csrrs

Atomic Read and Set Bits in CSR

This instruction is defined by:

Encoding

Synopsis

Atomically read and set bits in a CSR.

Reads the value of the CSR, zero-extends the value to XLEN bits, and writes it to integer register rd. The initial value in integer register rs1 is treated as a bit mask that specifies bit positions to be set in the CSR. Any bit that is high in rs1 will cause the corresponding bit to be set in the CSR, if that CSR bit is writable. Other bits in the CSR are not explicitly written.

Access

Always

Decode Variables

Bits<12> csr = $encoding[31:20];
Bits<5> rs1 = $encoding[19:15];
Bits<5> rd = $encoding[11:7];

Execution

Pruned, XLEN == 64
Original

XReg initial_csr_value = CSR[csr].sw_read();
XReg mask = X[rs1];
CSR[csr].sw_write(initial_csr_value | mask);
X[rd] = initial_csr_value;

XReg initial_csr_value = CSR[csr].sw_read();
XReg mask = X[rs1];
CSR[csr].sw_write(initial_csr_value | mask);
X[rd] = initial_csr_value;