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Okay, now it's somehow working:

  % guile
  guile> (use-modules (vm vm))
  guile> (use-modules (vm compile))
  guile> (define (vm-repl)
           (display "vm> ")
           (display (vm-run (make-vm) (compile (read))))
  guile> (vm-repl)
  vm> (+ 1 2)
  vm> (define (make-counter) (let ((n 0)) (lambda () (set! n (1+ n)) n)))
  vm> (define counter (make-counter))
  vm> (counter)
  vm> (counter)
  vm> (counter)

For those of you who might be interested, this is a snapshot of my
Guile VM implementation (requires the latest Guile and GOOPS):

There are lots of bugs and it's totally useless.  Non-GCC support is
not included yet.  Hopefully, I'll release a better version soon..

And this is a rough design document.  I'd appreciate it if someone
could advice me about a better design idea and terminology.

Keisuke Nishida

Guile VM Specification					-*- outline -*-
Updated: $Date: 2000/07/29 03:04:58 $

* Introduction

A Guile VM has a set of registers and its own stack memory.  Guile may
have more than one VM's.  Each VM may execute at most one program at a
time.  Guile VM is a CISC system so designed as to execute Scheme and
other languages efficiently.

** Registers

  pc - Program counter    ;; ip (instruction poiner) is better?
  sp - Stack pointer
  bp - Base pointer
  ac - Accumulator

** Engine

A VM may have one of three engines: reckless, regular, or debugging.
Reckless engine is fastest but dangerous.  Regular engine is normally
fail-safe and reasonably fast.  Debugging engine is safest and
functional but very slow.

** Memory

Stack is the only memory that each VM owns.  The other memory is shared
memory that is shared among every VM and other part of Guile.

** Program

A VM program consists of a bytecode that is executed and an environment
in which execution is done.  Each program is allocated in the shared
memory and may be executed by any VM.  A program may call other programs
within a VM.

** Instruction

Guile VM has dozens of system instructions and (possibly) hundreds of
functional instructions.  Some Scheme procedures such as cons and car
are implemented as VM's builtin functions, which are very efficient.
Other procedures defined outside of the VM are also considered as VM's
functional features, since they do not change the state of VM.
Procedures defined within the VM are called subprograms.

Most instructions deal with the accumulator (ac).  The VM stores all
results from functions in ac, instead of pushing them into the stack.
I'm not sure whether this is a good thing or not.

* Variable Management

A program may have access to local variables, external variables, and
top-level variables.

** Local/external variables

A stack is logically divided into several blocks during execution.  A
"block" is such a unit that maintains local variables and dynamic chain.
A "frame" is an upper level unit that maintains subprogram calls.

  dynamic |          |  |        |
    chain +==========+  -        =
        | |local vars|  |        |
        `-|block data|  | block  |
         /|frame data|  |        |
        | +----------+  -        |
        | |local vars|  |        | frame
        `-|block data|  |        |
         /+----------+  -        |
        | |local vars|  |        |
        `-|block data|  |        |
         /+==========+  -        =
        | |local vars|  |        |
        `-|block data|  |        |
         /|frame data|  |        |
        | +----------+  -        |
        | |          |  |        |

The first block of each frame may look like this:

       Address  Data
       -------  ----
       xxx0028  Local variable 2
       xxx0024  Local variable 1
  bp ->xxx0020  Local variable 0
       xxx001c  Local link       (block data)
       xxx0018  External link    (block data)
       xxx0014  Stack pointer    (block data)
       xxx0010  Return address   (frame data)
       xxx000c  Parent program   (frame data)

The base pointer (bp) always points to the lowest address of local
variables of the recent block.  Local variables are referred as "bp[n]".
The local link field has a pointer to the dynamic parent of the block.
The parent's variables are referred as "bp[-1][n]", and grandparent's
are "bp[-1][-1][n]".  Thus, any local variable is represented by its
depth and offset from the current bp.

A variable may be "external", which is allocated in the shared memory.
The external link field of a block has a pointer to such a variable set,
which I call "fragment" (what should I call?).  A fragment has a set of
variables and its own chain.

    local                    external
    chain|     |              chain
       | +-----+     .--------, |
        /+-----+  |  `--------'\,
       `-|block|--'             |
        /+-----+     .--------, |
         +-----+     `--------'
         |     |

An external variable is referred as "bp[-2]->variables[n]" or
"bp[-2]->link->...->variables[n]".  This is also represented by a pair
of depth and offset.  At any point of execution, the value of bp
determines the current local link and external link, and thus the
current environment of a program.

Other data fields are described later.

** Top-level variables

Guile VM uses the same top-level variables as the regular Guile.  A
program may have direct access to vcells.  Currently this is done by
calling scm_intern0, but a program is possible to have any top-level
environment defined by the current module.

*** Scheme and VM variable

Let's think about the following Scheme code as an example:

  (define (foo a)
    (lambda (b) (list foo a b)))

In the lambda expression, "foo" is a top-level variable, "a" is an
external variable, and "b" is a local variable.

When a VM executes foo, it allocates a block for "a".  Since "a" may be
externally referred from the closure, the VM creates a fragment with a
copy of "a" in it.  When the VM evaluates the lambda expression, it
creates a subprogram (closure), associating the fragment with the
subprogram as its external environment.  When the closure is executed,
its environment will look like this:

      block          Top-level: foo
  |local var: b |       fragment
  +-------------+     .-----------,
  |external link|---->|variable: a|
  +-------------+     `-----------'

The fragment remains as long as the closure exists.

** Addressing mode

Guile VM has five addressing modes:

  o Real address
  o Local position
  o External position
  o Top-level location
  o Immediate object

Real address points to the address in the real program and is only used
with the program counter (pc).

Local position and external position are represented as a pair of depth
and offset from bp, as described above.  These are base relative
addresses, and the real address may vary during execution.

Top-level location is represented as a Guile's vcell.  This location is
determined at loading time, so the use of this address is efficient.

Immediate object is not an address but gives an instruction an Scheme
object directly.

[ We'll also need dynamic scope addressing to support Emacs Lisp? ]

*** At a Glance

Guile VM has a set of instructions for each instruction family.  `%load'
is, for example, a family to load an object from memory and set the
accumulator (ac).  There are four basic `%load' instructions:

  %loadl - Local addressing
  %loade - External addressing
  %loadt - Top-level addressing
  %loadi - Immediate addressing

A possible program code may look like this:

  %loadl (0 . 1)                ; ac = local[0][1]
  %loade (2 . 3)                ; ac = external[2][3]
  %loadt (foo . #<undefined>)   ; ac = #<undefined>
  %loadi "hello"                ; ac = "hello"

One instruction that uses real addressing is `%jump', which changes the
value of the program counter:

  %jump  0x80234ab8             ; pc = 0x80234ab8

* Program Execution

Overall procedure:

 1. A source program is compiled into a bytecode.

 2. A bytecode is given an environment and becomes a program.

 3. A VM starts execution, creating a frame for it.

 4. Whenever a program calls a subprogram, a new frame is created for it.

 5. When a program finishes execution, it returns a value, and the VM
    continues execution of the parent program.

 6. When all programs terminated, the VM returns the final value and stops.

** Environment

Local variable:

 (let ((a 1) (b 2) (c 3)) (+ a b c)) ->

   %pushi 1       ; a
   %pushi 2       ; b
   %pushi 3       ; c
   %bind  3       ; create local bindings
   %pushl (0 . 0) ; local variable a
   %pushl (0 . 1) ; local variable b
   %pushl (0 . 2) ; local variable c
   add    3       ; ac = a + b + c
   %unbind        ; remove local bindings

External variable:

 (define foo (let ((n 0)) (lambda () n)))

   %pushi 0                      ; n
   %bind  1                      ; create local bindings
   %export [0]                   ; make it an external variable
   %make-program #<bytecode xxx> ; create a program in this environment
   %unbind                       ; remove local bindings
   %savet (foo . #<undefined>)   ; save the program in foo

 (foo) ->

   %loadt (foo . #<program xxx>) ; program has an external link
   %call  0                      ; change the current external link
   %loade (0 . 0)                ; external variable n
   %return                       ; recover the external link

Top-level variable:

 foo ->

   %loadt (foo . #<program xxx>) ; top-level variable foo

** Flow control

 (if #t 1 0) ->

      %loadi      #t
      %br-if-not  L1
      %loadi      1
      %jump       L2
  L1: %loadi      0

** Function call

Builtin function:

 (1+ 2) ->

   %loadi 2  ; ac = 2
   1+        ; one argument

 (+ 1 2) ->

   %pushi 1  ; 1 -> stack
   %loadi 2  ; ac = 2
   add2      ; two argument

 (+ 1 2 3) ->

   %pushi 1  ; 1 -> stack
   %pushi 2  ; 2 -> stack
   %pushi 3  ; 3 -> stack
   add    3  ; many argument

External function:

 (version) ->

   %func0 (version . #<primitive-procedure version>) ; no argument

 (display "hello") ->

   %loadi "hello"
   %func1 (display . #<primitive-procedure display>) ; one argument

 (open-file "file" "w") ->

   %pushi "file"
   %loadi "w"
   %func2 (open-file . #<primitive-procedure open-file>) ; two arguments

 (equal 1 2 3)

   %pushi 1
   %pushi 2
   %pushi 3
   %loadi 3                                      ; the number of arguments
   %func  (equal . #<primitive-procedure equal>) ; many arguments

** Subprogram call

 (define (plus a b) (+ a b))
 (plus 1 2) ->

   %pushi 1                       ; argument 1
   %pushi 2                       ; argument 2
   %loadt (plus . #<program xxx>) ; load the program
   %call  2                       ; call it with two arguments
   %pushl (0 . 0)                 ; argument 1
   %loadl (0 . 1)                 ; argument 2
   add2                           ; ac = 1 + 2
   %return                        ; result is 3

* Instruction Set

The Guile VM instruction set is roughly divided two groups: system
instructions and functional instructions.  System instructions control
the execution of programs, while functional instructions provide many
useful calculations.  By convention, system instructions begin with a
letter `%'.

** Environment control instructions

- %alloc
- %bind
- %export
- %unbind

** Subprogram control instructions

- %make-program
- %call
- %return

** Data control instructinos

- %push
- %pushi
- %pushl, %pushl:0:0, %pushl:0:1, %pushl:0:2, %pushl:0:3
- %pushe, %pushe:0:0, %pushe:0:1, %pushe:0:2, %pushe:0:3
- %pusht

- %loadi
- %loadl, %loadl:0:0, %loadl:0:1, %loadl:0:2, %loadl:0:3
- %loade, %loade:0:0, %loade:0:1, %loade:0:2, %loade:0:3
- %loadt

- %savei
- %savel, %savel:0:0, %savel:0:1, %savel:0:2, %savel:0:3
- %savee, %savee:0:0, %savee:0:1, %savee:0:2, %savee:0:3
- %savet

** Flow control instructions

- %br-if
- %br-if-not
- %jump

** Function call instructions

- %func, %func0, %func1, %func2

** Scheme buitin functions

- cons
- car
- cdr

** Mathematical buitin functions

- 1+
- 1-
- add, add2
- sub, sub2, minus
- mul2
- div2
- lt2
- gt2
- le2
- ge2
- num-eq2

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