csrrwi

Atomic Read/Write CSR Immediate

This instruction is defined by:

Encoding

Synopsis

Atomically write CSR using a 5-bit immediate, and load the previous value into 'rd'.

Read the old value of the CSR, zero-extends the value to XLEN bits, and then write it to integer register rd. The 5-bit uimm field is zero-extended and written to the CSR. If rd=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.

Access

Always

Decode Variables

Bits<12> csr = $encoding[31:20];
Bits<5> imm = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

Pruned, XLEN == 64
Original

if (rd != 0) {
  X[rd] = CSR[csr].sw_read();
}
CSR[csr].sw_write({{59{1'b0}}, imm});

if (rd != 0) {
  X[rd] = CSR[csr].sw_read();
}
CSR[csr].sw_write({{XLEN - 5{1'b0}}, imm});