What is meant by read-modify -write in register access generally?

UVM
2

In reply to Arun_Rajha:

  
 /*Let's assume 32bit ctrl_reg register with 2 different bit-field.
   data_ctrl(22bit), clk_ctrl(10bit)*/
  _______________________
  | 31  :  10 | 9 : 0    | 
  | data_ctrl | clk_ctrl |
  |___________|__________|  

  // you want to modify data_ctrl bit-field and don't want to change the clk_ctrl bit-field value. 
  // But when you write the register, it will modify all 32bit value.
  
  // so first read the register data, modify the read data then write the register

  tmp_data = *ctlr_reg; // ctrl_reg read operation
  tmp_data = data_ctrl | (tmp_data & 0x0000_003FF); // modify the data
  *ctlr_reg = tmp_data;  // ctrl_reg write operation