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

CSR Address	0x351
Defining extension	Smcsrind
Length	64-bit
Privilege Mode	M

CSR Address

0x351

Defining extension

Smcsrind

Length

64-bit

Privilege Mode

Format

Figure 1. mireg format

Field Summary

Name Location Type Reset Value

Name	Location	Type	Reset Value
`VALUE`	63:0	RW	UNDEFINED_LEGAL

VALUE

63:0

UNDEFINED_LEGAL

Fields

`VALUE`

Location: mireg[63:0]
Description: Register state of the CSR selected by the current miselect value
Type

RW	Read-Write Field is writable by software. Any value that fits in the field is acceptable and shall be retained for subsequent reads.

Reset value

UNDEFINED_LEGAL

Software write

This field has special behavior when written by software (e.g., through csrrw).

When software tries to write csr_value, the field will be written with the return value of the function below.

Csr handle = indirect_csr_lookup(PrivilegeMode::M, 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.

Pruned
Original

Csr handle = indirect_csr_lookup(PrivilegeMode::M, CSR[miselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
return csr_sw_read(handle);

Csr handle = indirect_csr_lookup(PrivilegeMode::M, CSR[miselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
return csr_sw_read(handle);