ARM Directives - Using as
Description: Using as

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#### 9.3.4 ARM Machine Directives

`.2byte` expression`[,` expression`]*`
`.4byte` expression`[,` expression`]*`
`.8byte` expression`[,` expression`]*`
  These directives write 2, 4 or 8 byte values to the output section.

`.align` expression`[,` expression`]`
  This is the generic .align directive. For the ARM however if the first argument is zero (ie no alignment is needed) the assembler will behave as if the argument had been 2 (ie pad to the next four byte boundary). This is for compatibility with ARM's own assembler.

`.arch` name
  Select the target architecture. Valid values for name are the same as for the `-march` commandline option.

  Specifying `.arch` clears any previously selected architecture extensions.

`.arch_extension` name
  Add or remove an architecture extension to the target architecture. Valid values for name are the same as those accepted as architectural extensions by the `-mcpu` commandline option.

  `.arch_extension` may be used multiple times to add or remove extensions incrementally to the architecture being compiled for.

`.arm`
  This performs the same action as .code 32.

`.pad #`count
  Generate unwinder annotations for a stack adjustment of count bytes. A positive value indicates the function prologue allocated stack space by decrementing the stack pointer.

`.bss`
  This directive switches to the `.bss` section.

`.cantunwind`
  Prevents unwinding through the current function. No personality routine or exception table data is required or permitted.

`.code [16|32]`
  This directive selects the instruction set being generated. The value 16 selects Thumb, with the value 32 selecting ARM.

`.cpu` name
  Select the target processor. Valid values for name are the same as for the `-mcpu` commandline option.

  Specifying `.cpu` clears any previously selected architecture extensions.

name`.dn` register name`[`.type`] [[`index`]]`
name`.qn` register name`[`.type`] [[`index`]]`
  The `dn` and `qn` directives are used to create typed and/or indexed register aliases for use in Advanced SIMD Extension (Neon) instructions. The former should be used to create aliases of double-precision registers, and the latter to create aliases of quad-precision registers.

  If these directives are used to create typed aliases, those aliases can be used in Neon instructions instead of writing types after the mnemonic or after each operand. For example:

                        x .dn d2.f32
                        y .dn d3.f32
                        z .dn d4.f32[1]
                        vmul x,y,z

  This is equivalent to writing the following:

                        vmul.f32 d2,d3,d4[1]

  Aliases created using `dn` or `qn` can be destroyed using `unreq`.

`.eabi_attribute` tag`,` value
  Set the EABI object attribute tag to value.

  The tag is either an attribute number, or one of the following: `Tag_CPU_raw_name`, `Tag_CPU_name`, `Tag_CPU_arch`, `Tag_CPU_arch_profile`, `Tag_ARM_ISA_use`, `Tag_THUMB_ISA_use`, `Tag_FP_arch`, `Tag_WMMX_arch`, `Tag_Advanced_SIMD_arch`, `Tag_PCS_config`, `Tag_ABI_PCS_R9_use`, `Tag_ABI_PCS_RW_data`, `Tag_ABI_PCS_RO_data`, `Tag_ABI_PCS_GOT_use`, `Tag_ABI_PCS_wchar_t`, `Tag_ABI_FP_rounding`, `Tag_ABI_FP_denormal`, `Tag_ABI_FP_exceptions`, `Tag_ABI_FP_user_exceptions`, `Tag_ABI_FP_number_model`, `Tag_ABI_align_needed`, `Tag_ABI_align_preserved`, `Tag_ABI_enum_size`, `Tag_ABI_HardFP_use`, `Tag_ABI_VFP_args`, `Tag_ABI_WMMX_args`, `Tag_ABI_optimization_goals`, `Tag_ABI_FP_optimization_goals`, `Tag_compatibility`, `Tag_CPU_unaligned_access`, `Tag_FP_HP_extension`, `Tag_ABI_FP_16bit_format`, `Tag_MPextension_use`, `Tag_DIV_use`, `Tag_nodefaults`, `Tag_also_compatible_with`, `Tag_conformance`, `Tag_T2EE_use`, `Tag_Virtualization_use`

  The value is either a `number`, `"string"`, or `number, "string"` depending on the tag.

  Note - the following legacy values are also accepted by tag: `Tag_VFP_arch`, `Tag_ABI_align8_needed`, `Tag_ABI_align8_preserved`, `Tag_VFP_HP_extension`,

`.even`
  This directive aligns to an even-numbered address.

`.extend` expression`[,` expression`]*`
`.ldouble` expression`[,` expression`]*`
  These directives write 12byte long double floating-point values to the output section. These are not compatible with current ARM processors or ABIs.

`.fnend`
  Marks the end of a function with an unwind table entry. The unwind index table entry is created when this directive is processed.

  If no personality routine has been specified then standard personality routine 0 or 1 will be used, depending on the number of unwind opcodes required.

`.fnstart`
  Marks the start of a function with an unwind table entry.

`.force_thumb`
  This directive forces the selection of Thumb instructions, even if the target processor does not support those instructions

`.fpu` name
  Select the floating-point unit to assemble for. Valid values for name are the same as for the `-mfpu` commandline option.

`.handlerdata`
  Marks the end of the current function, and the start of the exception table entry for that function. Anything between this directive and the `.fnend` directive will be added to the exception table entry.

  Must be preceded by a `.personality` or `.personalityindex` directive.

`.inst` opcode`[ , ... ]`
`.inst.n` opcode`[ , ... ]`
`.inst.w` opcode`[ , ... ]`
  Generates the instruction corresponding to the numerical value opcode. `.inst.n` and `.inst.w` allow the Thumb instruction size to be specified explicitly, overriding the normal encoding rules.
`.ldouble` expression`[,` expression`]*`
  See `.extend`.

`.ltorg`
  This directive causes the current contents of the literal pool to be dumped into the current section (which is assumed to be the .text section) at the current location (aligned to a word boundary). `GAS` maintains a separate literal pool for each section and each sub-section. The `.ltorg` directive will only affect the literal pool of the current section and sub-section. At the end of assembly all remaining, un-empty literal pools will automatically be dumped.

  Note - older versions of `GAS` would dump the current literal pool any time a section change occurred. This is no longer done, since it prevents accurate control of the placement of literal pools.

`.movsp` reg`[, #`offset`]`
  Tell the unwinder that reg contains an offset from the current stack pointer. If offset is not specified then it is assumed to be zero.

`.object_arch` name
  Override the architecture recorded in the EABI object attribute section. Valid values for name are the same as for the `.arch` directive. Typically this is useful when code uses runtime detection of CPU features.

`.packed` expression`[,` expression`]*`
  This directive writes 12-byte packed floating-point values to the output section. These are not compatible with current ARM processors or ABIs.

`.pad #`count
  Generate unwinder annotations for a stack adjustment of count bytes. A positive value indicates the function prologue allocated stack space by decrementing the stack pointer.

`.personality` name
  Sets the personality routine for the current function to name.

`.personalityindex` index
  Sets the personality routine for the current function to the EABI standard routine number index

`.pool`
  This is a synonym for .ltorg.

name`.req` register name
  This creates an alias for register name called name. For example:

                        foo .req r0

`.save` reglist
  Generate unwinder annotations to restore the registers in reglist. The format of reglist is the same as the corresponding store-multiple instruction.

      core registers

                  .save {r4, r5, r6, lr}
                  stmfd sp!, {r4, r5, r6, lr}

      FPA registers

                  .save f4, 2
                  sfmfd f4, 2, [sp]!

      VFP registers

                  .save {d8, d9, d10}
                  fstmdx sp!, {d8, d9, d10}

      iWMMXt registers

                  .save {wr10, wr11}
                  wstrd wr11, [sp, #-8]!
                  wstrd wr10, [sp, #-8]!
                or
                  .save wr11
                  wstrd wr11, [sp, #-8]!
                  .save wr10
                  wstrd wr10, [sp, #-8]!

`.setfp` fpreg`,` spreg`[, #`offset`]`
  Make all unwinder annotations relative to a frame pointer. Without this the unwinder will use offsets from the stack pointer.

  The syntax of this directive is the same as the `add` or `mov` instruction used to set the frame pointer. spreg must be either `sp` or mentioned in a previous `.movsp` directive.

                .movsp ip
                mov ip, sp
                ...
                .setfp fp, ip, #4
                add fp, ip, #4

`.secrel32` expression`[,` expression`]*`
  This directive emits relocations that evaluate to the section-relative offset of each expression's symbol. This directive is only supported for PE targets.

`.syntax [unified | divided]`
  This directive sets the Instruction Set Syntax as described in the ARM-Instruction-Set section.

`.thumb`
  This performs the same action as .code 16.

`.thumb_func`
  This directive specifies that the following symbol is the name of a Thumb encoded function. This information is necessary in order to allow the assembler and linker to generate correct code for interworking between Arm and Thumb instructions and should be used even if interworking is not going to be performed. The presence of this directive also implies `.thumb`

  This directive is not neccessary when generating EABI objects. On these targets the encoding is implicit when generating Thumb code.

`.thumb_set`
  This performs the equivalent of a `.set` directive in that it creates a symbol which is an alias for another symbol (possibly not yet defined). This directive also has the added property in that it marks the aliased symbol as being a thumb function entry point, in the same way that the `.thumb_func` directive does.

`.tlsdescseq` tls-variable
  This directive is used to annotate parts of an inlined TLS descriptor trampoline. Normally the trampoline is provided by the linker, and this directive is not needed.

`.unreq` alias-name
  This undefines a register alias which was previously defined using the `req`, `dn` or `qn` directives. For example:

                        foo .req r0
                        .unreq foo

  An error occurs if the name is undefined. Note - this pseudo op can be used to delete builtin in register name aliases (eg 'r0'). This should only be done if it is really necessary.

`.unwind_raw` offset`,` byte1`, ...`
  Insert one of more arbitary unwind opcode bytes, which are known to adjust the stack pointer by offset bytes.

  For example `.unwind_raw 4, 0xb1, 0x01` is equivalent to `.save {r0}`

`.vsave` vfp-reglist
  Generate unwinder annotations to restore the VFP registers in vfp-reglist using FLDMD. Also works for VFPv3 registers that are to be restored using VLDM. The format of vfp-reglist is the same as the corresponding store-multiple instruction.

      VFP registers

                  .vsave {d8, d9, d10}
                  fstmdd sp!, {d8, d9, d10}

      VFPv3 registers

                  .vsave {d15, d16, d17}
                  vstm sp!, {d15, d16, d17}

  Since FLDMX and FSTMX are now deprecated, this directive should be used in favour of `.save` for saving VFP registers for ARMv6 and above.