5    Assembler Directives

Assembler directives are instructions to the assembler to perform various bookkeeping tasks, storage reservation, and other control functions. To distinquish them from other instructions, directive names begin with a period. Table 5-1 lists the assembler directives by category.

Table 5-1:  Summary of Assembler Directives

The following list contains descriptions of the assembly directives (in alphabetical order):

.aent name

Sets an alternate entry point for the current procedure. Use this information when you want to generate information for the debugger. This directive must appear between a pair of .ent and .end directives.

.align expression

Sets low-order bits in the location counter to zero. The value of expression establishes the number of bits to be set to zero. The maximum value for expression is 16 (which produces 64K alignment).

If the .align directive advances the location counter, the assembler fills the skipped bytes with zeros in data sections and nop instructions in text sections.

Normally, the .word, .long, .quad, .float, .double, .extended, .d_floating, .f_floating, .g_floating, .s_floating, .t_floating, and .x_floating directives automatically align their data appropriately. For example, .word does an implicit .align 1, and .double does an implicit .align 3.

You can disable the automatic alignment feature with .align 0. The assembler reinstates automatic alignment at the next .text, .data, .rdata, or .sdata directive that it encounters.

Labels immediately preceding an automatic or explicit alignment are also realigned. For example:

foo: .align 3
     .word 0

This is equivalent to:

     .align 3
foo: .word 0

.arch model

Specifies the version of the Alpha architecture that the assembler is to generate instructions for. The valid values for model are identical to those you can specify with the -arch flag on the cc command line. See cc(1) for details.

.ascii string [ , string ] ...

Assembles each string from the list into successive locations. The .ascii directive does not pad the string with null characters. You must put double quotation marks (") around each string. You can optionally use the backslash escape characters. For a list of the backslash characters, see Section 2.4.3.

.asciiz string [ , string ] ...

Assembles each string in the list into successive locations and adds a null character. You can optionally use the backslash escape characters. For a list of the backslash characters, see Section 2.4.3.

.byte expression1 [,expression2] [expressionN]

Truncates the values of the expressions specified in the comma-separated list to 8-bit values, and assembles the values in successive locations. The values of the expressions must be absolute.

The operands for the .byte directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is an 8-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

.comm name, expression1[,expression2]

Unless defined elsewhere, name becomes a global common symbol at the head of a block of at least expression1 bytes of storage. The linker overlays like-named common blocks, using the expression value of the largest block as the byte size of the overlay. The expression2 operand has the same effect on alignment as the operand for the .align directive.

.data

Directs the assembler to add all subsequent data to the .data section.

.d_floating expression1 [,expression2] [expressionN]

Initializes memory to double-precision (64-bit) VAX D_floating numbers. The values of the expressions must be absolute.

The operands for the .d_floating directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 64-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .d_floating directive automatically aligns its data and any preceding labels on a double-word boundary. You can disable this feature with the .align 0 directive.

.double expression1 [,expression2] [expressionN]

Synonym for .t_floating.

.edata 0

.edata 1 lang-handler relocatable-expression

.edata 2lang-handler constant-expression

Marks data related to exception handling.

If the flag is 0, the assembler adds all subsequent data to the .xdata section.

If the flag is 1 or 2, the assembler creates a function table entry for the next .ent directive. The function table entry contains the language-specific handler (lang-handler) and data (relocatable-expression or constant-expression).

.eflag flags

Encodes exception-related flags to be stored in the PDSC_RPD_FLAGS field of the procedure's run-time procedure descriptor. See the Calling Standard for Alpha Systems for a description of the individual flags.

.end [ proc_name ]

Sets the end of a procedure. The .ent directive sets the beginning of a procedure. Use the .ent and .end directives when you want to generate information for the debugger.

.endr

Signals the end of a repeat block. The .repeat directive starts a repeat block.

.ent proc_name [ lex-level ]

Sets the beginning of the procedure proc_name. Use this directive when you want to generate information for the debugger. The .end directive sets the end of a procedure.

The lex-level operand indicates the number of procedures that statically surround the current procedure. This operand is only informational. It does not affect the assembly process; the assembler ignores it.

.err

For use only by compilers.This directive causes the assembler to signal an error. Any compiler frontend that detects an error condition puts this directive in the input stream. When the assembler encounters a .err directive, it issues an error message and ceases to assemble the source file. This prevents the assembler from continuing to process a program that is incorrect.

.extended expression1 [,expression2] [expressionN]

Synonym for .x_floating.

.extern [(THREADS)] name [ number ]

Indicates that the specified symbol is global and external; that is, the symbol is defined in another object module and cannot be defined until link time. The name operand is a global undefined symbol and number is the expected size of the external object. If the THREADS argument is specified, the symbol is treated as a tls (thread local storage) global undefined symbol.

.f_floating expression1 [,expression2] [expressionN]

Initializes memory to single-precision (32-bit) VAX F_floating numbers. The values of the expressions must be absolute.

The operands for the .f_floating directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 32-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .f_floating directive automatically aligns its data and preceding labels on a longword boundary. You can disable this feature by using the .align 0 directive.

.file file_number file_name_string

For use only by compilers. Specifies the source file from which the assembly instructions that follow originated. This directive causes the assembler to stop generating line numbers that are used by the debugger. A subsequent .loc directive causes the assembler to resume generating line numbers.

.float expression1 [,expression2] [expressionN]

Synonym for .s_floating.

.fmask mask offset

Sets a mask with a bit turned on for each floating-point register that the current routine saved. The least-significant bit corresponds to register $f0. The offset is the distance, in bytes, from the virtual frame pointer to where the floating-point registers are saved.

You must use .ent before .fmask, and you can use only one .fmask for each .ent. Space should be allocated for those registers specified in the .fmask.

.frame frame-reg frame-size return_pc-reg [local_offset]

Describes a stack frame. The first register is the frame register, andframe-size is the size of the stack frame, that is, the number of bytes between the frame register and the virtual frame pointer. The second register specifies the register that contains the return address. The local_offset parameter, which is for use only by compilers, specifies the number of bytes between the virtual frame pointer and the local variables.

You must use .ent before .frame, and you can use only one .frame for each .ent. No stack traces can be done in the debugger without the .frame directive.

.g_floating expression1 [,expression2] [expressionN]

Initializes memory to double-precision (64-bit) VAX G_floating numbers. The values of the expressions must be absolute.

The operands for the .g_floating directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 64-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .g_floating directive automatically aligns its data and any preceding labels on a quadword boundary. You can disable this feature with the .align 0 directive.

.globl name

Identifies name as an external symbol. If the name is otherwise defined (for example, by its appearance as a label), the assembler exports the symbol; otherwise, it imports the symbol. In general, the assembler imports undefined symbols; that is, it gives them the UNIX storage class "global undefined" and requires the linker to resolve them.

.gprel32 address1[, address2] [,addressN]

Truncates the signed displacement between the global pointer value and the addresses specified in the comma-separated list to 32-bit values, and assembles the values in successive locations.

The operands for the .gprel32 directive can optionally have the following form:

addressVal [ : addressRep ]

The addressVal is the address value. The optional addressRep is a non-negative expression that specifies how many times to replicate the value of addressVal. The expression value (addressVal) and repetition count (addressRep) must be absolute.

The .gprel32 directive automatically aligns its data and preceding labels on a longword boundary. You can disable this feature with the .align 0 directive.

.ident string

Allows the specification of a string that the assembler stores in the .o file created during assembly. This string can be searched for in a .o file or in an executable using the what(1) command.

.lab label_name

For use only by compilers. Associates a named label with the current location in the program text.

.lcomm name, expression1[,expression2]

Gives the named symbol (name) a data type of bss. The assembler allocates the named symbol to the bss area, and expression1 defines the named symbol's length. If a .globl directive also specifies the name, the assembler allocates the named symbol as an external symbol. The expression2 operand has the same effect on alignment as the operand for the .align directive. If expression2 is not specified, the alignment defaults to quadword alignment.

The assembler puts bss symbols in one of two bss areas. If the defined size is less than or equal to the size specified by the assembler or compiler's -G command-line option, the assembler puts the symbols in the sbss area.

.lit4

Allows 4-byte constants to be generated and placed in the lit4 section. This directive is only valid for .long (with nonrelocatable expressions), .f_floating, .float, and .s_floating.

.lit8

Allows 8-byte constants to be generated and placed in the lit8 section. This directive is only valid for .quad (with nonrelocatable expressions), .d_floating, .g_floating, .double, and .t_floating.

.loc file_number line_number

For use only by compilers. Specifies the source file and the line within it that corresponds to the assembly instructions that follow. The assembler ignores the file number when this directive appears in the assembly source file. Then, the assembler assumes that the directive refers to the most recent .file directive.

.long expression1 [,expression2] [expressionN]

Truncates the values of the expressions specified in the comma-separated list to 32-bit values, and assembles the values in successive locations. The values of the expression can be relocatable.

The operands for the .long directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 32-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .long directive automatically aligns its data and preceding labels on a longword boundary. You can disable this feature with the .align 0 directive.

.mask mask, offset

Sets a mask with a bit turned on for each general-purpose register that the current routine saved. The least significant bit corresponds to register $0. The offset is the distance, in bytes, from the virtual frame pointer to where the registers are saved.

You must use .ent before .mask, and you can use only one .mask for each .ent. Space should be allocated for those registers specified in the .mask.

.option options

For use only by compilers. Instructs the assembler to replace an optimization level that was specified on the command with the one specified in the options argument. Valid entries for this new optimization level are O, O1 - O4.

.prologue flag

Marks the end of the prologue section of a procedure.

A flag of 0 indicates that the procedure does not use $gp; the caller does not need to set up $pv prior to calling the procedure or restore $gp on return from the procedure.

A flag of 1 indicates that the procedure does use $gp; the caller must set up $pv prior to calling the procedure and restore $gp on return from the procedure.

If flag is not specified, the behavior is as if a value of 1 was specified.

.quad expression1 [,expression2] [expressionN]

Truncates the values of the expressions specified in the comma-separated list to 64-bit values, and assembles the values in successive locations. The values of the expressions can be relocatable.

The operands for the .quad directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 64-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .quad directive automatically aligns its data and preceding labels on a quadword boundary. You can disable this feature with the .align 0 directive.

.rconst

Instructs the assembler to add subsequent data into the .rconst section. (This is the same as the .rdata directive except that the entries cannot be relocatable.)

.rdata

Instructs the assembler to add subsequent data into the .rdata section.

.repeat expression

Repeats all instructions or data between the .repeat and .endr directives. The expression defines how many times the enclosing text and data repeats. With the .repeat directive, you cannot use labels, branch instructions, or values that require relocation in the block. Also note that nesting .repeat directives is not allowed.

.save_ra saved_ra_register

Specifies that saved_ra_register is the register in which the return address is saved during the execution of the procedure. If .save_ra is not used, the saved return address register is assumed to be the same as the return_pc_register argument of the frame directive. The .save_ra directive is valid only for register frame procedures.

.sdata

Instructs the assembler to add subsequent data to the .sdata section.

.set option

Instructs the assembler to enable or disable certain options. The assembler has the following default options: reorder, macro, move, novolatile, and at. Only one option can be specified by a single .set directive. The effects of the options are as follows:

• The reorder option permits the assembler to reorder machine-language instructions to improve performance.

  The noreorder option prevents the assembler from reordering machine-language instructions. If a machine-language instruction violates the hardware pipeline constraints, the assembler issues a warning message.

• The macro option permits the assembler to generate multiple machine-language instructions from a single assembler instruction.

  The nomacro option causes the assembler to print a warning whenever an assembler operation generates more than one machine-language instruction. You must select the noreorder option before using the nomacro option; otherwise, an error results.

• The at option permits the assembler to use the $at register for macros, but generates warnings if the source program uses $at.

  When you use the noat option and an assembler operation requires the $at register, the assembler issues a warning message; however, the noat option does permit source programs to use $at without warnings being issued.

• The nomove options instructs the assembler to mark each subsequent instruction so that it cannot be moved during reorganization. The assembler can still move instructions from below the nomove region to above the region or vice versa. The nomove option has part of the effect of the "volatile" C declaration; it prevents otherwise independent loads or stores from occurring in a different order than intended.

  The move option cancels the effect of nomove.

• The volatile option instructs the assembler that subsequent load and store instructions may not be moved in relation to each other or removed by redundant load removal or other optimization. The volatile option is less restrictive than noreorder; it allows the assembler to move other instructions (that is, instructions other than load and store instructions) without restrictions.

  The novolatile option cancels the effect of the volatile option.

.s_floating expression1 [,expression2] [expressionN]

Initializes memory to single-precision (32-bit) IEEE floating-point numbers. The values of the expressions must be absolute.

The operands for the .s_floating directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 32-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .s_floating directive automatically aligns its data and preceding labels on a longword boundary. You can disable this feature with the .align 0 directive.

.space expression

Advances the location counter by the number of bytes specified by the value of expression. The assembler fills the space with zeros.

.struct expression

Permits you to lay out a structure using labels plus directives such as .word or .byte. It ends at the next segment directive (.data, .text, and so forth). It does not emit any code or data, but defines the labels within it to have values that are the sum of expression plus their offsets from the .struct itself.

symbolic equate

Takes one of the following forms: name = expression or name = register. You must define the name only once in the assembly, and you cannot redefine it. The expression must be computable when you assemble the program, and the expression must involve only operators, constants, or equated symbols. You can use the name as a constant in any later statement.

.text

Instructs the assembler to add subsequent code to the .text section. (This is the default.)

.tlscomm name,expression

The name operand becomes a global tls common symbol at the head of a block of expression bytes of storage. This directive is analogous to the .comm directive.

.tlsdata

Directs the assembler to add all subsequent data to the .tlsdata section. This directive is analogous to the .data directive.

.tlslcomm name,expression

The name operand becomes a symbol of type tlsbss. The assembler allocates the symbol to the tlsbss section and the expression defines the named symbol's length. If a .globl directive also specifies the symbol name, the assembler allocates the named symbol as an external symbol.

Unlike non-tls symbols, thread local storage's bss data is allocated in only one area. There is no sbss area for tls symbols. This directive is analogous to the .lcomm directive.

.tlslcomm  b 8         /* TlsBss      stStatic */
.lcomm     B 8         /* SBss        stStatic */
 
.globl     c           /* TlsBss      stGlobal */
.tlslcomm  c 8
.globl     C           /* SBss        stGlobal */
.lcomm     C 8

.t_floating expression1 [,expression2] [expressionN]

Initializes memory to double-precision (64-bit) IEEE floating-point numbers. The values of the expressions must be absolute.

The operands for the .t_floating directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 64-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .t_floating directive automatically aligns its data and any preceding labels on a quadword boundary. You can disable this feature with the .align 0 directive.

.tune option

Selects processor-specific instruction tuning for various implementations of the Alpha architecture. Regardless of the setting of the .arch directive, the generated code will run correctly on all implementations of the Alpha architecture. The valid values for option are identical to those you can specify with the -arch flag on the cc command line. See cc(1) for details.

.verstamp major minor

Specifies the major and minor version numbers; for example, version 0.15 would be .verstamp 0 15.

.weakext name1 [,name2]

Sets name1 to be a weak symbol during linking. If name2 is specified, name1 is created as a weak symbol with the same value as name2. Weak symbols can be silently redefined at link time.

.word expression1 [,expression2] [expressionN]

Truncates the values of the expressions specified in the comma-separated list to 16-bit values, and assembles the values in successive locations. The values of the expressions must be absolute.

The operands for the .word directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 16-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .word directive automatically aligns its data and preceding labels on a word boundary. You can disable this feature with the .align 0 directive.

.x_floating expression1 [,expression2] [expressionN]

Initializes memory to quad-precision (128-bit) IEEE floating-point numbers. The values of the expressions must be absolute.

The operands for the .x_floating directive can optionally have the following form:

expressionVal [ : expressionRep ]

The expressionVal is a 128-bit value. The optional expressionRep is a non-negative expression that specifies how many times to replicate the value of expressionVal. The expression value (expressionVal) and repetition count (expressionRep) must be absolute.

The .x_floating directive automatically aligns its data and preceding labels on an octaword boundary. You can disable this feature with the .align 0 directive.