F94

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\annex{Compatibility analysis of ANS Forth} % D (informative annex)

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\cbstart

\annex[Compatibility analysis]{Compatibility analysis of ANS Forth\strike{4}{80}} % D (informative annex)

\label{annex:diff}

\cbend

\setwordlist{core}

Prior to ANS Forth, there were several industry standards for Forth.

Prior to \replace{F94}{ANS Forth}{this standard}, there were several industry standards for Forth.

The most influential are listed here in chronological order, along

with the major differences between ANS Forth and the most recent,

Forth 83.

with the major differences between \replace{F94}{ANS Forth}{this standard} and the most recent,

\replace{F94}{Forth 83}{ANS Forth}.

\section{FIG Forth (circa 1978)} % D.1

model'' implied by existing Forth standards.

\section{ANS Forth approach} % D.5

\cbstart\patch{F94}

\section[ANS Forth (1994)]{ANS Forth approach\strike{4}{60} (1994)\strike{-2}{40}} % D.5

\label{diff:ans}

\cbend

The ANS Forth committee addressed the serious fragmentation of the

Forth community caused by the differences between Forth 79 and

Forth 83, and the divergence from either of these two industry

standards caused by marketplace pressures.

Consequently, the committee has chosen to base its compatibility

Consequently, the committee \replace{F94}{has chosen}{chose} to base its compatibility

decisions not upon a strict comparison with the Forth-83 Standard,

but instead upon consideration of the variety of existing

implementations, especially those with substantial user bases and/or

considerable success in the marketplace.

The committee feels that, if ANS Forth prescribes stringent

The committee \replace{F94}{feels}{felt} that, if ANS Forth prescribes stringent

requirements upon the virtual machine model, as did the previous

standards, then many implementors will chose not to comply with

ANS Forth. The committee hopes that ANS Forth will serve to unify

rather than to further divide the Forth community, and thus has

chosen to encompass rather than invalidate popular implementation

ANS Forth. The committee \replace{F94}{hopes}{hoped} that ANS Forth \replace{F94}{will}{would} serve to unify

rather than to further divide the Forth community, and thus

\replace{F94}{has chosen}{chose} to encompass rather than invalidate popular implementation

techniques.

Many of the changes from Forth 83 are justified by this rationale.

Many of the changes from Forth 83 \replace{F94}{are}{where} justified by this rationale.

Most fall into the category that ``an ANS Forth Standard Program may

not assume x'', where ``x'' is an entitlement resulting from the

virtual machine model prescribed by the Forth-83 Standard. The

committee feels that these restrictions are reasonable, especially

committee \replace{F94}{feels}{felt} that these restrictions are reasonable, especially

considering that a substantial number of existing Forth implementations

do not correctly implement the Forth-83 virtual model, thus the Forth-83

\replace{F94}{do}{that did} not correctly implement the Forth-83 virtual model, thus the Forth-83

entitlements exist ``in theory'' but not ``in practice''.

Another way of looking at this is that while ANS Forth acknowledges

the diversity of current Forth practice, it attempts to document the

similarity therein. In some sense, ANS Forth is thus a ``description

the diversity of \remove{F94}{current} Forth practice, it \replace{F94}{attempts}{attempted} to document the

similarity therein. In some sense, ANS Forth \replace{F94}{is}{was} thus a ``description

of reality'' rather than a ``prescription for a particular virtual

machine''.

\remove{F94}{%

Since there is no previous American National Standard for Forth, the

action requirements prescribed by section 3.4 of X3/SD-9,

``Policy and Guidelines'', regarding previous standards do not apply.

}

\remove{F94}{%

The following discussion describes differences between ANS Forth and

Forth 83. In most cases, Forth 83 is representative of Forth 79 and

FIG Forth for the purposes of this discussion. In many of these cases,

however, ANS Forth is more representative of the existing state of the

Forth industry than the previously-published standards.

}

\place{F94}{The American National Standards Institution (ANSI) published

the ANS Forth Standard in 1994 with the title ``\emph{ANSI X3.215-1994

Information Systems --- Programming Language FORTH}''.  This is referenced

throughout this document as Forth 94.}

\cbstart\patch{F94}

\section[ISO Forth (1997)]{ISO Forth (1997)\strike{-2}{110}}

\label{diff:iso}

\uline{%

ANSI submitted the Forth 94 Standard to the

ISO (International Organization for Standardization) and

IEC (International Electrotechnical Commission) joint committee for

consideration as an international standard.

The ISO/IEC adopted the Forth 94 document as an international standard

in 1997, publishing it under the title ``\emph{ISO/IEC 15145:1997

Information technology.  Programming languages.  FORTH}''.

}

\cbend

\section{Differences from Forth 83} % D.6

\place{F94}{%

The following discussion describes differences between this standard and

Forth 83. In most cases, Forth 83 is representative of Forth 79 and

FIG Forth for the purposes of this discussion. In many of these cases,

however, this standard is more representative of the existing state of the

Forth industry than the previously-published standards.

}

\subsection{Stack width} % D.6.1

Forth 83 specifies that stack items occupy 16 bits. This includes

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\cbstart

\section{ANS Forth '94\strike{4}{90} Forth 94\strike{-3}{50}} % D.7

\cbstart\patch{F94}

\section[Differences from Forth 94]{ANS Forth '94\strike{4.5}{90} Differences from Forth 94\strike{-2}{160}} % D.7

\cbend

\label{diff:forth94}

    Reuse of these names is strongly discouraged.

\cbstart\patch{ed11}

\subsection{Obsolescent features}

\label{diff:94:obsolete}

\uline{This standard has designated the following practice as obsolescent:}

\begin{itemize}

\item \uline{Requiring floating-point numbers to be kept on the data stack.

	(This has always been an environmental dependency.)}

\item \uline{Using \word{ENVIRONMENTq} to enquire whether a word set is present.}

\end{itemize}

\cbend

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