Kernel4 Activity

Solutions

Advance work

1. Familiarize yourself with the WeensyOS source code as described in lecture and the initial parts of the problem set handout.

2. Read the “About virtual memory iterators” and “About physical memory iterators” sections of the problem set handout.

3. Read about the log_printf function in the problem set handout.

The remaining portions of this handout are taken up with section problems.

P1. Examining memory

Use vmiter and log_printf to print the physical addresses that are mapped to the following virtual addresses by kernel_pagetable:

1. The syscall_entry function
2. The kernel_pagetable
3. The p-hello process’s process_main

Print these physical addresses both (A) immediately before the call to process_setup, and (B) immediately after that call. Do the values change? Why or why not?

P2. Examining permissions

Use vmiter and log_printf to print the permissions active for the following virtual addresses in kernel_pagetable:

1. The syscall_entry function
2. The kernel_pagetable
3. The p-hello process’s process_main

Print these physical addresses both (A) immediately before the call to process_setup, and (B) immediately after that call. Do the values change? Why or why not?

P3. Examining page table structures

Use ptiter to print the page table pages active in kernel_pagetable. Then use GDB (or the QEMU monitor, or log_printf) to examine the memory stored in those page table pages. All page table pages have the same structure; can you deduce anything about that structure from their contents? Try adding some vmiter::map calls to investigate further.

P4. Warped virtual memory

Run make run-bigdata. Boo! Examine p-bigdata.cc. Now add one line of virtual memory manipulation code to process_startup so that run-bigdata prints CS 61 Is Amazing. (Don’t modify process memory directly; just modify memory mappings.)

Make sure you remove your VM manipulation code before moving on to another problem.

P5. Examining faults

Now run make run-recurse. This will run p-recurse instead of p-hello. Examine p-recurse and understand how it works.

1. What is the size of a stack frame for the f function? Try to determine this by examining obj/p-recurse.asm, then check your understanding by adding console_printf lines that print the address of some local variable (say i).

2. Find the smallest DEPTH constant that causes p-recurse to fail with a page fault exception. What is that constant?

3. What is your explanation for the page fault?

4. Change the kernel so that p-recurse supports a larger DEPTH.