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#### 9.2.5 Alpha Assembler Directives

`as` for the Alpha supports many additional directives for compatibility with the native assembler. This section describes them only briefly.

These are the additional directives in `as` for the Alpha:

`.arch cpu`

  Specifies the target processor. This is equivalent to the `-mcpu` command-line option. See Options, for a list of values for cpu.

`.ent function[, n]`

  Mark the beginning of function. An optional number may follow for compatibility with the OSF/1 assembler, but is ignored. When generating `.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 `.type` directive.

`.end function`

  Mark the end of function. In ELF, it will set the size of the symbol a-la the generic `.size` directive.

`.mask mask, offset`

  Indicate which of the integer registers are saved in the current function’s stack frame. mask is interpreted a bit mask in which bit n set indicates that register n is saved. The registers are saved in a block located offset bytes from the 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 `$26`) is saved first.

  This and the other directives that describe the stack frame are currently only used when generating `.mdebug` information. They may in the future be used to generate DWARF2 `.debug_frame` unwind information for hand written assembly.

`.fmask mask, offset`

  Indicate which of the floating-point registers are saved in the current stack frame. The mask and offset parameters are interpreted as with `.mask`.

`.frame framereg, frameoffset, retreg[, argoffset]`

  Describes the shape of the stack frame. The frame pointer in use is framereg; normally this is either `$fp` or `$sp`. The frame pointer is frameoffset bytes below the CFA. The return address is initially located in retreg until it is saved as indicated in `.mask`. For compatibility with OSF/1 an optional argoffset parameter is accepted and ignored. It is believed to indicate the offset from the CFA to the saved argument registers.

`.prologue n`

  Indicate that the stack frame is set up and all registers have been spilled. The argument n indicates whether and how the function uses the incoming procedure vector (the address of the called function) in `$27`. 0 indicates that `$27` is not used; 1 indicates that the first two instructions of the function use `$27` to perform a load of the GP register; 2 indicates that `$27` is used in some non-standard way and so the linker cannot elide the load of the procedure vector during relaxation.

`.usepv function, which`

  Used to indicate the use of the `$27` register, similar to `.prologue`, but without the other semantics of needing to be inside an open `.ent`/`.end` block.

  The which argument should be either `no`, indicating that `$27` is not used, or `std`, indicating that the first two instructions of the function perform a GP load.

  One might use this directive instead of `.prologue` if you are also using dwarf2 CFI directives.

`.gprel32 expression`

  Computes the difference between the address in 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.

`.t_floating expression`

  Stores expression as an IEEE double precision value.

`.s_floating expression`

  Stores expression as an IEEE single precision value.

`.f_floating expression`

  Stores expression as a VAX F format value.

`.g_floating expression`

  Stores expression as a VAX G format value.

`.d_floating expression`

  Stores expression as a VAX D format value.

`.set feature`

  Enables or disables various assembler features. Using the positive name of the feature enables while using ‘`nofeature`’ disables.

  `at`

    Indicates that macro expansions may clobber the assembler temporary (`$at` or `$28`) register. Some macros may not be expanded without this and will generate an error message if `noat` is in effect. When `at` is in effect, a warning will be generated if `$at` is used by the programmer.

  `macro`

    Enables the expansion of macro instructions. Note that variants of real instructions, such as `br label` vs `br $31,label` are considered alternate forms and not macros.

  `move`
  `reorder`
  `volatile`

    These control whether and how the assembler may re-order instructions. Accepted for compatibility with the OSF/1 assembler, but `as` does not do instruction scheduling, so these features are ignored.

The following directives are recognized for compatibility with the OSF/1 assembler but are ignored.

    .proc           .aproc
    .reguse         .livereg
    .option         .aent
    .ugen           .eflag
    .alias          .noalias

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