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 = $encoding[11:7];Execution
Csr csr_handle = direct_csr_lookup(csr);
Bits<MXLEN> initial_value = X[xs1];
if (csr_handle.valid == false) {
unimplemented_csr($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);