Base register directive

    Format

      .BASE  Rn [,base_expression]

1  –  Parameters

 Rn

    One of the base registers, R0 through R30, FP, and SP.

 base_expression

    The base address, which is optional, and can be one of the
    following:

    o  An absolute expression

    o  A relocatable expression

    o  An external expression

    An expression must not contain forward references or implicit
    external symbols. An implicitly defined external symbol is a
    symbol that the assembler defaults to an external symbol. This
    occurs when the assembler encounters references to a symbol, but
    does not encounter a definition for the symbol or an .EXTERNAL
    directive that declares the symbol.

2  –  Description

    The .BASE directive is used to inform the assembler that a
    specified base register contains a specified base address. Later
    in your program, the assembler allows you to implicitly reference
    the specified base register. When the assembler knows which
    base addresses are stored in one or more base registers, it can
    convert an expression to an offset from one of the base registers
    previously specified in a .BASE directive. .BASE provides a
    convenient and more readable shorthand for accessing memory and
    constant values using base registers. .BASE also makes it easier
    for you to change your register assignments if you later modify
    your code.

    The base expression is optional. If the base expression is
    specified, this base address value is assumed by the assembler
    to be in the specified register, Rn. If the base expression is
    omitted, the contents of the specified base register, Rn, is
    considered undefined until a new base expression is associated
    with the base register.

    R31 is defined to always contain 0, according to the architecture
    definition. Therefore, R31 is known to be a predefined base
    register containing 0. For every assembly, the assembler assumes
    the following statement:

        .BASE R31, 0

    Because the contents of R31 cannot change, you cannot specify a
    base address for R31.

    You can use the .BASE directive to implicitly reference base
    registers. You can also automatically compute offsets from a base
    address known to be in a register to a base address you use in an
    instruction argument.

    Most of the memory format Alpha instructions are defined
    such that one of their arguments must have a base register and
    an offset. If the assembler encounters only an expression with
    no base register, the assembler attempts to find a base register
    that contains a base address or constant within a 16-bit signed
    offset of the value of the expression. If it finds such a base
    register, the assembler computes an offset that, when added to
    the value of the base register, results in a value equal to the
    expression specified in the instruction argument.

3  –  Examples

      Example 1
        .EXTERNAL COMM_AREA       1
        .BASE R1, COMM_AREA       2
        CURR_LINE       = COMM_AREA + 0
        CURR_COLUMN     = COMM_AREA + 4
        CURR_MODE       = COMM_AREA + 8
        LDA     R4, 17                  ; LDA R4, 17(R31)  3
        LDL     R2, CURR_LINE           ; LDL R2, 0(R1)    4
        LDL     R3, CURR_COLUMN         ; LDL R3, 4(R1)
        STL     R4, CURR_MODE           ; STL R4, 8(R1)

      1  This statement declares an external symbol, COMM_AREA.
         COMM_AREA is a global symbol that represents the base
         address of a three-longword communication area that is used
         by different routines in the program.

      2  This statement informs the assembler that base register R1
         contains the base address, COMM_AREA, of this communication
         area. The next three statements define variables within the
         communication area.

      3  The first instruction shows how you can load registers
         with constant values in the range -32,768 to +32,767 by
         implicitly using R31 as the base register.

      4  The last three statements show how the .BASE directive
         allows you to implicitly reference base registers
         and automatically compute offsets. In each of these
         instructions, the second argument is defined to require
         an offset and a base register.

         Since no base register is specified, the assembler attempts
         to imply the base register and compute the offset based upon
         information given in previous .BASE directives.

         In the last three instructions, the address argument is
         within -32,768 to +32,767 of the base address known to
         be in R1 (that is, COMM_AREA). Therefore, R1 is selected
         as the base register. The assembler also computes the
         correct offset from the base address known to be in R1 to
         the address specified in the instruction argument.

      Example 2
      The assembler performs a sequential search through the list
      of possible base registers, R0 through R31. It uses the first
      definition possible if multiple base registers are valid. For
      example:

             .BASE R5, 300
             :
             LDQ  R10, 100

      The assembler outputs the LDQ instruction as follows:

             LDQ  R10, -200(R5)

      Both R31 and R5 are defined as base registers that can be used
      in constructing the instruction argument. R31 always contains
      0. In this example, R5 is also known to contain the constant
      300. The assembler uses the first base register, starting at
      R0 and progressing to R31, which provides a known value within
      -32,768 to +32,767 of the specified argument value. Since the
      assembler considers R5 before it considers R31, R5 is used
      rather than R31.