7 %% http://www.michaelshell.org/
8 %% for current contact information.
10 %% This is a skeleton file demonstrating the use of IEEEtran.cls
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42 %% File list of work: IEEEtran.cls, IEEEtran_HOWTO.pdf, bare_adv.tex,
43 %% bare_conf.tex, bare_jrnl.tex, bare_jrnl_compsoc.tex
44 %%*************************************************************************
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63 % should be used if it is desired that the figures are to be displayed in
66 \documentclass[conference,pdf,a4paper,10pt,final,twoside,twocolumn]{IEEEtran}
67 % Add the compsoc option for Computer Society conferences.
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76 % *** MISC UTILITY PACKAGES ***
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234 % IEEEtran contains the IEEEeqnarray family of commands that can be used to
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239 %\usepackage{eqparbox}
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243 % http://www.ctan.org/tex-archive/macros/latex/contrib/eqparbox/
249 % *** SUBFIGURE PACKAGES ***
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262 %\usepackage[caption=false]{caption}
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264 % subfig.sty, also written by Steven Douglas Cochran, is the modern
265 % replacement for subfigure.sty. However, subfig.sty requires and
266 % automatically loads Axel Sommerfeldt's caption.sty which will override
267 % IEEEtran.cls handling of captions and this will result in nonIEEE style
268 % figure/table captions. To prevent this problem, be sure and preload
269 % caption.sty with its "caption=false" package option. This is will preserve
270 % IEEEtran.cls handing of captions. Version 1.3 (2005/06/28) and later
271 % (recommended due to many improvements over 1.2) of subfig.sty supports
272 % the caption=false option directly:
273 %\usepackage[caption=false,font=footnotesize]{subfig}
275 % The latest version and documentation can be obtained at:
276 % http://www.ctan.org/tex-archive/macros/latex/contrib/subfig/
277 % The latest version and documentation of caption.sty can be obtained at:
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283 % *** FLOAT PACKAGES ***
285 %\usepackage{fixltx2e}
286 % fixltx2e, the successor to the earlier fix2col.sty, was written by
287 % Frank Mittelbach and David Carlisle. This package corrects a few problems
288 % in the LaTeX2e kernel, the most notable of which is that in current
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297 %\usepackage{stfloats}
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300 % as the top. (e.g., "\begin{figure*}[!b]" is not normally possible in
301 % LaTeX2e). It also provides a command:
303 % to enable the placement of footnotes below bottom floats (the standard
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321 % *** PDF, URL AND HYPERLINK PACKAGES ***
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328 % Read the url.sty source comments for usage information. Basically,
335 % *** Do not adjust lengths that control margins, column widths, etc. ***
336 % *** Do not use packages that alter fonts (such as pslatex). ***
337 % There should be no need to do such things with IEEEtran.cls V1.6 and later.
338 % (Unless specifically asked to do so by the journal or conference you plan
339 % to submit to, of course. )
341 % correct bad hyphenation here
342 \hyphenation{op-tical net-works semi-conduc-tor}
344 % Macro for certain acronyms in small caps. Doesn't work with the
345 % default font, though (it contains no smallcaps it seems).
346 \def\VHDL{\textsc{vhdl}}
347 \def\GHC{\textsc{ghc}}
348 \def\CLaSH{\textsc{C$\lambda$aSH}}
350 % Macro for pretty printing haskell snippets. Just monospaced for now, perhaps
351 % we'll get something more complex later on.
352 \def\hs#1{\texttt{#1}}
353 \def\quote#1{``{#1}"}
358 % can use linebreaks \\ within to get better formatting as desired
359 \title{\CLaSH: Structural Descriptions \\ of Synchronous Hardware using Haskell}
362 % author names and affiliations
363 % use a multiple column layout for up to three different
365 \author{\IEEEauthorblockN{Christiaan P.R. Baaij, Matthijs Kooijman, Jan Kuper, Marco E.T. Gerards, Bert Molenkamp, Sabih H. Gerez}
366 \IEEEauthorblockA{University of Twente, Department of EEMCS\\
367 P.O. Box 217, 7500 AE, Enschede, The Netherlands\\
368 c.p.r.baaij@utwente.nl, matthijs@stdin.nl}}
370 % \IEEEauthorblockN{Homer Simpson}
371 % \IEEEauthorblockA{Twentieth Century Fox\\
373 % Email: homer@thesimpsons.com}
375 % \IEEEauthorblockN{James Kirk\\ and Montgomery Scott}
376 % \IEEEauthorblockA{Starfleet Academy\\
377 % San Francisco, California 96678-2391\\
378 % Telephone: (800) 555--1212\\
379 % Fax: (888) 555--1212}}
381 % conference papers do not typically use \thanks and this command
382 % is locked out in conference mode. If really needed, such as for
383 % the acknowledgment of grants, issue a \IEEEoverridecommandlockouts
384 % after \documentclass
386 % for over three affiliations, or if they all won't fit within the width
387 % of the page, use this alternative format:
389 %\author{\IEEEauthorblockN{Michael Shell\IEEEauthorrefmark{1},
390 %Homer Simpson\IEEEauthorrefmark{2},
391 %James Kirk\IEEEauthorrefmark{3},
392 %Montgomery Scott\IEEEauthorrefmark{3} and
393 %Eldon Tyrell\IEEEauthorrefmark{4}}
394 %\IEEEauthorblockA{\IEEEauthorrefmark{1}School of Electrical and Computer Engineering\\
395 %Georgia Institute of Technology,
396 %Atlanta, Georgia 30332--0250\\ Email: see http://www.michaelshell.org/contact.html}
397 %\IEEEauthorblockA{\IEEEauthorrefmark{2}Twentieth Century Fox, Springfield, USA\\
398 %Email: homer@thesimpsons.com}
399 %\IEEEauthorblockA{\IEEEauthorrefmark{3}Starfleet Academy, San Francisco, California 96678-2391\\
400 %Telephone: (800) 555--1212, Fax: (888) 555--1212}
401 %\IEEEauthorblockA{\IEEEauthorrefmark{4}Tyrell Inc., 123 Replicant Street, Los Angeles, California 90210--4321}}
406 % use for special paper notices
407 %\IEEEspecialpapernotice{(Invited Paper)}
412 % make the title area
418 The abstract goes here.
420 % IEEEtran.cls defaults to using nonbold math in the Abstract.
421 % This preserves the distinction between vectors and scalars. However,
422 % if the conference you are submitting to favors bold math in the abstract,
423 % then you can use LaTeX's standard command \boldmath at the very start
424 % of the abstract to achieve this. Many IEEE journals/conferences frown on
425 % math in the abstract anyway.
432 % For peer review papers, you can put extra information on the cover
434 % \ifCLASSOPTIONpeerreview
435 % \begin{center} \bfseries EDICS Category: 3-BBND \end{center}
438 % For peerreview papers, this IEEEtran command inserts a page break and
439 % creates the second title. It will be ignored for other modes.
440 \IEEEpeerreviewmaketitle
443 \section{Introduction}
444 Hardware description languages has allowed the productivity of hardware engineers to keep pace with the development of chip technology. Standard Hardware description languages, like \VHDL\ and Verilog, allowed an engineer to describe circuits using a programming language. These standard languages are very good at describing detailed hardware properties such as timing behavior, but are generally cumbersome in expressing higher-level abstractions. These languages also tend to have a complex syntax and a lack of formal semantics. To overcome these complexities, and raise the abstraction level, a great number of approaches based on functional languages has been proposed \cite{T-Ruby,Hydra,HML2,Hawk1,Lava,ForSyDe1,Wired,reFLect}. The idea of using functional languages started in the early 1980s \cite{Cardelli1981,muFP,DAISY,FHDL}, a time which also saw the birth of the currently popular hardware description languages such as \VHDL.
446 What gives functional languages as hardware description languages their merits is the fact that basic combinatorial circuits are equivalent to mathematical function, and that functional languages lend themselves very well to describe and compose these mathematical functions.
447 \section{Hardware description in Haskell}
449 \subsection{Function application}
450 The basic syntactic elements of a functional program are functions
451 and function application. These have a single obvious \VHDL\
452 translation: each top level function becomes a hardware component,
453 where each argument is an input port and the result value is the
454 (single) output port. This output port can have a complex type (such
455 as a tuple), so having just a single output port does not create a
458 Each function application in turn becomes component instantiation.
459 Here, the result of each argument expression is assigned to a
460 signal, which is mapped to the corresponding input port. The output
461 port of the function is also mapped to a signal, which is used as
462 the result of the application itself.
464 Since every top level function generates its own component, the
465 hierarchy of of function calls is reflected in the final \VHDL\
466 output as well, creating a hierarchical \VHDL\ description of the
467 hardware. This separation in different components makes the
468 resulting \VHDL\ output easier to read and debug.
470 Example that defines the \texttt{mac} function by applying the
471 \texttt{add} and \texttt{mul} functions to calculate $a * b + c$:
474 mac a b c = add (mul a b) c
479 \subsection{Choices }
480 Although describing components and connections allows describing a
481 lot of hardware designs already, there is an obvious thing missing:
482 choice. We need some way to be able to choose between values based
483 on another value. In Haskell, choice is achieved by \hs{case}
484 expressions, \hs{if} expressions, pattern matching and guards.
486 The easiest of these are of course case expressions (and \hs{if}
487 expressions, which can be very directly translated to \hs{case}
488 expressions). A \hs{case} expression can in turn simply be
489 translated to a conditional assignment in \VHDL, where the
490 conditions use equality comparisons against the constructors in the
491 \hs{case} expressions.
493 A slightly more complex (but very powerful) form of choice is
494 pattern matching. A function can be defined in multiple clauses,
495 where each clause specifies a pattern. When the arguments match the
496 pattern, the corresponding clause will be used.
498 A pattern match (with optional guards) can also be implemented using
499 conditional assignments in \VHDL, where the condition is the logical
500 and of comparison results of each part of the pattern as well as the
503 Contrived example that sums two values when they are equal or
504 non-equal (depending on the predicate given) and returns 0
505 otherwise. This shows three implementations, one using and if
506 expression, one using only case expressions and one using pattern
510 sumif pred a b = if pred == Eq && a == b || pred == Neq && a != b
516 sumif pred a b = case pred of
520 Neq -> case a != b of
526 sumif Eq a b | a == b = a + b
527 sumif Neq a b | a != b = a + b
534 Translation of two most basic functional concepts has been
535 discussed: function application and choice. Before looking further
536 into less obvious concepts like higher-order expressions and
537 polymorphism, the possible types that can be used in hardware
538 descriptions will be discussed.
540 Some way is needed to translate every values used to its hardware
541 equivalents. In particular, this means a hardware equivalent for
542 every \emph{type} used in a hardware description is needed
544 Since most functional languages have a lot of standard types that
545 are hard to translate (integers without a fixed size, lists without
546 a static length, etc.), a number of \quote{built-in} types will be
547 defined first. These types are built-in in the sense that our
548 compiler will have a fixed VHDL type for these. User defined types,
549 on the other hand, will have their hardware type derived directly
550 from their Haskell declaration automatically, according to the rules
553 \subsection{Built-in types}
554 The language currently supports the following built-in types. Of these,
555 only the \hs{Bool} type is supported by Haskell out of the box (the
556 others are defined by the \CLaSH\ package, so they are user-defined types
557 from Haskell's point of view).
561 This is the most basic type available. It is mapped directly onto
562 the \texttt{std\_logic} \VHDL\ type. Mapping this to the
563 \texttt{bit} type might make more sense (since the Haskell version
564 only has two values), but using \texttt{std\_logic} is more standard
565 (and allowed for some experimentation with don't care values)
568 This is the only built-in Haskell type supported and is translated
569 exactly like the Bit type (where a value of \hs{True} corresponds to a
570 value of \hs{High}). Supporting the Bool type is particularly
571 useful to support \hs{if ... then ... else ...} expressions, which
572 always have a \hs{Bool} value for the condition.
574 A \hs{Bool} is translated to a \texttt{std\_logic}, just like \hs{Bit}.
575 \item[\hs{SizedWord}, \hs{SizedInt}]
576 These are types to represent integers. A \hs{SizedWord} is unsigned,
577 while a \hs{SizedInt} is signed. These types are parametrized by a
578 length type, so you can define an unsigned word of 32 bits wide as
582 type Word32 = SizedWord D32
585 Here, a type synonym \hs{Word32} is defined that is equal to the
586 \hs{SizedWord} type constructor applied to the type \hs{D32}. \hs{D32}
587 is the \emph{type level representation} of the decimal number 32,
588 making the \hs{Word32} type a 32-bit unsigned word.
590 These types are translated to the \small{VHDL} \texttt{unsigned} and
591 \texttt{signed} respectively.
593 This is a vector type, that can contain elements of any other type and
594 has a fixed length. It has two type parameters: its
595 length and the type of the elements contained in it. By putting the
596 length parameter in the type, the length of a vector can be determined
597 at compile time, instead of only at run-time for conventional lists.
599 The \hs{Vector} type constructor takes two type arguments: the length
600 of the vector and the type of the elements contained in it. The state
601 type of an 8 element register bank would then for example be:
604 type RegisterState = Vector D8 Word32
607 Here, a type synonym \hs{RegisterState} is defined that is equal to
608 the \hs{Vector} type constructor applied to the types \hs{D8} (The type
609 level representation of the decimal number 8) and \hs{Word32} (The 32
610 bit word type as defined above). In other words, the
611 \hs{RegisterState} type is a vector of 8 32-bit words.
613 A fixed size vector is translated to a \VHDL\ array type.
614 \item[\hs{RangedWord}]
615 This is another type to describe integers, but unlike the previous
616 two it has no specific bit-width, but an upper bound. This means that
617 its range is not limited to powers of two, but can be any number.
618 A \hs{RangedWord} only has an upper bound, its lower bound is
619 implicitly zero. There is a lot of added implementation complexity
620 when adding a lower bound and having just an upper bound was enough
621 for the primary purpose of this type: type-safely indexing vectors.
623 To define an index for the 8 element vector above, we would do:
626 type RegisterIndex = RangedWord D7
629 Here, a type synonym \hs{RegisterIndex} is defined that is equal to
630 the \hs{RangedWord} type constructor applied to the type \hs{D7}. In
631 other words, this defines an unsigned word with values from
632 0 to 7 (inclusive). This word can be be used to index the
633 8 element vector \hs{RegisterState} above.
635 This type is translated to the \texttt{unsigned} \VHDL type.
637 \subsection{User-defined types}
638 There are three ways to define new types in Haskell: algebraic
639 data-types with the \hs{data} keyword, type synonyms with the \hs{type}
640 keyword and type renamings with the \hs{newtype} keyword. \GHC\
641 offers a few more advanced ways to introduce types (type families,
642 existential typing, \small{GADT}s, etc.) which are not standard
643 Haskell. These will be left outside the scope of this research.
645 Only an algebraic datatype declaration actually introduces a
646 completely new type, for which we provide the \VHDL\ translation
647 below. Type synonyms and renamings only define new names for
648 existing types (where synonyms are completely interchangeable and
649 renamings need explicit conversion). Therefore, these do not need
650 any particular \VHDL\ translation, a synonym or renamed type will
651 just use the same representation as the original type. The
652 distinction between a renaming and a synonym does no longer matter
653 in hardware and can be disregarded in the generated \VHDL.
655 For algebraic types, we can make the following distinction:
660 A product type is an algebraic datatype with a single constructor with
661 two or more fields, denoted in practice like (a,b), (a,b,c), etc. This
662 is essentially a way to pack a few values together in a record-like
663 structure. In fact, the built-in tuple types are just algebraic product
664 types (and are thus supported in exactly the same way).
666 The ``product'' in its name refers to the collection of values belonging
667 to this type. The collection for a product type is the Cartesian
668 product of the collections for the types of its fields.
670 These types are translated to \VHDL\ record types, with one field for
671 every field in the constructor. This translation applies to all single
672 constructor algebraic data-types, including those with just one
673 field (which are technically not a product, but generate a VHDL
674 record for implementation simplicity).
675 \item[Enumerated types]
676 An enumerated type is an algebraic datatype with multiple constructors, but
677 none of them have fields. This is essentially a way to get an
678 enumeration-like type containing alternatives.
680 Note that Haskell's \hs{Bool} type is also defined as an
681 enumeration type, but we have a fixed translation for that.
683 These types are translated to \VHDL\ enumerations, with one value for
684 each constructor. This allows references to these constructors to be
685 translated to the corresponding enumeration value.
687 A sum type is an algebraic datatype with multiple constructors, where
688 the constructors have one or more fields. Technically, a type with
689 more than one field per constructor is a sum of products type, but
690 for our purposes this distinction does not really make a
691 difference, so this distinction is note made.
693 The ``sum'' in its name refers again to the collection of values
694 belonging to this type. The collection for a sum type is the
695 union of the the collections for each of the constructors.
697 Sum types are currently not supported by the prototype, since there is
698 no obvious \VHDL\ alternative. They can easily be emulated, however, as
699 we will see from an example:
702 data Sum = A Bit Word | B Word
705 An obvious way to translate this would be to create an enumeration to
706 distinguish the constructors and then create a big record that
707 contains all the fields of all the constructors. This is the same
708 translation that would result from the following enumeration and
709 product type (using a tuple for clarity):
713 type Sum = (SumC, Bit, Word, Word)
716 Here, the \hs{SumC} type effectively signals which of the latter three
717 fields of the \hs{Sum} type are valid (the first two if \hs{A}, the
718 last one if \hs{B}), all the other ones have no useful value.
720 An obvious problem with this naive approach is the space usage: the
721 example above generates a fairly big \VHDL\ type. Since we can be
722 sure that the two \hs{Word}s in the \hs{Sum} type will never be valid
723 at the same time, this is a waste of space.
725 Obviously, duplication detection could be used to reuse a
726 particular field for another constructor, but this would only
727 partially solve the problem. If two fields would be, for
728 example, an array of 8 bits and an 8 bit unsigned word, these are
729 different types and could not be shared. However, in the final
730 hardware, both of these types would simply be 8 bit connections,
731 so we have a 100\% size increase by not sharing these.
735 \section{\CLaSH\ prototype}
739 \section{Related work}
740 Many functional hardware description languages have been developed over the years. Early work includes such languages as \textsc{$\mu$fp}~\cite{muFP}, an extension of Backus' \textsc{fp} language to synchronous streams, designed particularly for describing and reasoning about regular circuits. The Ruby~\cite{Ruby} language uses relations, instead of functions, to describe circuits, and has a particular focus on layout. \textsc{hml}~\cite{HML2} is a hardware modeling language based on the strict functional language \textsc{ml}, and has support for polymorphic types and higher-order functions. Published work suggests that there is no direct simulation support for \textsc{hml}, and that the translation to \VHDL\ is only partial.
742 Like this work, many functional hardware description languages have some sort of foundation in the functional programming language Haskell. Hawk~\cite{Hawk1} uses Haskell to describe system-level executable specifications used to model the behavior of superscalar microprocessors. Hawk specifications can be simulated, but there seems to be no support for automated circuit synthesis. The ForSyDe~\cite{ForSyDe2} system uses Haskell to specify abstract system models, which can (manually) be transformed into an implementation model using semantic preserving transformations. ForSyDe has several simulation and synthesis backends, though synthesis is restricted to the synchronous subset of the ForSyDe language.
744 Lava~\cite{Lava} is a hardware description language that focuses on the structural representation of hardware. Besides support for simulation and circuit synthesis, Lava descriptions can be interfaced with formal method tools for formal verification. Lava descriptions are actually circuit generators when viewed from a synthesis viewpoint, in that the language elements of Haskell, such as choice, can be used to guide the circuit generation. If a developer wants to insert a choice element inside an actual circuit he will have to specify this explicitly as a component. In this respect \CLaSH\ differs from Lava, in that all the choice elements, such as case-statements and patter matching, are synthesized to choice elements in the eventual circuit. As such, richer control structures can both be specified and synthesized in \CLaSH\ compared to any of the languages mentioned in this section.
746 The merits of polymorphic typing, combined with higher-order functions, are now also recognized in the `main-stream' hardware description languages, exemplified by the new \VHDL\ 2008 standard~\cite{VHDL2008}. \VHDL-2008 has support to specify types as generics, thus allowing a developer to describe polymorphic components. Note that those types still require an explicit generic map, whereas type-inference and type-specialization are implicit in \CLaSH.
748 Wired~\cite{Wired},, T-Ruby~\cite{T-Ruby}, Hydra~\cite{Hydra}.
750 A functional language designed specifically for hardware design is $re{\mathit{FL}}^{ect}$~\cite{reFLect}, which draws experience from earlier language called \textsc{fl}~\cite{FL} to la
752 % An example of a floating figure using the graphicx package.
753 % Note that \label must occur AFTER (or within) \caption.
754 % For figures, \caption should occur after the \includegraphics.
755 % Note that IEEEtran v1.7 and later has special internal code that
756 % is designed to preserve the operation of \label within \caption
757 % even when the captionsoff option is in effect. However, because
758 % of issues like this, it may be the safest practice to put all your
759 % \label just after \caption rather than within \caption{}.
761 % Reminder: the "draftcls" or "draftclsnofoot", not "draft", class
762 % option should be used if it is desired that the figures are to be
763 % displayed while in draft mode.
767 %\includegraphics[width=2.5in]{myfigure}
768 % where an .eps filename suffix will be assumed under latex,
769 % and a .pdf suffix will be assumed for pdflatex; or what has been declared
770 % via \DeclareGraphicsExtensions.
771 %\caption{Simulation Results}
775 % Note that IEEE typically puts floats only at the top, even when this
776 % results in a large percentage of a column being occupied by floats.
779 % An example of a double column floating figure using two subfigures.
780 % (The subfig.sty package must be loaded for this to work.)
781 % The subfigure \label commands are set within each subfloat command, the
782 % \label for the overall figure must come after \caption.
783 % \hfil must be used as a separator to get equal spacing.
784 % The subfigure.sty package works much the same way, except \subfigure is
785 % used instead of \subfloat.
788 %\centerline{\subfloat[Case I]\includegraphics[width=2.5in]{subfigcase1}%
789 %\label{fig_first_case}}
791 %\subfloat[Case II]{\includegraphics[width=2.5in]{subfigcase2}%
792 %\label{fig_second_case}}}
793 %\caption{Simulation results}
797 % Note that often IEEE papers with subfigures do not employ subfigure
798 % captions (using the optional argument to \subfloat), but instead will
799 % reference/describe all of them (a), (b), etc., within the main caption.
802 % An example of a floating table. Note that, for IEEE style tables, the
803 % \caption command should come BEFORE the table. Table text will default to
804 % \footnotesize as IEEE normally uses this smaller font for tables.
805 % The \label must come after \caption as always.
808 %% increase table row spacing, adjust to taste
809 %\renewcommand{\arraystretch}{1.3}
810 % if using array.sty, it might be a good idea to tweak the value of
811 % \extrarowheight as needed to properly center the text within the cells
812 %\caption{An Example of a Table}
813 %\label{table_example}
815 %% Some packages, such as MDW tools, offer better commands for making tables
816 %% than the plain LaTeX2e tabular which is used here.
817 %\begin{tabular}{|c||c|}
827 % Note that IEEE does not put floats in the very first column - or typically
828 % anywhere on the first page for that matter. Also, in-text middle ("here")
829 % positioning is not used. Most IEEE journals/conferences use top floats
830 % exclusively. Note that, LaTeX2e, unlike IEEE journals/conferences, places
831 % footnotes above bottom floats. This can be corrected via the \fnbelowfloat
832 % command of the stfloats package.
837 The conclusion goes here.
842 % conference papers do not normally have an appendix
845 % use section* for acknowledgement
846 \section*{Acknowledgment}
849 The authors would like to thank...
855 % trigger a \newpage just before the given reference
856 % number - used to balance the columns on the last page
857 % adjust value as needed - may need to be readjusted if
858 % the document is modified later
859 %\IEEEtriggeratref{8}
860 % The "triggered" command can be changed if desired:
861 %\IEEEtriggercmd{\enlargethispage{-5in}}
865 % can use a bibliography generated by BibTeX as a .bbl file
866 % BibTeX documentation can be easily obtained at:
867 % http://www.ctan.org/tex-archive/biblio/bibtex/contrib/doc/
868 % The IEEEtran BibTeX style support page is at:
869 % http://www.michaelshell.org/tex/ieeetran/bibtex/
870 \bibliographystyle{IEEEtran}
871 % argument is your BibTeX string definitions and bibliography database(s)
872 \bibliography{IEEEabrv,cλash.bib}
874 % <OR> manually copy in the resultant .bbl file
875 % set second argument of \begin to the number of references
876 % (used to reserve space for the reference number labels box)
877 % \begin{thebibliography}{1}
879 % \bibitem{IEEEhowto:kopka}
880 % H.~Kopka and P.~W. Daly, \emph{A Guide to \LaTeX}, 3rd~ed.\hskip 1em plus
881 % 0.5em minus 0.4em\relax Harlow, England: Addison-Wesley, 1999.
883 % \end{thebibliography}
891 % vim: set ai sw=2 sts=2 expandtab: