csrrw
Atomic Read/Write CSR
This instruction is defined by:
Synopsis
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 rd.
The initial value in rs1 is 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.