Using as: Xtensa Registers

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#### 9.53.2.2 Register Names

The assembly syntax for a register file entry is the “short” name for a TIE register file followed by the index into that register file. For example, the general-purpose `AR` register file has a short name of `a`, so these registers are named `a0`…`a15`. As a special feature, `sp` is also supported as a synonym for `a1`. Additional registers may be added by processor configuration options and by designer-defined TIE extensions. An initial ‘`$`’ character is optional in all register names.