grammar::me::cpu::gasm(n) Grammar operations and usage grammar::me::cpu::gasm(n) 







grammar::me::cpu::gasm - ME assembler








package require grammar::me::cpu::gasm ?0.1?


::grammar::me::cpu::gasm::begin g n
    ?mode? ?note?


::grammar::me::cpu::gasm::done --> t


::grammar::me::cpu::gasm::state


::grammar::me::cpu::gasm::state! s


::grammar::me::cpu::gasm::lift t dst =
    src


::grammar::me::cpu::gasm::Inline t node
    label


::grammar::me::cpu::gasm::Cmd cmd
  ?arg...?


::grammar::me::cpu::gasm::Bra


::grammar::me::cpu::gasm::Nop text


::grammar::me::cpu::gasm::Note text


::grammar::me::cpu::gasm::Jmp label


::grammar::me::cpu::gasm::Exit


::grammar::me::cpu::gasm::Who label


::grammar::me::cpu::gasm::/Label name


::grammar::me::cpu::gasm::/Clear


::grammar::me::cpu::gasm::/Ok


::grammar::me::cpu::gasm::/Fail


::grammar::me::cpu::gasm::/At name


::grammar::me::cpu::gasm::/CloseLoop












This package provides a simple in-memory assembler. Its origin is
    that of a support package for use by packages converting PEG and other
    grammars into a corresponding matcher based on the ME virtual machine, like
    page::compiler::peg::mecpu. Despite that it is actually mostly
    agnostic regarding the instructions, users can choose any instruction set
    they like.


The program under construction is held in a graph structure (See
    package struct::graph) during assembly and subsequent manipulation,
    with instructions represented by nodes, and the flow of execution between
    instructions explicitly encoded in the arcs between them.


In this model jumps are not encoded explicitly, they are implicit
    in the arcs. The generation of explicit jumps is left to any code converting
    the graph structure into a more conventional representation. The same goes
    for branches. They are implicitly encoded by all instructions which have two
    outgoing arcs, whereas all other instructions have only one outgoing arc.
    Their conditonality is handled by tagging their outgoing arcs with
    information about the conditions under which they are taken.


While the graph the assembler operates on is supplied from the
    outside, i.e. external, it does manage some internal state, namely:



  
[1]

  
The handle of the graph node most assembler operations will work on, the
      anchor.

  
[2]

  
A mapping from arbitrary strings to instructions. I.e. it is possible to
      label an instruction during assembly, and later recall that
      instruction by its label.

  
[3]

  
The condition code to use when creating arcs between instructions, which
      is one of always, ok, and fail.

  
[4]

  
The current operation mode, one of halt, okfail, and
      !okfail.

  
[5]

  
The name of a node in a tree. This, and the operation mode above are the
      parts most heavily influenced by the needs of a grammar compiler, as they
      assume some basic program structures (selected through the operation
      mode), and intertwine the graph with a tree, like the AST for the grammar
      to be compiled.










As the graph the assembler is operating on, and the tree it is
    intertwined with, are supplied to the assembler from the outside it is
    necessary to specify the API expected from them, and to describe the
    structures expected and/or generated by the assembler in either.



  
[1]

  
Any graph object command used by the assembler has to provide the API as
      specified in the documentation for the package struct::graph.

  
[2]

  
Any tree object command used by the assembler has to provide the API as
      specified in the documentation for the package struct::tree.

  
[3]

  
Any instruction (node) generated by the assembler in a graph will have at
      least two, and at most three attributes:







  

  
The value of this attribute is the name of the instruction. The only names
      currently defined by the assembler are the three pseudo-instructions







  

  
This instruction does nothing. Useful for fixed framework nodes,
      unchanging jump destinations, and the like. No arguments.

  

  
A .NOP to allow the insertion of arbitrary comments into the instruction
      stream, i.e. a comment node. One argument, the text of the comment.

  

  
A .NOP serving as explicitly coded conditional branch. No arguments.





However we reserve the space of all instructions whose names begin with a
  "." (dot) for future use by the assembler.


  

  
The value of this attribute is a list of strings, the arguments of the
      instruction. The contents are dependent on the actual instruction and the
      assembler doesn't know or care about them. This means for example that it
      has no builtin knowledge about what instruction need which arguments and
      thus doesn't perform any type of checking.

  

  
This attribute is optional. When it is present its value is the name of a
      node in the tree intertwined with the graph.







  
[4]

  
Any arc between two instructions will have one attribute:







  

  
The value of this attribute determines under which condition execution
      will take this arc. It is one of always, ok, and
      fail. The first condition is used for all arcs which are the single
      outgoing arc of an instruction. The other two are used for the two
      outgoing arcs of an instruction which implicitly encode a branch.







  
[5]

  
A tree node given to the assembler for cross-referencing will be written
      to and given the following attributes, some fixed, some dependent on the
      operation mode. All values will be references to nodes in the instruction
      graph. Some of the instruction will expect some or specific sets of these
      attributes.







  

  
Always written.

  

  
Written for all modes but okfail.

  

  
Written for mode okfail.

  

  
Written for mode okfail.













  
::grammar::me::cpu::gasm::begin g n ?mode?
    ?note?

  
This command starts the assembly of an instruction sequence, and
      (re)initializes the state of the assembler. After completion of the
      instruction sequence use ::grammar::me::cpu::gasm::done to finalize
      the assembler.
    
It will operate on the graph g in the specified
        mode (Default is okfail). As part of the initialization it
        will always create a standard .NOP instruction and label it
        "entry". The creation of the remaining standard instructions
        is mode-dependent:

  







  

  
An "icf_halt" instruction labeled "exit/return".

  
!okfail

  
An "icf_ntreturn" instruction labeled
    "exit/return".

  

  
Two .NOP instructions labeled "exit/ok" and
      "exit/fail" respectively.






The note, if specified (default is not), is given to the
    "entry" .NOP instruction.


The node reference n is simply stored for use by
    ::grammar::me::cpu::gasm::done. It has to refer to a node in the tree
    t argument of that command.


After the initialization is done the "entry" instruction
    will be the anchor, and the condition code will be set to
    always.


The command returns the empy string as its result.



  
::grammar::me::cpu::gasm::done --> t

  
This command finalizes the creation of an instruction sequence and then
      clears the state of the assembler. NOTE that this does not
      delete any of the created instructions. They can be made available to
      future begin/done cycles. Further assembly will be possible only after
      reinitialization of the system via ::grammar::me::cpu::gasm::begin.
    
Before the state is cleared selected references to selected
        instructions will be written to attributes of the node n in the
        tree t. Which instructions are saved is mode-dependent.
        Both mode and the destination node n were specified during
        invokation of ::grammar::me::cpu::gasm::begin.

    
Independent of the mode a reference to the instruction labeled
        "entry" will be saved to the attribute gas::entry of
        n. The reference to the node n will further be saved into
        the attribute "expr" of the "entry" instruction.
        Beyond that

  







  

  
A reference to the instruction labeled "exit/return" will be
      saved to the attribute gas::exit of n.

  

  
See halt.

  
!okfail

  
Reference to the two instructions labeled "exit/ok" and
      "exit/fail" will be saved to the attributes gas::exit::ok
      and gas::exit::fail of n respectively.






The command returns the empy string as its result.



  
::grammar::me::cpu::gasm::state

  
This command returns the current state of the assembler. Its format is not
      documented and considered to be internal to the package.

  
::grammar::me::cpu::gasm::state! s

  
This command takes a serialized assembler state s as returned by
      ::grammar::me::cpu::gasm::state and makes it the current state of
      the assembler.
    
Note that this may overwrite label definitions, however
        all non-conflicting label definitions in the state before are not
        touched and merged with s.

    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::lift t dst =
    src

  
This command operates on the tree t. It copies the contents of the
      attributes gas::entry, gas::exit::ok and
      gas::exit::fail from the node src to the node dst. It
      returns the empty string as its result.

  
::grammar::me::cpu::gasm::Inline t node
    label

  
This command links an instruction sequence created by an earlier
      begin/done pair into the current instruction sequence.
    
To this end it

  







  
[1]

  
reads the instruction references from the attributes gas::entry,
      gas::exit::ok, and gas::exit::fail from the node n of
      the tree t and makes them available to assembler und the labels
      label/entry, label/exit::ok, and label/exit::fail
      respectively.

  
[2]

  
Creates an arc from the anchor to the node labeled
      label/entry, and tags it with the current condition code.

  
[3]

  
Makes the node labeled label/exit/ok the new anchor.





The command returns the empty string as its result.


  
::grammar::me::cpu::gasm::Cmd cmd ?arg...?

  
This is the basic command to add instructions to the graph. It creates a
      new instruction of type cmd with the given arguments arg...
      If the anchor was defined it will also create an arc from the
      anchor to the new instruction using the current condition code.
      After the call the new instruction will be the anchor and the
      current condition code will be set to always.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::Bra

  
This is a convenience command to create a .BRA pseudo-instruction. It uses
      ::grammar::me::cpu::gasm::Cmd to actually create the instruction
      and inherits its behaviour.

  
::grammar::me::cpu::gasm::Nop text

  
This is a convenience command to create a .NOP pseudo-instruction. It uses
      ::grammar::me::cpu::gasm::Cmd to actually create the instruction
      and inherits its behaviour. The text will be saved as the first and
      only argument of the new instruction.

  
::grammar::me::cpu::gasm::Note text

  
This is a convenience command to create a .C pseudo-instruction, i.e. a
      comment. It uses ::grammar::me::cpu::gasm::Cmd to actually create
      the instruction and inherits its behaviour. The text will be saved
      as the first and only argument of the new instruction.

  
::grammar::me::cpu::gasm::Jmp label

  
This command creates an arc from the anchor to the instruction
      labeled with label, and tags with the the current condition code.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::Exit

  
This command creates an arc from the anchor to one of the exit
      instructions, based on the operation mode (see
      ::grammar::me::cpu::gasm::begin), and tags it with current
      condition code.
    
For mode okfail it links to the instruction labeled
        either "exit/ok" or "exit/fail", depending on the
        current condition code, and tagging it with the current condition code
        For the other two modes it links to the instruction labeled
        "exit/return", tagging it condition code always,
        independent the current condition code.

    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::Who label

  
This command returns a reference to the instruction labeled with
      label.

  
::grammar::me::cpu::gasm::/Label name

  
This command labels the anchor with name. Note that
      an instruction can have more than one label.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::/Clear

  
This command clears the anchor, leaving it undefined, and further
      resets the current condition code to always.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::/Ok

  
This command sets the current condition code to ok.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::/Fail

  
This command sets the current condition code to fail.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::/At name

  
This command sets the anchor to the instruction labeled with
      name, and further resets the current condition code to
      always.
    
The command returns the empty string as its result.

  

  
::grammar::me::cpu::gasm::/CloseLoop

  
This command marks the anchor as the last instruction in a loop
      body, by creating the attribute LOOP.
    
The command returns the empty string as its result.

  










This document, and the package it describes, will undoubtedly
    contain bugs and other problems. Please report such in the category
    grammar_me of the Tcllib SF Trackers
    [http://sourceforge.net/tracker/?group_id=12883]. Please also report any
    ideas for enhancements you may have for either package and/or
  documentation.








assembler, grammar, graph, parsing, tree, virtual machine








Grammars and finite automata








Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>








  

    0.1
    grammar_me