#+title: Aryadev's Virtual Machine (AVM)
#+author: Aryadev Chavali
#+date: 2023-10-15

A virtual machine in C11, stack oriented with a dynamic register.
Deals primarily in bytes, doesn't make assertions about typing and is
very simple to target.

This repository contains both a library ([[file:lib/][lib]]) to
(de)serialize bytecode and a program ([[file:vm/][vm]]) to execute
said bytecode.

* How to build
Requires =GNU make= and a compliant C11 compiler.  Code base has been
tested against =gcc= and =clang=, but given how the project has been
written without use of GNU'isms (that I'm aware of) it shouldn't be an
issue to compile using something like =tcc= or another compiler (look
at [[file:Makefile::CC=gcc][here]] to change the compiler used).

To build a release version simply run ~make all RELEASE=1~.  To build
a debug version run ~make all VERBOSE=<n>~ where n can be 0, 1 or 2
depending on how verbose you want logs to standard output to be.  This
will build:
+ [[file:lib/][instruction bytecode system]] which provides a shared
  library for serialising and deserialising bytecode
+ [[file:vm/][VM executable]] to execute bytecode

You may also build each component individually through the
corresponding recipe:
+ ~make lib~
+ ~make vm~
* Targeting the virtual machine
Link with the shared library =libavm.so= which should be located in
the =build= folder.  The general idea is to construct a ~prog_t~
structure, which consists of:
1) A program header with some essential properties of the program
   (start address, count, etc)
2) An array of type ~inst_t~, ordered instructions for execution

This structure may be executed in two ways.
** Compilation then separate execution
The ~prog_t~ structure along with a sufficiently sized buffer of bytes
(using ~prog_bytecode_size~ to get the size necessary) can be used to
call ~prog_write_bytecode~, which will populate the buffer with the
corresponding bytecode.

The buffer is written to some file then executed using the =avm=
executable.  This is the classical way I expect languages to target
the virtual machine.
** In memory virtual machine
This method is more involved, introducing the virtual machine runtime
into the program itself.  After constructing a ~prog_t~ structure, it
can be fit into a ~vm_t~ structure.  This ~vm_t~ structure also must
have a stack, heap and call stack (look at [[file:vm/main.c]] to see
this in practice). This structure can then be used with
~vm_execute_all~ to execute the program.

Note that this skips the serialising process (i.e. the /compilation/)
by utilising the runtime directly.  I could see this approach being
used when writing an interpreted language such as Lisp where code
should be executed immediately after parsing.  Furthermore,
introducing the runtime directly into the calling program gives much
greater control over parameters such as stack/heap size and step by
step execution which can be useful in dynamic contexts.  Furthermore,
the ~prog_t~ can still be compiled into bytecode whenever required.
* Related projects
[[https://github.com/aryadev-software/aal][Assembler]] program which
can compile an assembly-like language to bytecode.
* Lines of code
#+begin_src sh :results table :exports results
wc -lwc $(find vm/ lib/ -regex ".*\.[ch]\(pp\)?")
#+end_src

#+RESULTS:
|------------------+-------+-------+------------|
| File             | Lines | Words | Characters |
|------------------+-------+-------+------------|
| vm/runtime.h     |   266 |   699 |       7250 |
| vm/main.c        |   135 |   375 |       3448 |
| vm/runtime.c     |   802 |  2441 |      23634 |
| vm/struct.c      |   262 |   783 |       7050 |
| vm/struct.h      |    69 |   196 |       1531 |
| lib/inst.c       |   493 |  1215 |      13043 |
| lib/darr.h       |   149 |   709 |       4482 |
| lib/inst.h       |   248 |   519 |       4964 |
| lib/inst-macro.h |    71 |   281 |       2806 |
| lib/heap.h       |   125 |   453 |       3050 |
| lib/base.h       |   190 |   710 |       4633 |
| lib/heap.c       |    79 |   214 |       1647 |
| lib/base.c       |    61 |   226 |       1583 |
| lib/darr.c       |    76 |   219 |       1746 |
|------------------+-------+-------+------------|
| total            |  3026 |  9040 |      80867 |
|------------------+-------+-------+------------|