Predefined symbols are MACRO-64 register symbols that are not
    permanently reserved. You can delete the definition of any of
    these predefined register symbols. You can also define your own
    register symbols.

    You can express the 32 general registers and the 32 floating-
    point registers of the Alpha processor in a source program as
    follows:

    Register
    Name   Description

    R0     General register 0.
    R1     General register 1.
    .      .
    .      .
    .      .
    R29    General register 29 or frame pointer. If you use R29 as a
    or FP  frame pointer, Digital recommends you use the name FP. If
           you use R29 as a general register, Digital recommends you
           use the name R29.
    R30    General register 30 or stack pointer. If you use R30 as a
    or     stack pointer, the name SP is recommended; if you use R30
    SP     as a general register, the name R30 is recommended.
    R31    General register 31.
    F0     Floating-point register 0.
    .      .
    .      .
    .      .
    F31    Floating-point register 31.

                                   NOTE

       When MACRO-64 operates in /NAMES=AS_IS mode, all of the
       previous register symbols are defined in all uppercase and
       all lowercase.

    To define your own register symbols, use either the .DEFINE_
    FREG or .DEFINE_IREG directive for floating-point or integer
    registers, respectively. For more information about the .DEFINE_
    FREG or .DEFINE_IREG directives, see .DEFINE_FREG and .DEFINE_
    IREG, respectively.

    You can delete a register symbol definition with the .UNDEFINE_
    REG directive. For more information about the .UNDEFINE_REG
    directive, see .UNDEFINE_REG.

    While an identifier is defined as a register symbol, it can only
    be used in those contexts that allow a register.