# grammar::me::cpu::gasm(n) 0.1 grammar_me "Grammar operations and usage"

## Name

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

## Table Of Contents

## 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. 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 © 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>