Lab 3: Check-in


  1. Explain how the lr register is used as part of making a function call. Which instruction writes to the lr register? Which instruction reads from it? What commands could you use in gdb to observe the changes to the lr register during execution of a function call?

  2. Why will loopback mode only work if you plug in both TX and RX?

  3. On a hosted system, executing an incorrect call to strlen (e.g. argument is an invalid address or unterminated string) can result in a runtime error (crash/halt). But when running bare metal on the Pi, every call to strlen (whether well-formed or not) will complete “successfully” and return a value. Explain the difference in behavior. What is the return value for an erroneous call?