sireg

Supervisor Indirect Register Alias

Access to sireg from M-mode or S-mode while siselect holds a number in a standard-defined and implemented range results in specific behavior that, for each combination of siselect and sireg, is defined by the extension to which the siselect value is allocated.

Ordinarily, sireg will access register state, access read-only 0 state, or, unless executing in a virtual machine (covered in the next section), raise an illegal instruction exception.

Note that the widths of siselect and sireg are always the current XLEN rather than SXLEN. Hence, for example, if MXLEN = 64 and SXLEN = 32, then this register is 64 bits when the current privilege mode is M (running RV64 code) but 32 bits when the privilege mode is S (RV32 code).

The behavior upon accessing sireg from M-mode or S-mode, while siselect holds a value that is not implemented at supervisor level, is UNSPECIFIED.

It is recommended that implementations raise an illegal instruction exception for such accesses, to facilitate possible emulation (by M-mode) of these accesses.

An extension is considered not to be implemented at supervisor level if machine level has disabled the extension for S-mode, such as by the settings of certain fields in CSR menvcfg, for example.

Attributes

CSR Address	0x151
Defining extension	Sscsrind
Length	64-bit
Privilege Mode	S

CSR Address

0x151

Defining extension

Sscsrind

Length

64-bit

Privilege Mode

Format

Figure 1. sireg 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: sireg[63:0]
Description: The data read from or written to the register selected by the current siselect 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::S, CSR[siselect].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::S, CSR[siselect].VALUE, 1);
if (!handle.valid) {
  unimplemented_csr($encoding);
}
return csr_sw_read(handle);

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