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@c Copyright 2002, 2003, 2005, 2009

@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{-replace} command line option, Alpha

@cindex @code{-noreplace} command line option, Alpha

@item -replace

@item -noreplace

Enables or disables the optimization of procedure calls, both at assemblage

and at link time.  These options are only available for VMS targets and

@code{-replace} is the default.  See section 1.4.1 of the OpenVMS Linker

Utility Manual.

@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 referred to as @samp{$@var{n}} or

@samp{$r@var{n}}.  In addition, registers 15, 28, 29, and 30 may

be referred to by the symbols @samp{$fp}, @samp{$at}, @samp{$gp},

and @samp{$sp} respectively.

The 32 floating-point registers are referred 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 Digital 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_jsrdirect!@var{N}

Similar to @code{lituse_jsr}, but also that this call cannot be vectored

through a PLT entry.  This is useful for functions with special calling

conventions which do not allow the normal call-clobbered registers to be

clobbered.

@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

Like @code{gprel} relocations except they compute tp-relative offsets.

@end table

@node Alpha Floating Point

@section Floating Point

@cindex floating point, Alpha (@sc{ieee})

@cindex Alpha floating point (@sc{ieee})

The Alpha family uses both @sc{ieee} and VAX floating-point numbers.

@node Alpha Directives

@section Alpha Assembler Directives

@command{@value{AS}} for the Alpha supports many additional directives for

compatibility with the native assembler.  This section describes them only

briefly.

@cindex Alpha-only directives

These are the additional directives in @code{@value{AS}} for the Alpha:

@table @code

@item .arch @var{cpu}

Specifies the target processor.  This is equivalent to the

@option{-m@var{cpu}} command-line option.  @xref{Alpha Options, Options},

for a list of values for @var{cpu}.

@item .ent @var{function}[, @var{n}]

Mark the beginning of @var{function}.  An optional number may follow for

compatibility with the OSF/1 assembler, but is ignored.  When generating

@code{.mdebug} information, this will create a procedure descriptor for

the function.  In ELF, it will mark the symbol as a function a-la the

generic @code{.type} directive.

@item .end @var{function}

Mark the end of @var{function}.  In ELF, it will set the size of the symbol

a-la the generic @code{.size} directive.

@item .mask @var{mask}, @var{offset}

Indicate which of the integer registers are saved in the current

function's stack frame.  @var{mask} is interpreted a bit mask in which

bit @var{n} set indicates that register @var{n} is saved.  The registers

are saved in a block located @var{offset} bytes from the @dfn{canonical

frame address} (CFA) which is the value of the stack pointer on entry to

the function.  The registers are saved sequentially, except that the

return address register (normally @code{$26}) is saved first.

This and the other directives that describe the stack frame are

currently only used when generating @code{.mdebug} information.  They

may in the future be used to generate DWARF2 @code{.debug_frame} unwind

information for hand written assembly.

@item .fmask @var{mask}, @var{offset}

Indicate which of the floating-point registers are saved in the current

stack frame.  The @var{mask} and @var{offset} parameters are interpreted

as with @code{.mask}.

@item .frame @var{framereg}, @var{frameoffset}, @var{retreg}[, @var{argoffset}]

Describes the shape of the stack frame.  The frame pointer in use is

@var{framereg}; normally this is either @code{$fp} or @code{$sp}.  The

frame pointer is @var{frameoffset} bytes below the CFA.  The return

address is initially located in @var{retreg} until it is saved as

indicated in @code{.mask}.  For compatibility with OSF/1 an optional

@var{argoffset} parameter is accepted and ignored.  It is believed to

indicate the offset from the CFA to the saved argument registers.

@item .prologue @var{n}

Indicate that the stack frame is set up and all registers have been

spilled.  The argument @var{n} indicates whether and how the function

uses the incoming @dfn{procedure vector} (the address of the called

function) in @code{$27}.  0 indicates that @code{$27} is not used; 1

indicates that the first two instructions of the function use @code{$27}

to perform a load of the GP register; 2 indicates that @code{$27} is

used in some non-standard way and so the linker cannot elide the load of

the procedure vector during relaxation.

@item .usepv @var{function}, @var{which}

Used to indicate the use of the @code{$27} register, similar to

@code{.prologue}, but without the other semantics of needing to

be inside an open @code{.ent}/@code{.end} block.

The @var{which} argument should be either @code{no}, indicating that

@code{$27} is not used, or @code{std}, indicating that the first two

instructions of the function perform a GP load.

One might use this directive instead of @code{.prologue} if you are

also using dwarf2 CFI directives.

@item .gprel32 @var{expression}

Computes the difference between the address in @var{expression} and the

GP for the current object file, and stores it in 4 bytes.  In addition

to being smaller than a full 8 byte address, this also does not require

a dynamic relocation when used in a shared library.

@item .t_floating @var{expression}

Stores @var{expression} as an @sc{ieee} double precision value.

@item .s_floating @var{expression}

Stores @var{expression} as an @sc{ieee} single precision value.

@item .f_floating @var{expression}

Stores @var{expression} as a VAX F format value.

@item .g_floating @var{expression}

Stores @var{expression} as a VAX G format value.

@item .d_floating @var{expression}

Stores @var{expression} as a VAX D format value.

@item .set @var{feature}

Enables or disables various assembler features.  Using the positive

name of the feature enables while using @samp{no@var{feature}} disables.

@table @code

@item at

Indicates that macro expansions may clobber the @dfn{assembler

temporary} (@code{$at} or @code{$28}) register.  Some macros may not be

expanded without this and will generate an error message if @code{noat}

is in effect.  When @code{at} is in effect, a warning will be generated

if @code{$at} is used by the programmer.

@item macro

Enables the expansion of macro instructions.  Note that variants of real

instructions, such as @code{br label} vs @code{br $31,label} are

considered alternate forms and not macros.

@item move

@itemx reorder

@itemx volatile

These control whether and how the assembler may re-order instructions.

Accepted for compatibility with the OSF/1 assembler, but @command{@value{AS}}

does not do instruction scheduling, so these features are ignored.

@end table

@end table

The following directives are recognized for compatibility with the OSF/1

assembler but are ignored.

@example

.proc           .aproc

.reguse         .livereg

.option         .aent

.ugen           .eflag

.alias          .noalias

@end example

@node Alpha Opcodes

@section Opcodes

For detailed information on the Alpha machine instruction set, see the

@c Attempt to work around a very overfull hbox.

@iftex

Alpha Architecture Handbook located at

@smallfonts

@example

ftp://ftp.digital.com/pub/Digital/info/semiconductor/literature/alphaahb.pdf

@end example

@textfonts

@end iftex

@ifnottex

@uref{ftp://ftp.digital.com/pub/Digital/info/semiconductor/literature/alphaahb.pdf,Alpha Architecture Handbook}.

@end ifnottex