files.

lib/

Where compiled libraries (static and dynamic) go.

src/

Source code for the virtual machine and tools.

src/build/

Source code for tools used during build-time, and not thereafter.

tests/

Tests.

Tour of the source

assemble.c

Main program for the assembler.

chain.c
chain.h

Utilities to accumulate a linked list of values, then turn them into a tuple when accumulation has finished.

cmdline.c
cmdline.h

Harness which translates command-line arguments to dictionaries, and otherwise provides facilities so that programs can be written in a "Kosheri view" of the outside world.

disasm.c

Main program for the disassembler.

discern.c
discern.h

Routines to parse human-readable representation of tuples into the internal format.

file.c
file.h

Native implementation of processes supporting the stream-like interface and which are backed with C's stdio.

freeze.c

Main program for the human-readable-value-to-compact-binary serializer.

gen.c
gen.h

Routines for generating VM instructions (used by assembler, but would also be useful for compilers targeting this VM.)

geninstr.c

A build tool which generates instrtab.c and instrenum.h from vm.c.

instrtab.h

Header file for the generated instrtab.c.

lib.c
lib.h

Clean-room re-implementations of the few libc-supplied functions which the VM uses.

load.c
load.h

Routines to parse (unserialize) the compact binary representation of values.

portray.c
portray.h

Routines to format values into a human-readable representation.

process.c
process.h

Routines for communicating and switching between co-operative lightweight concurrent processes.

render.c
render.h

Routines for rendering terms to file-like processes.

report.c
report.h

Routines for reporting errors, successes, etc. for the assembler, disassembler, freezer and thawer.

run.c

Main program for the VM runner. Takes a VM program in the compact binary representation and executes it.

save.c
save.h

Routines to generate (serialize) the compact binary representation of values.

scan.c
scan.h

Lexical scanner, used by the assembler, and by the discern routines. Fairly general, so could also be used to parse a language being compiled to the VM.

stream.c
stream.h

Routines to communicate with processes which support the stream-like interface; you can read from them, write to them, check for eof, and close them.

thaw.c

Main program for the compact-binary-to-human-readable-value unserializer.

value.c
value.h

Data structure (and associated functions) which internally represents values, including atomic values (booleans, integers, process id's, direct-threaded opcodes) and structured values (tagged tuples and symbols.)

vm.c
vm.h

The virtual machine implementation itself.

vmproc.c
vmproc.h

The implementation of lightweight processes which uses the virtual machine to execute the process.

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