Explain volatile

Author: cpq

README.md

@@ -913,12 +913,13 @@ and compare generated assembly code:
    bx      lr                                                  bx      lr
 ```
 
-Long story short: if there is no `volalile`, the `delay()` function will
-loop forever and never return. Because it caches (optimises) the value of
-`s_ticks` in a register and never updates it. The generated code with
-`volatile`, on the other hand, loads `s_ticks` value on each iteration.
-So, the rule of thumb: those values in memory that get updated by interrupt
-handlers, or by the hardware, declare as `volatile`.
+Long story short: if there is no `volalile`, the `delay()` function will loop
+forever and never return. Because it caches (optimises) the value of `s_ticks`
+in a register and never updates it. A compiler does that because it doesn't
+know that `s_ticks` can be updated elsewhere - by the interrupt handler!  The
+generated code with `volatile`, on the other hand, loads `s_ticks` value on
+each iteration.  So, the rule of thumb: those values in memory that get updated
+by interrupt handlers, or by the hardware, declare as `volatile`.
 
 Now we should add `SysTick_Handler()` interrupt handler to the vector table: