grammar::me::cpu::gasm(n)Grammar operations and usagegrammar::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
       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>




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