Provided by: tcllib_1.14-dfsg-1_all bug

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 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.

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 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:

              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 arguments.

                     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 ::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

              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  ::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.

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://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>