4–134 Alpha Architecture Handbook
• On 21164A (EV56), 21164PC (PCA56), and 21264 (EV6), AMASK correctly indicates
support for architecture extensions by copying Rbv to Rc and clearing appropriate bits.
Bits are assigned and placed in Appendix D for architecture extensions as ECOs for those
extensions are passed. The low 8 bits are reserved for standard architecture extensions so
they can be tested with a literal; application-specific extensions are assigned from bit 8
upward.