Chapter 2 code sample fixes

These fixes get code samples to compile without warnings. Other changes
include:
* Fix the SysTick code snippet on the "Memory Mapped Registers" page so
  that it:
  * Compiles without errors/warnings.
  * Sets up the SysTick peripheral correctly for counting down 1000
    ticks.
* Added a reminder in the "Semihosting" section about how to enable
  semihosting in openOCD.

src/start/panicking.md

@@ -81,7 +81,7 @@ use cortex_m_rt::entry;
 fn main() -> ! {
     let xs = [0, 1, 2];
     let i = xs.len() + 1;
-    let y = xs[i]; // out of bounds access
+    let _y = xs[i]; // out of bounds access
 
     loop {}
 }

src/start/registers.md

@@ -26,15 +26,18 @@ use cortex_m::peripheral::{syst, Peripherals};
 use cortex_m_rt::entry;
 
 #[entry]
-fn main() {
+fn main() -> ! {
     let mut peripherals = Peripherals::take().unwrap();
     let mut systick = peripherals.SYST;
     systick.set_clock_source(syst::SystClkSource::Core);
+    systick.set_reload(1_000);
     systick.clear_current();
     systick.enable_counter();
-    while systick.get_current() < 1_000 {
+    while !systick.has_wrapped() {
         // Loop
     }
+
+    loop {}
 }
 ```
 

src/start/semihosting.md

@@ -18,8 +18,6 @@ world!":
 
 extern crate panic_halt;
 
-use core::fmt::Write;
-
 use cortex_m_rt::entry;
 use cortex_m_semihosting::hprintln;
 
@@ -41,6 +39,12 @@ Hello, world!
 (..)
 ```
 
+You do need to enable semihosting in OpenOCD from GDB first:
+``` console
+(gdb) monitor arm semihosting enable
+semihosting is enabled
+```
+
 QEMU understands semihosting operations so the above program will also work with
 `qemu-system-arm` without having to start a debug session. Note that you'll
 need to pass the `-semihosting-config` flag to QEMU to enable semihosting