Function begin_task 

pub fn begin_task<H, L>(
    unit: &mut RunUnit<'_, H, L>,
    page: &CodePage,
    length: usize,
)
where
    H: Clone,
    L: Loader<H>,

Fills the EXEC interface block for the task’s first program: time, date, transaction, task number, terminal, COMMAREA length and the AID that started it.