Smcsrind/Sscsrind is an ISA extension that extends the indirect CSR access mechanism originally defined as part of the Smaia/Ssaia extensions, in order to make it available for use by other extensions without creating an unnecessary dependence on Smaia/Ssaia.
This extension confers two benefits:
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It provides a means to access an array of registers via CSRs without requiring allocation of large chunks of the limited CSR address space.
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It enables software to access each of an array of registers by index, without requiring a switch statement with a case for each register.
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Note
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CSRs are accessed indirectly via this extension using select values, in contrast to being accessed directly using standard CSR numbers. A CSR accessible via one method may or may not be accessible via the other method. Select values are a separate address space from CSR numbers, and from tselect values in the Sdtrig extension. If a CSR is both directly and indirectly accessible, the CSR’s select value is unrelated to its CSR number. Further, Machine-level and Supervisor-level select values are separate address spaces from each other; however, Machine-level and Supervisor-level CSRs with the same select value may be defined by an extension as partial or full aliases with respect to each other. This typically would be done for CSRs that can be delegated from Machine-level to Supervisor-level. |
The machine-level extension Smcsrind encompasses all added CSRs and all behavior modifications for a hart, over all privilege levels. For a supervisor-level environment, extension Sscsrind is essentially the same as Smcsrind except excluding the machine-level CSRs and behavior not directly visible to supervisor level.