Subversion Repositories openrisc_2011-10-31

@c Copyright 2002, 2004 Free Software Foundation, Inc.

@c This is part of the GAS manual.

@c For copying conditions, see the file as.texinfo.

@c CRIS description contributed by Axis Communications.

@ifset GENERIC

@page

@node CRIS-Dependent

@chapter CRIS Dependent Features

@end ifset

@ifclear GENERIC

@node Machine Dependencies

@chapter CRIS Dependent Features

@end ifclear

@cindex CRIS support

@menu

* CRIS-Opts::              Command-line Options

* CRIS-Expand::            Instruction expansion

* CRIS-Symbols::           Symbols

* CRIS-Syntax::            Syntax

@end menu

@node CRIS-Opts

@section Command-line Options

@cindex options, CRIS

@cindex CRIS options

The CRIS version of @code{@value{AS}} has these

machine-dependent command-line options.

@cindex @option{--emulation=criself} command line option, CRIS

@cindex @option{--emulation=crisaout} command line option, CRIS

@cindex CRIS @option{--emulation=criself} command line option

@cindex CRIS @option{--emulation=crisaout} command line option

The format of the generated object files can be either ELF or

a.out, specified by the command-line options

@option{--emulation=crisaout} and @option{--emulation=criself}.

The default is ELF (criself), unless @code{@value{AS}} has been

configured specifically for a.out by using the configuration

name @code{cris-axis-aout}.

@cindex @option{--underscore} command line option, CRIS

@cindex @option{--no-underscore} command line option, CRIS

@cindex CRIS @option{--underscore} command line option

@cindex CRIS @option{--no-underscore} command line option

There are two different link-incompatible ELF object file

variants for CRIS, for use in environments where symbols are

expected to be prefixed by a leading @samp{_} character and for

environments without such a symbol prefix.  The variant used for

GNU/Linux port has no symbol prefix.  Which variant to produce

is specified by either of the options @option{--underscore} and

@option{--no-underscore}.  The default is @option{--underscore}.

Since symbols in CRIS a.out objects are expected to have a

@samp{_} prefix, specifying @option{--no-underscore} when

generating a.out objects is an error.  Besides the object format

difference, the effect of this option is to parse register names

differently (@pxref{crisnous}).  The @option{--no-underscore}

option makes a @samp{$} register prefix mandatory.

@cindex @option{--pic} command line option, CRIS

@cindex CRIS @option{--pic} command line option

@cindex Position-independent code, CRIS

@cindex CRIS position-independent code

The option @option{--pic} must be passed to @code{@value{AS}} in

order to recognize the symbol syntax used for ELF (SVR4 PIC)

position-independent-code (@pxref{crispic}).  This will also

affect expansion of instructions.  The expansion with

@option{--pic} will use PC-relative rather than (slightly

faster) absolute addresses in those expansions.

@cindex @option{--march=@var{architecture}} command line option, CRIS

@cindex CRIS @option{--march=@var{architecture}} command line option

@cindex Architecture variant option, CRIS

@cindex CRIS architecture variant option

The option @option{--march=@var{architecture}}

@anchor{march-option}specifies the recognized instruction set

and recognized register names.  It also controls the

architecture type of the object file.  Valid values for

@var{architecture} are:

@table @code

@item v0_v10

All instructions and register names for any architecture variant

in the set v0@dots{}v10 are recognized.  This is the

default if the target is configured as cris-*.

@item v10

Only instructions and register names for CRIS v10 (as found in

ETRAX 100 LX) are recognized.  This is the default if the target

is configured as crisv10-*.

@item v32

Only instructions and register names for CRIS v32 (code name

Guinness) are recognized.  This is the default if the target is

configured as crisv32-*.  This value implies

@option{--no-mul-bug-abort}.  (A subsequent

@option{--mul-bug-abort} will turn it back on.)

@item common_v10_v32

Only instructions with register names and addressing modes with

opcodes common to the v10 and v32 are recognized.

@end table

@cindex @option{-N} command line option, CRIS

@cindex CRIS @option{-N} command line option

When @option{-N} is specified, @code{@value{AS}} will emit a

warning when a 16-bit branch instruction is expanded into a

32-bit multiple-instruction construct (@pxref{CRIS-Expand}).

@cindex @option{--no-mul-bug-abort} command line option, CRIS

@cindex @option{--mul-bug-abort} command line option, CRIS

@cindex CRIS @option{--no-mul-bug-abort} command line option

@cindex CRIS @option{--mul-bug-abort} command line option

Some versions of the CRIS v10, for example in the Etrax 100 LX,

contain a bug that causes destabilizing memory accesses when a

multiply instruction is executed with certain values in the

first operand just before a cache-miss.  When the

@option{--mul-bug-abort} command line option is active (the

default value), @code{@value{AS}} will refuse to assemble a file

containing a multiply instruction at a dangerous offset, one

that could be the last on a cache-line, or is in a section with

insufficient alignment.  This placement checking does not catch

any case where the multiply instruction is dangerously placed

because it is located in a delay-slot.  The

@option{--mul-bug-abort} command line option turns off the

checking.

@node CRIS-Expand

@section Instruction expansion

@cindex instruction expansion, CRIS

@cindex CRIS instruction expansion

@code{@value{AS}} will silently choose an instruction that fits

the operand size for @samp{[register+constant]} operands.  For

example, the offset @code{127} in @code{move.d [r3+127],r4} fits

in an instruction using a signed-byte offset.  Similarly,

@code{move.d [r2+32767],r1} will generate an instruction using a

16-bit offset.  For symbolic expressions and constants that do

not fit in 16 bits including the sign bit, a 32-bit offset is

generated.

For branches, @code{@value{AS}} will expand from a 16-bit branch

instruction into a sequence of instructions that can reach a

full 32-bit address.  Since this does not correspond to a single

instruction, such expansions can optionally be warned about.

@xref{CRIS-Opts}.

If the operand is found to fit the range, a @code{lapc} mnemonic

will translate to a @code{lapcq} instruction.  Use @code{lapc.d}

to force the 32-bit @code{lapc} instruction.

Similarly, the @code{addo} mnemonic will translate to the

shortest fitting instruction of @code{addoq}, @code{addo.w} and

@code{addo.d}, when used with a operand that is a constant known

at assembly time.

@node CRIS-Symbols

@section Symbols

@cindex Symbols, built-in, CRIS

@cindex Symbols, CRIS, built-in

@cindex CRIS built-in symbols

@cindex Built-in symbols, CRIS

Some symbols are defined by the assembler.  They're intended to

be used in conditional assembly, for example:

@smallexample

 .if ..asm.arch.cris.v32

 @var{code for CRIS v32}

 .elseif ..asm.arch.cris.common_v10_v32

 @var{code common to CRIS v32 and CRIS v10}

 .elseif ..asm.arch.cris.v10 | ..asm.arch.cris.any_v0_v10

 @var{code for v10}

 .else

 .error "Code needs to be added here."

 .endif

@end smallexample

These symbols are defined in the assembler, reflecting

command-line options, either when specified or the default.

They are always defined, to 0 or 1.

@table @code

@item ..asm.arch.cris.any_v0_v10

This symbol is non-zero when @option{--march=v0_v10} is specified

or the default.

@item ..asm.arch.cris.common_v10_v32

Set according to the option @option{--march=common_v10_v32}.

@item ..asm.arch.cris.v10

Reflects the option @option{--march=v10}.

@item ..asm.arch.cris.v32

Corresponds to @option{--march=v10}.

@end table

Speaking of symbols, when a symbol is used in code, it can have

a suffix modifying its value for use in position-independent

code. @xref{CRIS-Pic}.

@node CRIS-Syntax

@section Syntax

There are different aspects of the CRIS assembly syntax.

@menu

* CRIS-Chars::                  Special Characters

* CRIS-Pic::                    Position-Independent Code Symbols

* CRIS-Regs::                   Register Names

* CRIS-Pseudos::                Assembler Directives

@end menu

@node CRIS-Chars

@subsection Special Characters

@cindex line comment characters, CRIS

@cindex CRIS line comment characters

The character @samp{#} is a line comment character.  It starts a

comment if and only if it is placed at the beginning of a line.

A @samp{;} character starts a comment anywhere on the line,

causing all characters up to the end of the line to be ignored.

A @samp{@@} character is handled as a line separator equivalent

to a logical new-line character (except in a comment), so

separate instructions can be specified on a single line.

@node CRIS-Pic

@subsection Symbols in position-independent code

@cindex Symbols in position-independent code, CRIS

@cindex CRIS symbols in position-independent code

@cindex Position-independent code, symbols in, CRIS

When generating @anchor{crispic}position-independent code (SVR4

PIC) for use in cris-axis-linux-gnu or crisv32-axis-linux-gnu

shared libraries, symbol

suffixes are used to specify what kind of run-time symbol lookup

will be used, expressed in the object as different

@emph{relocation types}.  Usually, all absolute symbol values

must be located in a table, the @emph{global offset table},

leaving the code position-independent; independent of values of

global symbols and independent of the address of the code.  The

suffix modifies the value of the symbol, into for example an

index into the global offset table where the real symbol value

is entered, or a PC-relative value, or a value relative to the

start of the global offset table.  All symbol suffixes start

with the character @samp{:} (omitted in the list below).  Every

symbol use in code or a read-only section must therefore have a

PIC suffix to enable a useful shared library to be created.

Usually, these constructs must not be used with an additive

constant offset as is usually allowed, i.e.@: no 4 as in

@code{symbol + 4} is allowed.  This restriction is checked at

link-time, not at assembly-time.

@table @code

@item GOT

Attaching this suffix to a symbol in an instruction causes the

symbol to be entered into the global offset table.  The value is

a 32-bit index for that symbol into the global offset table.

The name of the corresponding relocation is

@samp{R_CRIS_32_GOT}.  Example: @code{move.d

[$r0+extsym:GOT],$r9}

@item GOT16

Same as for @samp{GOT}, but the value is a 16-bit index into the

global offset table.  The corresponding relocation is

@samp{R_CRIS_16_GOT}.  Example: @code{move.d

[$r0+asymbol:GOT16],$r10}

@item PLT

This suffix is used for function symbols.  It causes a

@emph{procedure linkage table}, an array of code stubs, to be

created at the time the shared object is created or linked

against, together with a global offset table entry.  The value

is a pc-relative offset to the corresponding stub code in the

procedure linkage table.  This arrangement causes the run-time

symbol resolver to be called to look up and set the value of the

symbol the first time the function is called (at latest;

depending environment variables).  It is only safe to leave the

symbol unresolved this way if all references are function calls.

The name of the relocation is @samp{R_CRIS_32_PLT_PCREL}.

Example: @code{add.d fnname:PLT,$pc}

@item PLTG

Like PLT, but the value is relative to the beginning of the

global offset table.  The relocation is

@samp{R_CRIS_32_PLT_GOTREL}.  Example: @code{move.d

fnname:PLTG,$r3}

@item GOTPLT

Similar to @samp{PLT}, but the value of the symbol is a 32-bit

index into the global offset table.  This is somewhat of a mix

between the effect of the @samp{GOT} and the @samp{PLT} suffix;

the difference to @samp{GOT} is that there will be a procedure

linkage table entry created, and that the symbol is assumed to

be a function entry and will be resolved by the run-time

resolver as with @samp{PLT}.  The relocation is

@samp{R_CRIS_32_GOTPLT}.  Example: @code{jsr

[$r0+fnname:GOTPLT]}

@item GOTPLT16

A variant of @samp{GOTPLT} giving a 16-bit value.  Its

relocation name is @samp{R_CRIS_16_GOTPLT}.  Example: @code{jsr

[$r0+fnname:GOTPLT16]}

@item GOTOFF

This suffix must only be attached to a local symbol, but may be

used in an expression adding an offset.  The value is the

address of the symbol relative to the start of the global offset

table.  The relocation name is @samp{R_CRIS_32_GOTREL}.

Example: @code{move.d [$r0+localsym:GOTOFF],r3}

@end table

@node CRIS-Regs

@subsection Register names

@cindex register names, CRIS

@cindex CRIS register names

A @samp{$} character may always prefix a general or special

register name in an instruction operand but is mandatory when

the option @option{--no-underscore} is specified or when the

@code{.syntax register_prefix} directive is in effect

(@pxref{crisnous}).  Register names are case-insensitive.

@node CRIS-Pseudos

@subsection Assembler Directives

@cindex assembler directives, CRIS

@cindex pseudo-ops, CRIS

@cindex CRIS assembler directives

@cindex CRIS pseudo-ops

There are a few CRIS-specific pseudo-directives in addition to

the generic ones.  @xref{Pseudo Ops}.  Constants emitted by

pseudo-directives are in little-endian order for CRIS.  There is

no support for floating-point-specific directives for CRIS.

@table @code

@item .dword EXPRESSIONS

@cindex assembler directive .dword, CRIS

@cindex pseudo-op .dword, CRIS

@cindex CRIS assembler directive .dword

@cindex CRIS pseudo-op .dword

The @code{.dword} directive is a synonym for @code{.int},

expecting zero or more EXPRESSIONS, separated by commas.  For

each expression, a 32-bit little-endian constant is emitted.

@item .syntax ARGUMENT

@cindex assembler directive .syntax, CRIS

@cindex pseudo-op .syntax, CRIS

@cindex CRIS assembler directive .syntax

@cindex CRIS pseudo-op .syntax

The @code{.syntax} directive takes as @var{ARGUMENT} one of the

following case-sensitive choices.

@table @code

@item no_register_prefix

The @code{.syntax no_register_prefix} @anchor{crisnous}directive

makes a @samp{$} character prefix on all registers optional.  It

overrides a previous setting, including the corresponding effect

of the option @option{--no-underscore}.  If this directive is

used when ordinary symbols do not have a @samp{_} character

prefix, care must be taken to avoid ambiguities whether an

operand is a register or a symbol; using symbols with names the

same as general or special registers then invoke undefined

behavior.

@item register_prefix

This directive makes a @samp{$} character prefix on all

registers mandatory.  It overrides a previous setting, including

the corresponding effect of the option @option{--underscore}.

@item leading_underscore

This is an assertion directive, emitting an error if the

@option{--no-underscore} option is in effect.

@item no_leading_underscore

This is the opposite of the @code{.syntax leading_underscore}

directive and emits an error if the option @option{--underscore}

is in effect.

@end table

@item .arch ARGUMENT

@cindex assembler directive .arch, CRIS

@cindex pseudo-op .arch, CRIS

@cindex CRIS assembler directive .arch

@cindex CRIS pseudo-op .arch

This is an assertion directive, giving an error if the specified

@var{ARGUMENT} is not the same as the specified or default value

for the @option{--march=@var{architecture}} option

(@pxref{march-option}).

@c If you compare with md_pseudo_table, you see that we don't

@c document ".file" and ".loc" here.  This is because we're just

@c wrapping the corresponding ELF function and emitting an error for

@c a.out.

@end table