Lab 4: Check-in

Each question below is a diagnostic question for a lab section. Test your understanding by answering the corresponding question as you finish an exercise. We encourage you to check in with a TA after completing a section or two, while the material is still fresh in your mind.

  1. How does a program change during the linking process? According to nm, what segment is the n variable in linking.o, and how is it different in linking.elf? How does the instruction bl _cstart change after linking?

  2. On a sheet of paper, with address 0x0 at the bottom of the page and 0x8000000 at the top, sketch a diagram of the program memory for code/linking. Include the contents of the stack at the point where execution has just entered the main function. Use the memory diagram for simple as a guide.

  3. How does the bootloader work? As a group, collectively add comments to bootloader.c explaining what it does.

  4. How does the saved pc in a function’s stack frame relate to the address of the first instruction for the function? If you take the saved pc in the stack frame for main and subtract it from the saved lr in the stack frame for diff, what do you get? Express your answer as a number, then explain conceptually what that computation represents. The memory diagram for simple is a helpful reference.

  5. How should the number of free calls relate to the number of malloc calls in a correctly-written program that uses heap allocation?

Before leaving the lab, check in with a TA and discuss any challenges to answering the questions in the lab. If you do not feel confident about your responses, please ask us about it too!