There are two types of write operations in 8253
(1) Writing a control word register
(2) Writing a count value into a count register

The control word register accepts data from the data buffer and initializes the counters, as required. The control word register contents are used for

(a) Initializing the operating modes (mode 0-mode4)
(b) Selection of counters (counter 0- counter 2)
(c) Choosing binary /BCD counters
(d) Loading of the counter registers.

The mode control register is a write only register and the CPU cannot read its contents.