The D3, D2, and D1 bits of the Control Word set the operating mode of the timer. There are total 6 modes; for modes 2 and 3, the D3 bit is avoided, so the missing modes 6 and 7 are aliases for modes 2 and 3. Notice down that, for modes 0, 2, 3 and 4, GATE should be set to HIGH to enable counting. For modes 5 and 1, the rising edge of GATE starts the count.
Mode 0 (000): Interrupt on Terminal Count
In this mode, the counter will begin counting from the initial COUNT value loaded into it, down to 0. Counting rate is equivalent to the input clock frequency. The OUT pin is set to low after the Control Word is written and counting begin one clock cycle after the COUNT programmed. OUT remains low till the counter attain 0, at which point OUT will be set high till the counter is reloaded or the Control Word is written.
Mode 1 (001): Hardware-Triggered One Shot
It is similar to mode 0, but counting is begun by a rising edge on the GATE input rather than immediately after programming. The GATE input is avoided while counting. The output is set to high as soon as the Control Word is written. Later on COUNT is written, the device will wait till the rising edge of the GATE input. One of clock cycle after this rising edge is detected; OUT will become and remain low till the counter reaches 0. Then OUT will go high, waiting for the next trigger.