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

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NAME

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

SYNOPSIS

       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

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DESCRIPTION

       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  con-
       verting	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.

DEFINITIONS

       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  nec-
       essary 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:

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

		     NOP    This  instruction does nothing. Useful for fixed framework nodes, unchanging jump destinations, and the like. No argu-
			    ments.

		     C	    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.

		     BRA    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.

	      arguments
		     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.

	      expr   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:

	      condition
		     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.

	      gas::entry
		     Always written.

	      gas::exit
		     Written for all modes but okfail.

	      gas::exit::ok
		     Written for mode okfail.

	      gas::exit::fail
		     Written for mode okfail.

API

       ::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:

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

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

	      okfail 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 ::gram-
	      mar::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

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

	      okfail 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 ::gram-
	      mar::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.

BUGS, IDEAS, FEEDBACK
       This document, and the package it describes, will undoubtedly contain bugs and other problems.  Please report such in  the  category  gram-
       mar_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.

KEYWORDS

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

CATEGORY

       Grammars and finite automata

COPYRIGHT

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

grammar_me								0.1						 grammar::me::cpu::gasm(n)