[CFU] add support for CFU-internal CSRs #681
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- add internal CSR logic - cleanup - comment updates
add CFU-CSR examples
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This PR allows to implement up to 2^32 CFU-internal (Custom Functions Subsystem for user-defined RISC-V instructions) control and status registers (CSR) for additional operand propagation, configuration and status checking (i.e. passing a 256-bit key to an encryption module implemented within the CFU).
The CFU-CSR access mechanism uses an indirect approach via just two CPU-CSRs:
cfusel(address0x800) - CFU-internal CSR select ("CFU-CSR address")cfureg(address0x801) - CFU-internal CSR alias register ("CFU-CSR read/write data")This concept is similar to the RISC-V indirect-csr-access proposal.