cyccnt.rs DONE

cargo_klee_examples/examples/cyccnt.rs

@@ -9,8 +9,8 @@ extern crate panic_klee;
 #[no_mangle]
 fn main() {
     let mut core = cortex_m::Peripherals::take().unwrap();
-    // core.DCB.enable_trace();
-    // core.DWT.enable_cycle_counter();
+    core.DCB.enable_trace();
+    core.DWT.enable_cycle_counter();
 
     let start: u32 = core.DWT.cyccnt.read();
 
@@ -208,9 +208,10 @@ fn main() {
 // Why does these values cause an error debug/dev build but not in a release build?
 //
 // [This line is causing the problem: let _time = end - start;
-// When taking substraction of values two values there is a high risk that it will overflow, 
-// therefore the error occurs in debug/dev build. 
-// In release build the substractions occurs and no overflow is found then errors will not occurs.
+// When taking substraction between two values with unsigned variable,
+// there is a possibility that the value becomes negative. The u32 cant represent negative
+// values therefore error occurs in debug/dev build.
+// In release build the substractions occurs and no negative value is found then errors will not occur.
 // It seems like the release build is not detecting potential problems, only problems that occurs.]
 //
 // C) Fix the problem!
@@ -223,8 +224,8 @@ fn main() {
 // There are numerous ways to solve the problem.
 // Argue for your solution in your own words.
 //
-// [The substraction can overflow, to remove the error and make sure the substraction is almost always correct,
-// we need to use wrapper_sub.]
+// [I added wrapper_sub to the caculation. It will remove the error and make sure the substraction wrap around instead
+//  of giving an error when the differance is negative. ]
 //
 // D) Learning outcomes and major takeaways.
 //
@@ -243,7 +244,13 @@ fn main() {
 //
 // How long time would lines 16/17 take to run to trigger the error?
 //
-// [your answer here]
+// [
+//  They are hard do find becasue the error appears when the timer has wrapped around.
+//  The error is triggered when end is smaller than start.
+//  The CYCCNT is a 32 bit unsigned integer, it will wrap around when the value has reached 2^32.
+//  2^32/8MHz = 537 s
+//  It would take at least 537s.
+// ]
 //
 // Of course this is a contrived example, and may not occur in practice.
 // But, it represents a class of problems/errors/bugs that is