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version 1.13, 2002/08/19 07:38:16	version 1.15, 2002/08/20 16:59:01
Line 615 The grammar is in EBNF format, with @cod	Line 615 The grammar is in EBNF format, with @cod
of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.	of @var{c} and @code{[@var{d}]} meaning 0 or 1 repetitions of @var{d}.

@cindex free-format, not	@cindex free-format, not
	@cindex newlines, significance in syntax
Vmgen input is not free-format, so you have to take care where you put	Vmgen input is not free-format, so you have to take care where you put
spaces and especially newlines; it's not as bad as makefiles, though:	newlines (and, in a few cases, white space).
any sequence of spaces and tabs is equivalent to a single space.

@example	@example
description: @{instruction\|comment\|eval-escape@}	description: @{instruction\|comment\|eval-escape\|c-escape@}

instruction: simple-inst\|superinst	instruction: simple-inst\|superinst

simple-inst: ident ' (' stack-effect ' )' newline c-code newline newline	simple-inst: ident '(' stack-effect ')' newline c-code newline newline

stack-effect: @{ident@} ' --' @{ident@}	stack-effect: @{ident@} '--' @{ident@}

super-inst: ident ' =' ident @{ident@}	super-inst: ident '=' ident @{ident@}

comment: '\ ' text newline	comment: '\ ' text newline

eval-escape: '\E ' text newline	eval-escape: '\E ' text newline

	c-escape: '\C ' text newline
@end example	@end example
@c \+ \- \g \f \c	@c \+ \- \g \f \c

Note that the @code{\}s in this grammar are meant literally, not as	Note that the @code{\}s in this grammar are meant literally, not as
C-style encodings for non-printable characters.	C-style encodings for non-printable characters.

The C code in @code{simple-inst} must not contain empty lines (because	There are two ways to delimit the C code in @code{simple-inst}:
Vmgen would mistake that as the end of the simple-inst. The text in
@code{comment} and @code{eval-escape} must not contain a newline.	@itemize @bullet
@code{Ident} must conform to the usual conventions of C identifiers
(otherwise the C compiler would choke on the Vmgen output).	@item
	If you start it with a @samp{@{} at the start of a line (i.e., not even
	white space before it), you have to end it with a @samp{@}} at the start
	of a line (followed by a newline). In this case you may have empty
	lines within the C code (typically used between variable definitions and
	statements).

	@item
	You do not start it with @samp{@{}. Then the C code ends at the first
	empty line, so you cannot have empty lines within this code.

	@end itemize

	The text in @code{comment}, @code{eval-escape} and @code{c-escape} must
	not contain a newline. @code{Ident} must conform to the usual
	conventions of C identifiers (otherwise the C compiler would choke on
	the Vmgen output), except that idents in @code{stack-effect} may have a
	stack prefix (for stack prefix syntax, @pxref{Eval escapes}).

	@cindex C escape
	@cindex @code{\C}
	@cindex conditional compilation of Vmgen output
	The @code{c-escape} passes the text through to each output file (without
	the @samp{\C}). This is useful mainly for conditional compilation
	(i.e., you write @samp{\C #if ...} etc.).

	@cindex sync lines
	@cindex @code{#line}
	In addition to the syntax given in the grammer, Vmgen also processes
	sync lines (lines starting with @samp{#line}), as produced by @samp{m4
	-s} (@pxref{Invoking m4, , Invoking m4, m4.info, GNU m4}) and similar
	tools. This allows associating C compiler error messages with the
	original source of the C code.

Vmgen understands a few extensions beyond the grammar given here, but	Vmgen understands a few extensions beyond the grammar given here, but
these extensions are only useful for building Gforth. You can find a	these extensions are only useful for building Gforth. You can find a
Line 652 description of the format used for Gfort	Line 686 description of the format used for Gfort
@subsection Eval escapes	@subsection Eval escapes
@cindex escape to Forth	@cindex escape to Forth
@cindex eval escape	@cindex eval escape
	@cindex @code{\E}


@c woanders?	@c woanders?
The text in @code{eval-escape} is Forth code that is evaluated when	The text in @code{eval-escape} is Forth code that is evaluated when
Line 677 Note that the syntax of this code is not	Line 710 Note that the syntax of this code is not
many other Forth program fragments that could be written in an	many other Forth program fragments that could be written in an
eval-escape).	eval-escape).

	A stack prefix can contain letters, digits, or @samp{:}, and may start
	with an @samp{#}; e.g., in Gforth the return stack has the stack prefix
	@samp{R:}. This restriction is not checked during the stack prefix
	definition, but it is enforced by the parsing rules for stack items
	later.

If you know Forth, the stack effects of the non-standard words involved	If you know Forth, the stack effects of the non-standard words involved
are:	are:
@findex stack	@findex stack
Line 687 are:	Line 726 are:
@example	@example
stack ( "name" "pointer" "type" -- )	stack ( "name" "pointer" "type" -- )
( name execution: -- stack )	( name execution: -- stack )
type-prefix ( addr u xt1 xt2 n stack "prefix" -- )	type-prefix ( addr u item-size stack "prefix" -- )
single ( -- xt1 xt2 n )	single ( -- item-size )
double ( -- xt1 xt2 n )	double ( -- item-size )
stack-prefix ( stack "prefix" -- )	stack-prefix ( stack "prefix" -- )
@end example	@end example

	An @var{item-size} takes three cells on the stack.

@c --------------------------------------------------------------------	@c --------------------------------------------------------------------
@node Simple instructions, Superinstructions, Input File Grammar, Input File Format	@node Simple instructions, Superinstructions, Input File Grammar, Input File Format

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