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grammar::me::cpu::gasm(3tGrammar operations and usgrammar::me::cpu::gasm(3tcl)

______________________________________________________________________________

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

_________________________________________________________________

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 gram‐
       mars 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	 pack‐
       age  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 convert‐
       ing  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  out‐
       going  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  instruc‐
	      tions, 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 inter‐
       twined 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 instruc‐
		     tion. 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 arguments.

		     C	    A  .NOP  to	 allow the insertion of arbitrary com‐
			    ments 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 argu‐
		     ments 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 condi‐
		     tion  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  ::gram‐
       mar::me::cpu::gasm::done. It has to refer to a node in the tree t argu‐
       ment 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  ::gram‐
	      mar::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 ref‐
	      erence  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 assem‐
		     bler 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 condi‐
	      tion 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-instruc‐
	      tion. 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-instruc‐
	      tion. 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 actu‐
	      ally 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 instruc‐
	      tion 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  fur‐
	      ther 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 grammar_me
       of	the	  Tcllib       SF	Trackers       [http://source‐
       forge.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(3tcl)
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