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>