Reading a write only register value

[Jonathan]

What's the expected behavior when reading a register that is write only? For example, I'm using an some code that is reading the value of the TASK_START register of the RNG (nRF52832).

I'm expecting this to be unpredictable so I'm surprised to see this code.

[Jonathan]

In the mynewt RTOS, there's a code that checks if the RNG started in the following manner:

if (NRF_RNG->TASKS_START == 0) {
        NRF_RNG->EVENTS_VALRDY = 0;
        if (g_ble_rng_isr_cb) {
            NRF_RNG->INTENSET = 1;
        }
        NRF_RNG->TASKS_START = 1;
    }