mireg

Machine Indirect Register Alias

The mireg machine indirect alias CSR is used to access another CSR’s state indirectly upon a read or write, as determined by the value of miselect.

The behavior upon accessing mireg from M-mode, while miselect holds a value that is not implemented, is UNSPECIFIED.

It is expected that implementations will typically raise an illegal instruction exception for such accesses, so that, for example, they can be identified as software bugs. Platform specs, profile specs, and/or the Privileged ISA spec may place more restrictions on behavior for such accesses.

Attempts to access mireg while miselect holds a number in an allocated and implemented range results in a specific behavior that, for each combination of miselect and mireg, is defined by the extension to which the miselect value is allocated.

Ordinarily, mireg will access register state, access read-only 0 state, or raise an illegal instruction exception.

For RV32, if an extension defines an indirectly accessed register as 64 bits wide, it is recommended that the lower 32 bits of the register are accessed through mireg, while the upper 32 bits are accessed through mireg4.

Attributes

Defining Extension

  • Smcsrind, version >= Smcsrind@1.0.0

CSR Address

0x351

Length

32 when CSR[misa].MXL == 0 64 when CSR[misa].MXL == 1

Privilege Mode

M

Format

This CSR format changes dynamically.

mireg Format when CSR[misa].MXL == 0
Figure 1. mireg Format when CSR[misa].MXL == 0
mireg Format when CSR[misa].MXL == 1
Figure 2. mireg Format when CSR[misa].MXL == 1

Field Summary

Name Location Type Reset Value

mireg.VALUE

* 31:0 when CSR[misa].MXL == 0 * 63:0 when CSR[misa].MXL == 1

RW

UNDEFINED_LEGAL

Fields

VALUE

Location

  • 31:0 when CSR[misa].MXL == 0

  • 63:0 when CSR[misa].MXL == 1

Description

Register state of the CSR selected by the current miselect value

Type

RW

Reset value

UNDEFINED_LEGAL

Software write

This CSR may store a value that is different from what software attempts to write.

When a software write occurs (e.g., through csrrw), the following determines the written value:

VALUE = Csr handle = indirect_csr_lookup(CSR[miselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
if (!handle.writable) {
  raise (ExceptionCode::IllegalInstruction, mode(), $encoding);
}
csr_sw_write(handle, csr_value.VALUE);
return csr_hw_read(handle);

Software read

This CSR may return a value that is different from what is stored in hardware.

Csr handle = indirect_csr_lookup(miselect.VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
return csr_sw_read(handle);