Normally, you only need to worry about access due to other CPU's or channels.
For an interruptible instruction, you also need to worry about access by other tasks on the same CPU. In the olden days, the interval timer at address 80 could be updated with an MVC instruction. The new value was stored at 84, and and 8 byte MVC move from 80 to 76 would update the timer and return the previous value without losing any counts. MVCL would not necessarily prevent the loss of counts. -- glen
