412 lines
15 KiB
Plaintext
412 lines
15 KiB
Plaintext
@c Copyright (C) 2002-2016 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 CRIS description contributed by Axis Communications.
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@ifset GENERIC
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@page
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@node CRIS-Dependent
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@chapter CRIS 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 CRIS Dependent Features
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@end ifclear
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@cindex CRIS support
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@menu
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* CRIS-Opts:: Command-line Options
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* CRIS-Expand:: Instruction expansion
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* CRIS-Symbols:: Symbols
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* CRIS-Syntax:: Syntax
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@end menu
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@node CRIS-Opts
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@section Command-line Options
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@cindex options, CRIS
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@cindex CRIS options
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The CRIS version of @code{@value{AS}} has these
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machine-dependent command-line options.
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@cindex @option{--emulation=criself} command line option, CRIS
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@cindex @option{--emulation=crisaout} command line option, CRIS
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@cindex CRIS @option{--emulation=criself} command line option
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@cindex CRIS @option{--emulation=crisaout} command line option
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The format of the generated object files can be either ELF or
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a.out, specified by the command-line options
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@option{--emulation=crisaout} and @option{--emulation=criself}.
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The default is ELF (criself), unless @code{@value{AS}} has been
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configured specifically for a.out by using the configuration
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name @code{cris-axis-aout}.
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@cindex @option{--underscore} command line option, CRIS
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@cindex @option{--no-underscore} command line option, CRIS
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@cindex CRIS @option{--underscore} command line option
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@cindex CRIS @option{--no-underscore} command line option
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There are two different link-incompatible ELF object file
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variants for CRIS, for use in environments where symbols are
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expected to be prefixed by a leading @samp{_} character and for
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environments without such a symbol prefix. The variant used for
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GNU/Linux port has no symbol prefix. Which variant to produce
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is specified by either of the options @option{--underscore} and
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@option{--no-underscore}. The default is @option{--underscore}.
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Since symbols in CRIS a.out objects are expected to have a
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@samp{_} prefix, specifying @option{--no-underscore} when
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generating a.out objects is an error. Besides the object format
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difference, the effect of this option is to parse register names
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differently (@pxref{crisnous}). The @option{--no-underscore}
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option makes a @samp{$} register prefix mandatory.
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@cindex @option{--pic} command line option, CRIS
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@cindex CRIS @option{--pic} command line option
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@cindex Position-independent code, CRIS
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@cindex CRIS position-independent code
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The option @option{--pic} must be passed to @code{@value{AS}} in
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order to recognize the symbol syntax used for ELF (SVR4 PIC)
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position-independent-code (@pxref{crispic}). This will also
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affect expansion of instructions. The expansion with
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@option{--pic} will use PC-relative rather than (slightly
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faster) absolute addresses in those expansions. This option is only
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valid when generating ELF format object files.
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@cindex @option{--march=@var{architecture}} command line option, CRIS
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@cindex CRIS @option{--march=@var{architecture}} command line option
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@cindex Architecture variant option, CRIS
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@cindex CRIS architecture variant option
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The option @option{--march=@var{architecture}}
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@anchor{march-option}specifies the recognized instruction set
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and recognized register names. It also controls the
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architecture type of the object file. Valid values for
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@var{architecture} are:
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@table @code
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@item v0_v10
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All instructions and register names for any architecture variant
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in the set v0@dots{}v10 are recognized. This is the
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default if the target is configured as cris-*.
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@item v10
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Only instructions and register names for CRIS v10 (as found in
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ETRAX 100 LX) are recognized. This is the default if the target
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is configured as crisv10-*.
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@item v32
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Only instructions and register names for CRIS v32 (code name
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Guinness) are recognized. This is the default if the target is
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configured as crisv32-*. This value implies
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@option{--no-mul-bug-abort}. (A subsequent
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@option{--mul-bug-abort} will turn it back on.)
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@item common_v10_v32
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Only instructions with register names and addressing modes with
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opcodes common to the v10 and v32 are recognized.
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@end table
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@cindex @option{-N} command line option, CRIS
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@cindex CRIS @option{-N} command line option
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When @option{-N} is specified, @code{@value{AS}} will emit a
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warning when a 16-bit branch instruction is expanded into a
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32-bit multiple-instruction construct (@pxref{CRIS-Expand}).
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@cindex @option{--no-mul-bug-abort} command line option, CRIS
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@cindex @option{--mul-bug-abort} command line option, CRIS
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@cindex CRIS @option{--no-mul-bug-abort} command line option
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@cindex CRIS @option{--mul-bug-abort} command line option
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Some versions of the CRIS v10, for example in the Etrax 100 LX,
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contain a bug that causes destabilizing memory accesses when a
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multiply instruction is executed with certain values in the
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first operand just before a cache-miss. When the
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@option{--mul-bug-abort} command line option is active (the
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default value), @code{@value{AS}} will refuse to assemble a file
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containing a multiply instruction at a dangerous offset, one
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that could be the last on a cache-line, or is in a section with
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insufficient alignment. This placement checking does not catch
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any case where the multiply instruction is dangerously placed
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because it is located in a delay-slot. The
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@option{--mul-bug-abort} command line option turns off the
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checking.
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@node CRIS-Expand
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@section Instruction expansion
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@cindex instruction expansion, CRIS
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@cindex CRIS instruction expansion
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@code{@value{AS}} will silently choose an instruction that fits
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the operand size for @samp{[register+constant]} operands. For
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example, the offset @code{127} in @code{move.d [r3+127],r4} fits
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in an instruction using a signed-byte offset. Similarly,
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@code{move.d [r2+32767],r1} will generate an instruction using a
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16-bit offset. For symbolic expressions and constants that do
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not fit in 16 bits including the sign bit, a 32-bit offset is
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generated.
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For branches, @code{@value{AS}} will expand from a 16-bit branch
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instruction into a sequence of instructions that can reach a
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full 32-bit address. Since this does not correspond to a single
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instruction, such expansions can optionally be warned about.
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@xref{CRIS-Opts}.
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If the operand is found to fit the range, a @code{lapc} mnemonic
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will translate to a @code{lapcq} instruction. Use @code{lapc.d}
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to force the 32-bit @code{lapc} instruction.
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Similarly, the @code{addo} mnemonic will translate to the
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shortest fitting instruction of @code{addoq}, @code{addo.w} and
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@code{addo.d}, when used with a operand that is a constant known
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at assembly time.
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@node CRIS-Symbols
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@section Symbols
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@cindex Symbols, built-in, CRIS
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@cindex Symbols, CRIS, built-in
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@cindex CRIS built-in symbols
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@cindex Built-in symbols, CRIS
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Some symbols are defined by the assembler. They're intended to
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be used in conditional assembly, for example:
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@smallexample
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.if ..asm.arch.cris.v32
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@var{code for CRIS v32}
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.elseif ..asm.arch.cris.common_v10_v32
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@var{code common to CRIS v32 and CRIS v10}
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.elseif ..asm.arch.cris.v10 | ..asm.arch.cris.any_v0_v10
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@var{code for v10}
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.else
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.error "Code needs to be added here."
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.endif
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@end smallexample
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These symbols are defined in the assembler, reflecting
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command-line options, either when specified or the default.
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They are always defined, to 0 or 1.
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@table @code
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@item ..asm.arch.cris.any_v0_v10
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This symbol is non-zero when @option{--march=v0_v10} is specified
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or the default.
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@item ..asm.arch.cris.common_v10_v32
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Set according to the option @option{--march=common_v10_v32}.
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@item ..asm.arch.cris.v10
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Reflects the option @option{--march=v10}.
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@item ..asm.arch.cris.v32
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Corresponds to @option{--march=v10}.
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@end table
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Speaking of symbols, when a symbol is used in code, it can have
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a suffix modifying its value for use in position-independent
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code. @xref{CRIS-Pic}.
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@node CRIS-Syntax
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@section Syntax
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There are different aspects of the CRIS assembly syntax.
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@menu
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* CRIS-Chars:: Special Characters
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* CRIS-Pic:: Position-Independent Code Symbols
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* CRIS-Regs:: Register Names
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* CRIS-Pseudos:: Assembler Directives
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@end menu
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@node CRIS-Chars
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@subsection Special Characters
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@cindex line comment characters, CRIS
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@cindex CRIS line comment characters
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The character @samp{#} is a line comment character. It starts a
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comment if and only if it is placed at the beginning of a line.
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A @samp{;} character starts a comment anywhere on the line,
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causing all characters up to the end of the line to be ignored.
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A @samp{@@} character is handled as a line separator equivalent
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to a logical new-line character (except in a comment), so
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separate instructions can be specified on a single line.
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@node CRIS-Pic
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@subsection Symbols in position-independent code
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@cindex Symbols in position-independent code, CRIS
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@cindex CRIS symbols in position-independent code
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@cindex Position-independent code, symbols in, CRIS
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When generating @anchor{crispic}position-independent code (SVR4
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PIC) for use in cris-axis-linux-gnu or crisv32-axis-linux-gnu
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shared libraries, symbol
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suffixes are used to specify what kind of run-time symbol lookup
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will be used, expressed in the object as different
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@emph{relocation types}. Usually, all absolute symbol values
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must be located in a table, the @emph{global offset table},
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leaving the code position-independent; independent of values of
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global symbols and independent of the address of the code. The
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suffix modifies the value of the symbol, into for example an
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index into the global offset table where the real symbol value
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is entered, or a PC-relative value, or a value relative to the
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start of the global offset table. All symbol suffixes start
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with the character @samp{:} (omitted in the list below). Every
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symbol use in code or a read-only section must therefore have a
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PIC suffix to enable a useful shared library to be created.
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Usually, these constructs must not be used with an additive
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constant offset as is usually allowed, i.e.@: no 4 as in
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@code{symbol + 4} is allowed. This restriction is checked at
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link-time, not at assembly-time.
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@table @code
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@item GOT
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Attaching this suffix to a symbol in an instruction causes the
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symbol to be entered into the global offset table. The value is
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a 32-bit index for that symbol into the global offset table.
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The name of the corresponding relocation is
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@samp{R_CRIS_32_GOT}. Example: @code{move.d
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[$r0+extsym:GOT],$r9}
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@item GOT16
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Same as for @samp{GOT}, but the value is a 16-bit index into the
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global offset table. The corresponding relocation is
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@samp{R_CRIS_16_GOT}. Example: @code{move.d
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[$r0+asymbol:GOT16],$r10}
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@item PLT
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This suffix is used for function symbols. It causes a
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@emph{procedure linkage table}, an array of code stubs, to be
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created at the time the shared object is created or linked
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against, together with a global offset table entry. The value
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is a pc-relative offset to the corresponding stub code in the
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procedure linkage table. This arrangement causes the run-time
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symbol resolver to be called to look up and set the value of the
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symbol the first time the function is called (at latest;
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depending environment variables). It is only safe to leave the
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symbol unresolved this way if all references are function calls.
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The name of the relocation is @samp{R_CRIS_32_PLT_PCREL}.
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Example: @code{add.d fnname:PLT,$pc}
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@item PLTG
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Like PLT, but the value is relative to the beginning of the
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global offset table. The relocation is
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@samp{R_CRIS_32_PLT_GOTREL}. Example: @code{move.d
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fnname:PLTG,$r3}
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@item GOTPLT
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Similar to @samp{PLT}, but the value of the symbol is a 32-bit
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index into the global offset table. This is somewhat of a mix
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between the effect of the @samp{GOT} and the @samp{PLT} suffix;
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the difference to @samp{GOT} is that there will be a procedure
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linkage table entry created, and that the symbol is assumed to
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be a function entry and will be resolved by the run-time
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resolver as with @samp{PLT}. The relocation is
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@samp{R_CRIS_32_GOTPLT}. Example: @code{jsr
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[$r0+fnname:GOTPLT]}
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@item GOTPLT16
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A variant of @samp{GOTPLT} giving a 16-bit value. Its
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relocation name is @samp{R_CRIS_16_GOTPLT}. Example: @code{jsr
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[$r0+fnname:GOTPLT16]}
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@item GOTOFF
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This suffix must only be attached to a local symbol, but may be
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used in an expression adding an offset. The value is the
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address of the symbol relative to the start of the global offset
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table. The relocation name is @samp{R_CRIS_32_GOTREL}.
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Example: @code{move.d [$r0+localsym:GOTOFF],r3}
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@end table
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@node CRIS-Regs
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@subsection Register names
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@cindex register names, CRIS
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@cindex CRIS register names
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A @samp{$} character may always prefix a general or special
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register name in an instruction operand but is mandatory when
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the option @option{--no-underscore} is specified or when the
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@code{.syntax register_prefix} directive is in effect
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(@pxref{crisnous}). Register names are case-insensitive.
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@node CRIS-Pseudos
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@subsection Assembler Directives
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@cindex assembler directives, CRIS
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@cindex pseudo-ops, CRIS
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@cindex CRIS assembler directives
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@cindex CRIS pseudo-ops
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There are a few CRIS-specific pseudo-directives in addition to
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the generic ones. @xref{Pseudo Ops}. Constants emitted by
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pseudo-directives are in little-endian order for CRIS. There is
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no support for floating-point-specific directives for CRIS.
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@table @code
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@item .dword EXPRESSIONS
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@cindex assembler directive .dword, CRIS
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@cindex pseudo-op .dword, CRIS
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@cindex CRIS assembler directive .dword
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@cindex CRIS pseudo-op .dword
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The @code{.dword} directive is a synonym for @code{.int},
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expecting zero or more EXPRESSIONS, separated by commas. For
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each expression, a 32-bit little-endian constant is emitted.
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@item .syntax ARGUMENT
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@cindex assembler directive .syntax, CRIS
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@cindex pseudo-op .syntax, CRIS
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@cindex CRIS assembler directive .syntax
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@cindex CRIS pseudo-op .syntax
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The @code{.syntax} directive takes as @var{ARGUMENT} one of the
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following case-sensitive choices.
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@table @code
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@item no_register_prefix
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The @code{.syntax no_register_prefix} @anchor{crisnous}directive
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makes a @samp{$} character prefix on all registers optional. It
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overrides a previous setting, including the corresponding effect
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of the option @option{--no-underscore}. If this directive is
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used when ordinary symbols do not have a @samp{_} character
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prefix, care must be taken to avoid ambiguities whether an
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operand is a register or a symbol; using symbols with names the
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same as general or special registers then invoke undefined
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behavior.
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@item register_prefix
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This directive makes a @samp{$} character prefix on all
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registers mandatory. It overrides a previous setting, including
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the corresponding effect of the option @option{--underscore}.
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@item leading_underscore
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This is an assertion directive, emitting an error if the
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@option{--no-underscore} option is in effect.
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@item no_leading_underscore
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This is the opposite of the @code{.syntax leading_underscore}
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directive and emits an error if the option @option{--underscore}
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is in effect.
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@end table
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@item .arch ARGUMENT
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@cindex assembler directive .arch, CRIS
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@cindex pseudo-op .arch, CRIS
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@cindex CRIS assembler directive .arch
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@cindex CRIS pseudo-op .arch
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This is an assertion directive, giving an error if the specified
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@var{ARGUMENT} is not the same as the specified or default value
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for the @option{--march=@var{architecture}} option
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(@pxref{march-option}).
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@c If you compare with md_pseudo_table, you see that we don't
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@c document ".file" and ".loc" here. This is because we're just
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@c wrapping the corresponding ELF function and emitting an error for
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@c a.out.
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@end table
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