


grammar::me::cpu::gGrammar operations angrammar::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

_________________________________________________________________

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



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       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



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              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.




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              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



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              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 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.



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       ::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.




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       ::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>







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