source: branches/libc-0.6/src/binutils/gas/doc/c-alpha.texi@ 2594

@c Copyright 2002
@c Free Software Foundation, Inc.
@c This is part of the GAS manual.
@c For copying conditions, see the file as.texinfo.

@ifset GENERIC
@page
@node Alpha-Dependent
@chapter Alpha Dependent Features
@end ifset

@ifclear GENERIC
@node Machine Dependencies
@chapter Alpha Dependent Features
@end ifclear

@cindex Alpha support
@menu
* Alpha Notes::                Notes
* Alpha Options::              Options
* Alpha Syntax::               Syntax
* Alpha Floating Point::       Floating Point
* Alpha Directives::           Alpha Machine Directives
* Alpha Opcodes::              Opcodes
@end menu

@node Alpha Notes
@section Notes
@cindex Alpha notes
@cindex notes for Alpha

The documentation here is primarily for the ELF object format.
@code{@value{AS}} also supports the ECOFF and EVAX formats, but
features specific to these formats are not yet documented.

@node Alpha Options
@section Options
@cindex Alpha options
@cindex options for Alpha

@table @option
@cindex @code{-m@var{cpu}} command line option, Alpha
@item -m@var{cpu}
This option specifies the target processor.  If an attempt is made to
assemble an instruction which will not execute on the target processor,
the assembler may either expand the instruction as a macro or issue an
error message.  This option is equivalent to the @code{.arch} directive.

The following processor names are recognized: 
@code{21064},
@code{21064a},
@code{21066},
@code{21068},
@code{21164},
@code{21164a},
@code{21164pc},
@code{21264},
@code{21264a},
@code{21264b},
@code{ev4},
@code{ev5},
@code{lca45},
@code{ev5},
@code{ev56},
@code{pca56},
@code{ev6},
@code{ev67},
@code{ev68}.
The special name @code{all} may be used to allow the assembler to accept
instructions valid for any Alpha processor.

In order to support existing practice in OSF/1 with respect to @code{.arch},
and existing practice within @command{MILO} (the Linux ARC bootloader), the
numbered processor names (e.g.@: 21064) enable the processor-specific PALcode
instructions, while the ``electro-vlasic'' names (e.g.@: @code{ev4}) do not.

@cindex @code{-mdebug} command line option, Alpha
@cindex @code{-no-mdebug} command line option, Alpha
@item -mdebug
@itemx -no-mdebug
Enables or disables the generation of @code{.mdebug} encapsulation for
stabs directives and procedure descriptors.  The default is to automatically
enable @code{.mdebug} when the first stabs directive is seen.

@cindex @code{-relax} command line option, Alpha
@item -relax
This option forces all relocations to be put into the object file, instead
of saving space and resolving some relocations at assembly time.  Note that
this option does not propagate all symbol arithmetic into the object file,
because not all symbol arithmetic can be represented.  However, the option
can still be useful in specific applications.

@cindex @code{-g} command line option, Alpha
@item -g
This option is used when the compiler generates debug information.  When
@command{gcc} is using @command{mips-tfile} to generate debug
information for ECOFF, local labels must be passed through to the object
file.  Otherwise this option has no effect.

@cindex @code{-G} command line option, Alpha
@item -G@var{size}
A local common symbol larger than @var{size} is placed in @code{.bss},
while smaller symbols are placed in @code{.sbss}.

@cindex @code{-F} command line option, Alpha
@cindex @code{-32addr} command line option, Alpha
@item -F
@itemx -32addr
These options are ignored for backward compatibility.
@end table

@cindex Alpha Syntax
@node Alpha Syntax
@section Syntax
The assembler syntax closely follow the Alpha Reference Manual;
assembler directives and general syntax closely follow the OSF/1 and
OpenVMS syntax, with a few differences for ELF.

@menu
* Alpha-Chars::                Special Characters
* Alpha-Regs::                 Register Names
* Alpha-Relocs::               Relocations
@end menu

@node Alpha-Chars
@subsection Special Characters

@cindex line comment character, Alpha
@cindex Alpha line comment character
@samp{#} is the line comment character.

@cindex line separator, Alpha
@cindex statement separator, Alpha
@cindex Alpha line separator
@samp{;} can be used instead of a newline to separate statements.

@node Alpha-Regs
@subsection Register Names
@cindex Alpha registers
@cindex register names, Alpha

The 32 integer registers are refered to as @samp{$@var{n}} or
@samp{$r@var{n}}.  In addition, registers 15, 28, 29, and 30 may
be refered to by the symbols @samp{$fp}, @samp{$at}, @samp{$gp},
and @samp{$sp} respectively.

The 32 floating-point registers are refered to as @samp{$f@var{n}}.

@node Alpha-Relocs
@subsection Relocations
@cindex Alpha relocations
@cindex relocations, Alpha

Some of these relocations are available for ECOFF, but mostly
only for ELF.  They are modeled after the relocation format 
introduced in Digial Unix 4.0, but there are additions.

The format is @samp{!@var{tag}} or @samp{!@var{tag}!@var{number}}
where @var{tag} is the name of the relocation.  In some cases
@var{number} is used to relate specific instructions.

The relocation is placed at the end of the instruction like so:

@example
ldah  $0,a($29)    !gprelhigh
lda   $0,a($0)     !gprellow
ldq   $1,b($29)    !literal!100
ldl   $2,0($1)     !lituse_base!100
@end example

@table @code
@item !literal
@itemx !literal!@var{N}
Used with an @code{ldq} instruction to load the address of a symbol
from the GOT.

A sequence number @var{N} is optional, and if present is used to pair
@code{lituse} relocations with this @code{literal} relocation.  The
@code{lituse} relocations are used by the linker to optimize the code
based on the final location of the symbol.

Note that these optimizations are dependent on the data flow of the
program.  Therefore, if @emph{any} @code{lituse} is paired with a
@code{literal} relocation, then @emph{all} uses of the register set by
the @code{literal} instruction must also be marked with @code{lituse}
relocations.  This is because the original @code{literal} instruction
may be deleted or transformed into another instruction.

Also note that there may be a one-to-many relationship between
@code{literal} and @code{lituse}, but not a many-to-one.  That is, if
there are two code paths that load up the same address and feed the
value to a single use, then the use may not use a @code{lituse}
relocation.

@item !lituse_base!@var{N}
Used with any memory format instruction (e.g.@: @code{ldl}) to indicate
that the literal is used for an address load.  The offset field of the
instruction must be zero.  During relaxation, the code may be altered
to use a gp-relative load.

@item !lituse_jsr!@var{N}
Used with a register branch format instruction (e.g.@: @code{jsr}) to
indicate that the literal is used for a call.  During relaxation, the
code may be altered to use a direct branch (e.g.@: @code{bsr}).

@item !lituse_bytoff!@var{N}
Used with a byte mask instruction (e.g.@: @code{extbl}) to indicate
that only the low 3 bits of the address are relevant.  During relaxation,
the code may be altered to use an immediate instead of a register shift.

@item !lituse_addr!@var{N}
Used with any other instruction to indicate that the original address
is in fact used, and the original @code{ldq} instruction may not be
altered or deleted.  This is useful in conjunction with @code{lituse_jsr}
to test whether a weak symbol is defined.

@example
ldq  $27,foo($29)   !literal!1
beq  $27,is_undef   !lituse_addr!1
jsr  $26,($27),foo  !lituse_jsr!1
@end example

@item !lituse_tlsgd!@var{N}
Used with a register branch format instruction to indicate that the
literal is the call to @code{__tls_get_addr} used to compute the 
address of the thread-local storage variable whose descriptor was
loaded with @code{!tlsgd!@var{N}}.

@item !lituse_tlsldm!@var{N}
Used with a register branch format instruction to indicate that the
literal is the call to @code{__tls_get_addr} used to compute the 
address of the base of the thread-local storage block for the current
module.  The descriptor for the module must have been loaded with
@code{!tlsldm!@var{N}}.

@item !gpdisp!@var{N}
Used with @code{ldah} and @code{lda} to load the GP from the current
address, a-la the @code{ldgp} macro.  The source register for the
@code{ldah} instruction must contain the address of the @code{ldah}
instruction.  There must be exactly one @code{lda} instruction paired
with the @code{ldah} instruction, though it may appear anywhere in 
the instruction stream.  The immediate operands must be zero.

@example
bsr  $26,foo
ldah $29,0($26)     !gpdisp!1
lda  $29,0($29)     !gpdisp!1
@end example

@item !gprelhigh
Used with an @code{ldah} instruction to add the high 16 bits of a
32-bit displacement from the GP.

@item !gprellow
Used with any memory format instruction to add the low 16 bits of a
32-bit displacement from the GP.

@item !gprel
Used with any memory format instruction to add a 16-bit displacement
from the GP.

@item !samegp
Used with any branch format instruction to skip the GP load at the
target address.  The referenced symbol must have the same GP as the
source object file, and it must be declared to either not use @code{$27}
or perform a standard GP load in the first two instructions via the
@code{.prologue} directive.

@item !tlsgd
@itemx !tlsgd!@var{N}
Used with an @code{lda} instruction to load the address of a TLS
descriptor for a symbol in the GOT.

The sequence number @var{N} is optional, and if present it used to
pair the descriptor load with both the @code{literal} loading the
address of the @code{__tls_get_addr} function and the @code{lituse_tlsgd}
marking the call to that function.

For proper relaxation, both the @code{tlsgd}, @code{literal} and
@code{lituse} relocations must be in the same extended basic block.
That is, the relocation with the lowest address must be executed
first at runtime.

@item !tlsldm
@itemx !tlsldm!@var{N}
Used with an @code{lda} instruction to load the address of a TLS
descriptor for the current module in the GOT.

Similar in other respects to @code{tlsgd}.

@item !gotdtprel
Used with an @code{ldq} instruction to load the offset of the TLS
symbol within its module's thread-local storage block.  Also known
as the dynamic thread pointer offset or dtp-relative offset.

@item !dtprelhi
@itemx !dtprello
@itemx !dtprel
Like @code{gprel} relocations except they compute dtp-relative offsets.

@item !gottprel
Used with an @code{ldq} instruction to load the offset of the TLS
symbol from the thread pointer.  Also known as the tp-relative offset.

@item !tprelhi
@itemx !tprello
@itemx !tprel