csrrw
Atomic Read/Write CSR
Atomically swap values in the CSRs and integer registers.
Read the old value of the CSR, zero-extends the value to XLEN bits,
and then write it to integer register xd.
The initial value in xs1 is written to the CSR.
If xd=x0, then the instruction shall not read the CSR and shall not
cause any of the side effects that might occur on a CSR read.
Decode Variables
Bits<12> csr = $encoding[31:20];
Bits<5> xs1 = $encoding[19:15];
Bits<5> xd = $encoding[11:7];Execution
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IDL
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Sail
Csr csr_handle = direct_csr_lookup(csr);
Bits<MXLEN> initial_value = X[xs1];
if (csr_handle.valid == false) {
unimplemented_csr($encoding);
} else if (!compatible_mode?(csr_handle.mode, mode())) {
raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
} else if (csr_handle.writable == false) {
raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
}
if (xd != 0) {
X[xd] = csr_sw_read(csr_handle);
}
csr_sw_write(csr_handle, initial_value);{
let rs1_val : xlenbits = if is_imm then zero_extend(rs1) else X(rs1);
let isWrite : bool = match op {
CSRRW => true,
_ => if is_imm then unsigned(rs1_val) != 0 else unsigned(rs1) != 0
};
if not(check_CSR(csr, cur_privilege, isWrite))
then { handle_illegal(); RETIRE_FAIL }
else if not(ext_check_CSR(csr, cur_privilege, isWrite))
then { ext_check_CSR_fail(); RETIRE_FAIL }
else {
let csr_val = readCSR(csr); /* could have side-effects, so technically shouldn't perform for CSRW[I] with rd == 0 */
if isWrite then {
let new_val : xlenbits = match op {
CSRRW => rs1_val,
CSRRS => csr_val | rs1_val,
CSRRC => csr_val & ~(rs1_val)
};
writeCSR(csr, new_val)
};
X(rd) = csr_val;
RETIRE_SUCCESS
}
}