590 lines
23 KiB
Plaintext
590 lines
23 KiB
Plaintext
@c Copyright (C) 2001-2020 Free Software Foundation, Inc.
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@c This is part of the GAS manual.
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@c For copying conditions, see the file as.texinfo.
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@c MMIX description by Hans-Peter Nilsson, hp@bitrange.com
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@ifset GENERIC
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@page
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@node MMIX-Dependent
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@chapter MMIX Dependent Features
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@end ifset
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@ifclear GENERIC
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@node Machine Dependencies
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@chapter MMIX Dependent Features
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@end ifclear
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@cindex MMIX support
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@menu
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* MMIX-Opts:: Command-line Options
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* MMIX-Expand:: Instruction expansion
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* MMIX-Syntax:: Syntax
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* MMIX-mmixal:: Differences to @code{mmixal} syntax and semantics
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@end menu
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@node MMIX-Opts
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@section Command-line Options
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@cindex options, MMIX
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@cindex MMIX options
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The MMIX version of @code{@value{AS}} has some machine-dependent options.
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@cindex @samp{--fixed-special-register-names} command-line option, MMIX
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When @samp{--fixed-special-register-names} is specified, only the register
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names specified in @ref{MMIX-Regs} are recognized in the instructions
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@code{PUT} and @code{GET}.
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@cindex @samp{--globalize-symbols} command-line option, MMIX
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You can use the @samp{--globalize-symbols} to make all symbols global.
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This option is useful when splitting up a @code{mmixal} program into
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several files.
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@cindex @samp{--gnu-syntax} command-line option, MMIX
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The @samp{--gnu-syntax} turns off most syntax compatibility with
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@code{mmixal}. Its usability is currently doubtful.
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@cindex @samp{--relax} command-line option, MMIX
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The @samp{--relax} option is not fully supported, but will eventually make
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the object file prepared for linker relaxation.
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@cindex @samp{--no-predefined-syms} command-line option, MMIX
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If you want to avoid inadvertently calling a predefined symbol and would
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rather get an error, for example when using @code{@value{AS}} with a
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compiler or other machine-generated code, specify
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@samp{--no-predefined-syms}. This turns off built-in predefined
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definitions of all such symbols, including rounding-mode symbols, segment
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symbols, @samp{BIT} symbols, and @code{TRAP} symbols used in @code{mmix}
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``system calls''. It also turns off predefined special-register names,
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except when used in @code{PUT} and @code{GET} instructions.
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@cindex @samp{--no-expand} command-line option, MMIX
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By default, some instructions are expanded to fit the size of the operand
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or an external symbol (@pxref{MMIX-Expand}). By passing
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@samp{--no-expand}, no such expansion will be done, instead causing errors
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at link time if the operand does not fit.
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@cindex @samp{--no-merge-gregs} command-line option, MMIX
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The @code{mmixal} documentation (@pxref{mmixsite}) specifies that global
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registers allocated with the @samp{GREG} directive (@pxref{MMIX-greg}) and
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initialized to the same non-zero value, will refer to the same global
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register. This isn't strictly enforceable in @code{@value{AS}} since the
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final addresses aren't known until link-time, but it will do an effort
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unless the @samp{--no-merge-gregs} option is specified. (Register merging
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isn't yet implemented in @code{@value{LD}}.)
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@cindex @samp{-x} command-line option, MMIX
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@code{@value{AS}} will warn every time it expands an instruction to fit an
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operand unless the option @samp{-x} is specified. It is believed that
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this behaviour is more useful than just mimicking @code{mmixal}'s
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behaviour, in which instructions are only expanded if the @samp{-x} option
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is specified, and assembly fails otherwise, when an instruction needs to
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be expanded. It needs to be kept in mind that @code{mmixal} is both an
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assembler and linker, while @code{@value{AS}} will expand instructions
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that at link stage can be contracted. (Though linker relaxation isn't yet
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implemented in @code{@value{LD}}.) The option @samp{-x} also implies
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@samp{--linker-allocated-gregs}.
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@cindex @samp{--no-pushj-stubs} command-line option, MMIX
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@cindex @samp{--no-stubs} command-line option, MMIX
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If instruction expansion is enabled, @code{@value{AS}} can expand a
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@samp{PUSHJ} instruction into a series of instructions. The shortest
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expansion is to not expand it, but just mark the call as redirectable to a
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stub, which @code{@value{LD}} creates at link-time, but only if the
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original @samp{PUSHJ} instruction is found not to reach the target. The
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stub consists of the necessary instructions to form a jump to the target.
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This happens if @code{@value{AS}} can assert that the @samp{PUSHJ}
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instruction can reach such a stub. The option @samp{--no-pushj-stubs}
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disables this shorter expansion, and the longer series of instructions is
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then created at assembly-time. The option @samp{--no-stubs} is a synonym,
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intended for compatibility with future releases, where generation of stubs
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for other instructions may be implemented.
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@cindex @samp{--linker-allocated-gregs} command-line option, MMIX
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Usually a two-operand-expression (@pxref{GREG-base}) without a matching
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@samp{GREG} directive is treated as an error by @code{@value{AS}}. When
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the option @samp{--linker-allocated-gregs} is in effect, they are instead
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passed through to the linker, which will allocate as many global registers
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as is needed.
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@node MMIX-Expand
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@section Instruction expansion
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@cindex instruction expansion, MMIX
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When @code{@value{AS}} encounters an instruction with an operand that is
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either not known or does not fit the operand size of the instruction,
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@code{@value{AS}} (and @code{@value{LD}}) will expand the instruction into
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a sequence of instructions semantically equivalent to the operand fitting
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the instruction. Expansion will take place for the following
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instructions:
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@table @asis
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@item @samp{GETA}
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Expands to a sequence of four instructions: @code{SETL}, @code{INCML},
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@code{INCMH} and @code{INCH}. The operand must be a multiple of four.
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@item Conditional branches
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A branch instruction is turned into a branch with the complemented
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condition and prediction bit over five instructions; four instructions
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setting @code{$255} to the operand value, which like with @code{GETA} must
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be a multiple of four, and a final @code{GO $255,$255,0}.
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@item @samp{PUSHJ}
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Similar to expansion for conditional branches; four instructions set
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@code{$255} to the operand value, followed by a @code{PUSHGO $255,$255,0}.
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@item @samp{JMP}
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Similar to conditional branches and @code{PUSHJ}. The final instruction
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is @code{GO $255,$255,0}.
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@end table
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The linker @code{@value{LD}} is expected to shrink these expansions for
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code assembled with @samp{--relax} (though not currently implemented).
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@node MMIX-Syntax
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@section Syntax
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The assembly syntax is supposed to be upward compatible with that
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described in Sections 1.3 and 1.4 of @samp{The Art of Computer
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Programming, Volume 1}. Draft versions of those chapters as well as other
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MMIX information is located at
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@anchor{mmixsite}@url{http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html}.
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Most code examples from the mmixal package located there should work
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unmodified when assembled and linked as single files, with a few
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noteworthy exceptions (@pxref{MMIX-mmixal}).
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Before an instruction is emitted, the current location is aligned to the
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next four-byte boundary. If a label is defined at the beginning of the
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line, its value will be the aligned value.
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In addition to the traditional hex-prefix @samp{0x}, a hexadecimal number
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can also be specified by the prefix character @samp{#}.
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After all operands to an MMIX instruction or directive have been
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specified, the rest of the line is ignored, treated as a comment.
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@menu
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* MMIX-Chars:: Special Characters
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* MMIX-Symbols:: Symbols
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* MMIX-Regs:: Register Names
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* MMIX-Pseudos:: Assembler Directives
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@end menu
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@node MMIX-Chars
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@subsection Special Characters
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@cindex line comment characters, MMIX
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@cindex MMIX line comment characters
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The characters @samp{*} and @samp{#} are line comment characters; each
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start a comment at the beginning of a line, but only at the beginning of a
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line. A @samp{#} prefixes a hexadecimal number if found elsewhere on a
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line. If a @samp{#} appears at the start of a line the whole line is
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treated as a comment, but the line can also act as a logical line
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number directive (@pxref{Comments}) or a preprocessor control command
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(@pxref{Preprocessing}).
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Two other characters, @samp{%} and @samp{!}, each start a comment anywhere
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on the line. Thus you can't use the @samp{modulus} and @samp{not}
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operators in expressions normally associated with these two characters.
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A @samp{;} is a line separator, treated as a new-line, so separate
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instructions can be specified on a single line.
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@node MMIX-Symbols
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@subsection Symbols
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The character @samp{:} is permitted in identifiers. There are two
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exceptions to it being treated as any other symbol character: if a symbol
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begins with @samp{:}, it means that the symbol is in the global namespace
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and that the current prefix should not be prepended to that symbol
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(@pxref{MMIX-prefix}). The @samp{:} is then not considered part of the
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symbol. For a symbol in the label position (first on a line), a @samp{:}
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at the end of a symbol is silently stripped off. A label is permitted,
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but not required, to be followed by a @samp{:}, as with many other
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assembly formats.
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The character @samp{@@} in an expression, is a synonym for @samp{.}, the
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current location.
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In addition to the common forward and backward local symbol formats
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(@pxref{Symbol Names}), they can be specified with upper-case @samp{B} and
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@samp{F}, as in @samp{8B} and @samp{9F}. A local label defined for the
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current position is written with a @samp{H} appended to the number:
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@smallexample
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3H LDB $0,$1,2
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@end smallexample
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This and traditional local-label formats cannot be mixed: a label must be
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defined and referred to using the same format.
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There's a minor caveat: just as for the ordinary local symbols, the local
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symbols are translated into ordinary symbols using control characters are
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to hide the ordinal number of the symbol. Unfortunately, these symbols
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are not translated back in error messages. Thus you may see confusing
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error messages when local symbols are used. Control characters
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@samp{\003} (control-C) and @samp{\004} (control-D) are used for the
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MMIX-specific local-symbol syntax.
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The symbol @samp{Main} is handled specially; it is always global.
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By defining the symbols @samp{__.MMIX.start..text} and
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@samp{__.MMIX.start..data}, the address of respectively the @samp{.text}
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and @samp{.data} segments of the final program can be defined, though when
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linking more than one object file, the code or data in the object file
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containing the symbol is not guaranteed to be start at that position; just
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the final executable. @xref{MMIX-loc}.
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@node MMIX-Regs
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@subsection Register names
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@cindex register names, MMIX
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@cindex MMIX register names
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Local and global registers are specified as @samp{$0} to @samp{$255}.
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The recognized special register names are @samp{rJ}, @samp{rA}, @samp{rB},
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@samp{rC}, @samp{rD}, @samp{rE}, @samp{rF}, @samp{rG}, @samp{rH},
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@samp{rI}, @samp{rK}, @samp{rL}, @samp{rM}, @samp{rN}, @samp{rO},
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@samp{rP}, @samp{rQ}, @samp{rR}, @samp{rS}, @samp{rT}, @samp{rU},
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@samp{rV}, @samp{rW}, @samp{rX}, @samp{rY}, @samp{rZ}, @samp{rBB},
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@samp{rTT}, @samp{rWW}, @samp{rXX}, @samp{rYY} and @samp{rZZ}. A leading
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@samp{:} is optional for special register names.
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Local and global symbols can be equated to register names and used in
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place of ordinary registers.
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Similarly for special registers, local and global symbols can be used.
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Also, symbols equated from numbers and constant expressions are allowed in
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place of a special register, except when either of the options
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@code{--no-predefined-syms} and @code{--fixed-special-register-names} are
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specified. Then only the special register names above are allowed for the
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instructions having a special register operand; @code{GET} and @code{PUT}.
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@node MMIX-Pseudos
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@subsection Assembler Directives
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@cindex assembler directives, MMIX
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@cindex pseudo-ops, MMIX
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@cindex MMIX assembler directives
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@cindex MMIX pseudo-ops
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@table @code
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@item LOC
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@cindex assembler directive LOC, MMIX
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@cindex pseudo-op LOC, MMIX
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@cindex MMIX assembler directive LOC
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@cindex MMIX pseudo-op LOC
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@anchor{MMIX-loc}
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The @code{LOC} directive sets the current location to the value of the
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operand field, which may include changing sections. If the operand is a
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constant, the section is set to either @code{.data} if the value is
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@code{0x2000000000000000} or larger, else it is set to @code{.text}.
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Within a section, the current location may only be changed to
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monotonically higher addresses. A LOC expression must be a previously
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defined symbol or a ``pure'' constant.
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An example, which sets the label @var{prev} to the current location, and
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updates the current location to eight bytes forward:
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@smallexample
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prev LOC @@+8
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@end smallexample
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When a LOC has a constant as its operand, a symbol
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@code{__.MMIX.start..text} or @code{__.MMIX.start..data} is defined
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depending on the address as mentioned above. Each such symbol is
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interpreted as special by the linker, locating the section at that
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address. Note that if multiple files are linked, the first object file
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with that section will be mapped to that address (not necessarily the file
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with the LOC definition).
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@item LOCAL
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@cindex assembler directive LOCAL, MMIX
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@cindex pseudo-op LOCAL, MMIX
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@cindex MMIX assembler directive LOCAL
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@cindex MMIX pseudo-op LOCAL
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@anchor{MMIX-local}
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Example:
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@smallexample
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LOCAL external_symbol
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LOCAL 42
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.local asymbol
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@end smallexample
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This directive-operation generates a link-time assertion that the operand
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does not correspond to a global register. The operand is an expression
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that at link-time resolves to a register symbol or a number. A number is
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treated as the register having that number. There is one restriction on
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the use of this directive: the pseudo-directive must be placed in a
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section with contents, code or data.
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@item IS
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@cindex assembler directive IS, MMIX
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@cindex pseudo-op IS, MMIX
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@cindex MMIX assembler directive IS
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@cindex MMIX pseudo-op IS
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@anchor{MMIX-is}
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The @code{IS} directive:
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@smallexample
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asymbol IS an_expression
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@end smallexample
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sets the symbol @samp{asymbol} to @samp{an_expression}. A symbol may not
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be set more than once using this directive. Local labels may be set using
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this directive, for example:
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@smallexample
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5H IS @@+4
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@end smallexample
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@item GREG
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@cindex assembler directive GREG, MMIX
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@cindex pseudo-op GREG, MMIX
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@cindex MMIX assembler directive GREG
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@cindex MMIX pseudo-op GREG
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@anchor{MMIX-greg}
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This directive reserves a global register, gives it an initial value and
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optionally gives it a symbolic name. Some examples:
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@smallexample
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areg GREG
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breg GREG data_value
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GREG data_buffer
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.greg creg, another_data_value
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@end smallexample
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The symbolic register name can be used in place of a (non-special)
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register. If a value isn't provided, it defaults to zero. Unless the
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option @samp{--no-merge-gregs} is specified, non-zero registers allocated
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with this directive may be eliminated by @code{@value{AS}}; another
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register with the same value used in its place.
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Any of the instructions
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@samp{CSWAP},
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@samp{GO},
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@samp{LDA},
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@samp{LDBU},
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@samp{LDB},
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@samp{LDHT},
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@samp{LDOU},
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@samp{LDO},
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@samp{LDSF},
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@samp{LDTU},
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@samp{LDT},
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@samp{LDUNC},
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@samp{LDVTS},
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@samp{LDWU},
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@samp{LDW},
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@samp{PREGO},
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@samp{PRELD},
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@samp{PREST},
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@samp{PUSHGO},
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@samp{STBU},
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@samp{STB},
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@samp{STCO},
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@samp{STHT},
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@samp{STOU},
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@samp{STSF},
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@samp{STTU},
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@samp{STT},
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@samp{STUNC},
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@samp{SYNCD},
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@samp{SYNCID},
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can have a value nearby @anchor{GREG-base}an initial value in place of its
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second and third operands. Here, ``nearby'' is defined as within the
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range 0@dots{}255 from the initial value of such an allocated register.
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@smallexample
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buffer1 BYTE 0,0,0,0,0
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buffer2 BYTE 0,0,0,0,0
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@dots{}
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GREG buffer1
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LDOU $42,buffer2
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@end smallexample
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In the example above, the @samp{Y} field of the @code{LDOUI} instruction
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(LDOU with a constant Z) will be replaced with the global register
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allocated for @samp{buffer1}, and the @samp{Z} field will have the value
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5, the offset from @samp{buffer1} to @samp{buffer2}. The result is
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equivalent to this code:
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@smallexample
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buffer1 BYTE 0,0,0,0,0
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buffer2 BYTE 0,0,0,0,0
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@dots{}
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tmpreg GREG buffer1
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LDOU $42,tmpreg,(buffer2-buffer1)
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@end smallexample
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Global registers allocated with this directive are allocated in order
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higher-to-lower within a file. Other than that, the exact order of
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register allocation and elimination is undefined. For example, the order
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is undefined when more than one file with such directives are linked
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together. With the options @samp{-x} and @samp{--linker-allocated-gregs},
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@samp{GREG} directives for two-operand cases like the one mentioned above
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can be omitted. Sufficient global registers will then be allocated by the
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linker.
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@item BYTE
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@cindex assembler directive BYTE, MMIX
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@cindex pseudo-op BYTE, MMIX
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@cindex MMIX assembler directive BYTE
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@cindex MMIX pseudo-op BYTE
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@anchor{MMIX-byte}
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The @samp{BYTE} directive takes a series of operands separated by a comma.
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If an operand is a string (@pxref{Strings}), each character of that string
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is emitted as a byte. Other operands must be constant expressions without
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forward references, in the range 0@dots{}255. If you need operands having
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expressions with forward references, use @samp{.byte} (@pxref{Byte}). An
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operand can be omitted, defaulting to a zero value.
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@item WYDE
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@itemx TETRA
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@itemx OCTA
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@cindex assembler directive WYDE, MMIX
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@cindex pseudo-op WYDE, MMIX
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@cindex MMIX assembler directive WYDE
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@cindex MMIX pseudo-op WYDE
|
|
@cindex assembler directive TETRA, MMIX
|
|
@cindex pseudo-op TETRA, MMIX
|
|
@cindex MMIX assembler directive TETRA
|
|
@cindex MMIX pseudo-op TETRA
|
|
@cindex assembler directive OCTA, MMIX
|
|
@cindex pseudo-op OCTA, MMIX
|
|
@cindex MMIX assembler directive OCTA
|
|
@cindex MMIX pseudo-op OCTA
|
|
|
|
@anchor{MMIX-constants}
|
|
The directives @samp{WYDE}, @samp{TETRA} and @samp{OCTA} emit constants of
|
|
two, four and eight bytes size respectively. Before anything else happens
|
|
for the directive, the current location is aligned to the respective
|
|
constant-size boundary. If a label is defined at the beginning of the
|
|
line, its value will be that after the alignment. A single operand can be
|
|
omitted, defaulting to a zero value emitted for the directive. Operands
|
|
can be expressed as strings (@pxref{Strings}), in which case each
|
|
character in the string is emitted as a separate constant of the size
|
|
indicated by the directive.
|
|
|
|
@item PREFIX
|
|
@cindex assembler directive PREFIX, MMIX
|
|
@cindex pseudo-op PREFIX, MMIX
|
|
@cindex MMIX assembler directive PREFIX
|
|
@cindex MMIX pseudo-op PREFIX
|
|
|
|
@anchor{MMIX-prefix}
|
|
The @samp{PREFIX} directive sets a symbol name prefix to be prepended to
|
|
all symbols (except local symbols, @pxref{MMIX-Symbols}), that are not
|
|
prefixed with @samp{:}, until the next @samp{PREFIX} directive. Such
|
|
prefixes accumulate. For example,
|
|
@smallexample
|
|
PREFIX a
|
|
PREFIX b
|
|
c IS 0
|
|
@end smallexample
|
|
defines a symbol @samp{abc} with the value 0.
|
|
|
|
@item BSPEC
|
|
@itemx ESPEC
|
|
@cindex assembler directive BSPEC, MMIX
|
|
@cindex pseudo-op BSPEC, MMIX
|
|
@cindex MMIX assembler directive BSPEC
|
|
@cindex MMIX pseudo-op BSPEC
|
|
@cindex assembler directive ESPEC, MMIX
|
|
@cindex pseudo-op ESPEC, MMIX
|
|
@cindex MMIX assembler directive ESPEC
|
|
@cindex MMIX pseudo-op ESPEC
|
|
|
|
@anchor{MMIX-spec}
|
|
A pair of @samp{BSPEC} and @samp{ESPEC} directives delimit a section of
|
|
special contents (without specified semantics). Example:
|
|
@smallexample
|
|
BSPEC 42
|
|
TETRA 1,2,3
|
|
ESPEC
|
|
@end smallexample
|
|
The single operand to @samp{BSPEC} must be number in the range
|
|
0@dots{}255. The @samp{BSPEC} number 80 is used by the GNU binutils
|
|
implementation.
|
|
@end table
|
|
|
|
@node MMIX-mmixal
|
|
@section Differences to @code{mmixal}
|
|
@cindex mmixal differences
|
|
@cindex differences, mmixal
|
|
|
|
The binutils @code{@value{AS}} and @code{@value{LD}} combination has a few
|
|
differences in function compared to @code{mmixal} (@pxref{mmixsite}).
|
|
|
|
The replacement of a symbol with a GREG-allocated register
|
|
(@pxref{GREG-base}) is not handled the exactly same way in
|
|
@code{@value{AS}} as in @code{mmixal}. This is apparent in the
|
|
@code{mmixal} example file @code{inout.mms}, where different registers
|
|
with different offsets, eventually yielding the same address, are used in
|
|
the first instruction. This type of difference should however not affect
|
|
the function of any program unless it has specific assumptions about the
|
|
allocated register number.
|
|
|
|
Line numbers (in the @samp{mmo} object format) are currently not
|
|
supported.
|
|
|
|
Expression operator precedence is not that of mmixal: operator precedence
|
|
is that of the C programming language. It's recommended to use
|
|
parentheses to explicitly specify wanted operator precedence whenever more
|
|
than one type of operators are used.
|
|
|
|
The serialize unary operator @code{&}, the fractional division operator
|
|
@samp{//}, the logical not operator @code{!} and the modulus operator
|
|
@samp{%} are not available.
|
|
|
|
Symbols are not global by default, unless the option
|
|
@samp{--globalize-symbols} is passed. Use the @samp{.global} directive to
|
|
globalize symbols (@pxref{Global}).
|
|
|
|
Operand syntax is a bit stricter with @code{@value{AS}} than
|
|
@code{mmixal}. For example, you can't say @code{addu 1,2,3}, instead you
|
|
must write @code{addu $1,$2,3}.
|
|
|
|
You can't LOC to a lower address than those already visited
|
|
(i.e., ``backwards'').
|
|
|
|
A LOC directive must come before any emitted code.
|
|
|
|
Predefined symbols are visible as file-local symbols after use. (In the
|
|
ELF file, that is---the linked mmo file has no notion of a file-local
|
|
symbol.)
|
|
|
|
Some mapping of constant expressions to sections in LOC expressions is
|
|
attempted, but that functionality is easily confused and should be avoided
|
|
unless compatibility with @code{mmixal} is required. A LOC expression to
|
|
@samp{0x2000000000000000} or higher, maps to the @samp{.data} section and
|
|
lower addresses map to the @samp{.text} section (@pxref{MMIX-loc}).
|
|
|
|
The code and data areas are each contiguous. Sparse programs with
|
|
far-away LOC directives will take up the same amount of space as a
|
|
contiguous program with zeros filled in the gaps between the LOC
|
|
directives. If you need sparse programs, you might try and get the wanted
|
|
effect with a linker script and splitting up the code parts into sections
|
|
(@pxref{Section}). Assembly code for this, to be compatible with
|
|
@code{mmixal}, would look something like:
|
|
@smallexample
|
|
.if 0
|
|
LOC away_expression
|
|
.else
|
|
.section away,"ax"
|
|
.fi
|
|
@end smallexample
|
|
@code{@value{AS}} will not execute the LOC directive and @code{mmixal}
|
|
ignores the lines with @code{.}. This construct can be used generally to
|
|
help compatibility.
|
|
|
|
Symbols can't be defined twice--not even to the same value.
|
|
|
|
Instruction mnemonics are recognized case-insensitive, though the
|
|
@samp{IS} and @samp{GREG} pseudo-operations must be specified in
|
|
upper-case characters.
|
|
|
|
There's no unicode support.
|
|
|
|
The following is a list of programs in @samp{mmix.tar.gz}, available at
|
|
@url{http://www-cs-faculty.stanford.edu/~knuth/mmix-news.html}, last
|
|
checked with the version dated 2001-08-25 (md5sum
|
|
c393470cfc86fac040487d22d2bf0172) that assemble with @code{mmixal} but do
|
|
not assemble with @code{@value{AS}}:
|
|
|
|
@table @code
|
|
@item silly.mms
|
|
LOC to a previous address.
|
|
@item sim.mms
|
|
Redefines symbol @samp{Done}.
|
|
@item test.mms
|
|
Uses the serial operator @samp{&}.
|
|
@end table
|