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Update "A Virtual Machine for Guile"

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* doc/ref/vm.texi (A Virtual Machine for Guile): Update.
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Andy Wingo 2018-09-28 12:14:09 +02:00
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@node A Virtual Machine for Guile
@section A Virtual Machine for Guile
Guile has both an interpreter and a compiler. To a user, the difference
is transparent---interpreted and compiled procedures can call each other
as they please.
Enough about data---how does Guile run code?
The difference is that the compiler creates and interprets bytecode
for a custom virtual machine, instead of interpreting the
S-expressions directly. Loading and running compiled code is faster
than loading and running source code.
Computer systems consist of towers of languages, each level defining a
higher level language and implemented using lower-level facilities.
Sometimes these languages are implemented using interpreters: programs
that run along-side the program being interpreted, dynamically
translating the high-level code to low-level code. Sometimes these
languages are implemented using compilers: programs that translate
high-level programs to equivalent low-level code, and pass on that
low-level code to the next level down. Each of these levels can be
throught to be virtual machines; they offer programs an abstract machine
on which to run.
The virtual machine that does the bytecode interpretation is a part of
Guile itself. This section describes the nature of Guile's virtual
machine.
Guile implements a number of interpreters and compilers on different
language levels. For example, there is an interpreter for the Scheme
language that is itself implemented as a Scheme program compiled to a
bytecode for a low-level virtual machine shipped with Guile. That
virtual machine is implemented by both an interpreter---a C program that
interprets the bytecodes---and a compiler---a C program that dynamically
translates bytecode programs to native machine code.
Even the lowest-level machine code can be thought to be interpreted by
the CPU, and indeed is often implemented by compiling machine
instructions to ``micro-operations''.
This section describes the language implemented by Guile's bytecode
virtual machine, as well as some examples of translations of Scheme
programs to Guile's VM.
@menu
* Why a VM?::