This is not the current version of the class.

Problem set 3: WeensyOS

In this assignment, you implement process memory isolation, virtual memory, and some system calls in a tiny operating system. This will introduce you to virtual memory and operating system design.

You may want to read Chapter 9 of the text. The 64-bit x86 virtual memory architecture is described in Section 9.7. The PTE_P, PTE_W, and PTE_U bits are shown in Figure 9.23 and discussed in Section 9.7.1.

Get the code

Get our code with

$ git pull; git pull handout main

or, alternately

$ git pull; git pull git:// main

This will merge our Problem Set 3 code with your previous work. If you have any “conflicts” from prior problem sets, resolve them before continuing further. Run git push to save your work back to your personal repository.

You may also create a new cs61-psets repository for this assignment. Don’t forget to enter your repository URL on the grading server.

Initial state

For this assignment, there is no handy make check functionality. Instead, you should run your instance of WeensyOS and visually compare it to the images you see below in the assignment.

Run make run in your pset3 directory. You should see something like this, which shows four related p-allocator processes running in parallel:

Initial WeensyOS state

This image loops forever; in an actual run, the bars will move to the right and stay there. Don’t worry if your image has different numbers of K’s or otherwise has different details.

If your bars run painfully slowly, edit the file and reduce the ALLOC_SLOWDOWN constant. Stop now to read and understand

Here’s what’s going on in the physical memory display.

Here are two labeled memory diagrams, showing what the characters mean and how memory is arranged.

Physical memory map 1

Physical memory map 2

The virtual memory display is similar.


You will implement complete and correct memory isolation for WeensyOS processes. Then you'll implement full virtual memory, which will improve utilization. You'll implement fork—creating new processes at runtime—and exit—destroying processes at runtime.

We need to provide a lot of support code for this assignment, but the code you write will all go in


Running WeensyOS

If QEMU’s default display causes accessibility problems, you will want to run make run-console.

There are several ways to debug WeensyOS. We recommend:

Memory system layout

WeensyOS memory system layout is described by several constants.

KERNEL_START_ADDR Start of kernel code.
KERNEL_STACK_TOP Top of kernel stack. The kernel stack is one page long.
CONSOLE_ADDR CGA console memory.
PROC_START_ADDR Start of application code. Applications should not be able to access memory below PROC_START_ADDR, except for the single page at console.
MEMSIZE_PHYSICAL Size of physical memory in bytes. WeensyOS does not support physical addresses ≥ MEMSIZE_PHYSICAL. Equals 0x200000 (2MB).
MEMSIZE_VIRTUAL Size of virtual memory. WeensyOS does not support virtual addresses ≥ MEMSIZE_VIRTUAL. Equals 0x300000 (3MB).
PAGESIZE Size of a memory page. Equals 4096 (or, equivalently, 1 << 12).
PAGEOFFMASK Mask for the offset portion of an address. Equals 4095 (PAGESIZE - 1). If (a & PAGEOFFMASK) == 0, then a is page-aligned.

Kernel and process address spaces

WeensyOS begins with the kernel and all processes sharing a single address space. This is defined by the kernel_pagetable page table. kernel_pagetable is initialized to the identity mapping: virtual address X maps to physical address X.

As you work through the pset, you will shift processes to use independent address spaces, where each process can access only a subset of physical memory.

The kernel, though, still needs the ability to access all of physical memory. Therefore, all kernel functions run using the kernel_pagetable page table. Thus, in kernel functions, each virtual address maps to the physical address with the same number. The exception_entry and syscall_entry assembly codes explicitly install kernel_pagetable when they begin, and exception_return and the syscall return path install the process’s page table as they exit.

Each process page table must contain kernel mappings for the kernel stack and for the exception_entry and syscall_entry code paths.

Step 1: Kernel isolation

WeensyOS processes could stomp all over the kernel’s memory if they wanted. Better stop that. Change kernel_start, the kernel initialization function, so that kernel memory is inaccessible to applications—except for the memory holding the CGA console (the single page at CONSOLE_ADDR == 0xB8000).

When you are done, WeensyOS should look like this. In the virtual map, kernel memory is no longer reverse-video, since the user can’t access it. Note the lonely CGA console memory block.

Kernel isolation

Use vmiter to create memory mappings. Start from the vmiter loop in the kernel_start function.

About virtual memory iterators (vmiter)

In addition, make sure that your sys_page_alloc system call preserves kernel isolation: Applications shouldn’t be able to use sys_page_alloc to screw up the kernel. This requires changes to the SYSCALL_PAGE_ALLOC case in syscall. Read the description of sys_page_alloc in u-lib.hh to get a feeling for the possible errors.

Step 2: Isolated address spaces

Implement process isolation by giving each process its own independent page table. Your OS should look like this:

Per-process isolation

Each process only has permission to access its own pages, which you can tell because only its own pages are shown in reverse video.

How to implement per-process page tables in process_setup:

Note the diagram now has four pages for each process in the kernel area, starting at 0x1000. These are the four-level page tables for each process. (The colored background indicates that these pages contain kernel-private page table data, even though the pages “belong” to the process.) The first page was allocated explicitly in process_setup; the other pages were allocated by vmiter::map as the page table was initialized.

One common solution, shown above, leaves addresses above PROC_START_ADDR totally unmapped by default, but other designs work too. As long as a virtual address mapping has no PTE_U bit, its process isolation properties are unchanged. For instance, this solution, in which all mappings are present but accessible only to the kernel, also implements process isolation correctly:

Per-process isolation, alternate

If you create an incorrect page table, WeensyOS might crazily reboot. Don’t panic; see the debugging hints above.

About program images and segments (program_image pgm, seg)

Step 3: Virtual page allocation

So far, WeensyOS processes use physical page allocation for process memory: Process code, data, stack, and heap pages with virtual address X always use the physical pages with physical address X. This is inflexible and limits utilization.

Change your operating system to allocate all process data, including its code, globals, stack, and heap, using kalloc instead of direct access to the pages array.

Here’s how our OS looks after this step.

Virtual page allocation

Virtual page allocation will complicate the code that initializes process code in process_setup. You’ll need to figure out why (hint: which page table is in force in process_setup?) and find a way around it (hint: vmiter or set_pagetable).

Step 4: Overlapping address spaces

Now the processes are isolated, which is awesome, but they’re still not taking full advantage of virtual memory. Isolated address spaces can use the same virtual addresses for different physical memory. There’s no need to keep the four process address spaces disjoint.

In this step, change each process’s stack to grow down starting at address 0x300000 == MEMSIZE_VIRTUAL. Now the processes have enough heap room to use up all of physical memory!

Overlapping address spaces

If there’s no physical memory available, sys_page_alloc should return an error code to the calling process, such as -1. Do not kill the calling process! Lack of memory is a potentially recoverable problem.

Step 5: Fork

The fork system call starts a new process as a copy of an existing process. The process that calls fork is called the parent process, and the newly created copy is called the child process. The system call appears to return twice: it returns 0 to the child, and returns the child’s process ID to the parent.

Run WeensyOS with make run or make run-console. At any time, press the ‘f’ key. This will soft-reboot WeensyOS and ask it to run a single p-fork process, rather than the gang of allocators. You should see something like this:

Initial fork state

This is because you haven’t implemented fork yet.

Implement fork.

When you’re done, you should see something like this after pressing ‘f’.

Fork works

An image like this means you forgot to copy the data for some pages, so the processes are actually sharing stack and/or data pages:

Incorrect fork

Step 6: Freeing memory

So far none of your test programs have ever freed memory or exited. Memory allocation’s pretty easy until you add free! So let’s add free by allowing applications to exit.

In this exercise you’ll implement the sys_exit system call, which exits the current process. This exercise is a capstone since freeing memory will tend to expose weaknesses and problems in your other code.

To test your work, use make run and then type ‘e’. This reboots WeensyOS to run the p-forkexit program. (Initially it’ll crash because sys_exit() isn’t implemented yet.) p-forkexit processes all alternate among forking new children, allocating memory, and exiting. The result is that once your code is correct, p-forkexit makes crazy patterns forever, like this:

Fork with exit

2021 note. Your picture will not have S characters until Step 7.

A fully correct OS can run p-forkexit indefinitely. An OS with a memory leak will display a persistent blank spot in the physical memory map—the leaked page—and if run long enough, blank spots will take over the screen. This OS has a pretty bad leak; within 10 seconds it has run out of memory:

Fork with exit and leak

This OS’s leak is slower, but if you look at the bottom row of the physical memory map, you should see a persistently unused pages just above and to the left of the “V” in “VIRTUAL”. Persistently unused pages are a hallmark of leaks.

Fork with exit and slow leak

Reducing ALLOC_SLOWDOWN in p-forkexit may encourage errors to manifest, but you may need to be patient.

Here’s your task.

  1. Complete kfree so that kfree frees memory. Make sure that kalloc can re-use freed memory.

  2. sys_exit should mark a process as free and free all of its memory. This includes the process’s code, data, heap, and stack pages, as well as the pages used for its page directory and page table pages. The memory should become available again for future allocations.

    Use vmiter and ptiter to enumerate the relevant pages. But be careful not to free the console.

  3. You will also need to change fork and perhaps other places. Think about all the places that allocate or share memory: what information must they maintain? In p-forkexit, unlike in previous parts of the pset, sys_fork and sys_page_alloc can run when there isn’t quite enough memory to create a new process or allocate or map a page. Your code should handle this cleanly, without leaking memory in any situation.

    • If there isn’t enough free memory to successfully complete sys_page_alloc, the system call should return -1 to the caller.

    • If there isn’t enough free memory to successfully complete sys_fork, the system call should clean up (i.e., free any memory that was allocated for the new process before memory ran out), mark the possibly-partially-initialized child process as free, and then return -1 to the parent.

    • If any allocation or mapping in sys_page_alloc or sys_fork fails, then the whole function must fail cleanly, leaving the state of allocated memory unchanged. When this happens, your code must clean up any previous “successful” allocations before returning -1 or there will be a memory leak. Be careful. Note that allocation failures should not exit the calling process!

There should be no memory leaks!

About physical memory iterators (ptiter)

Step 7: Shared read-only memory

It’s wasteful for fork() to copy all of a process’s memory. For example, most processes, including p-fork, never change their code. So what if we shared the memory containing the code? That’d be fine for process isolation, as long as neither process could write the code.

Change process_setup to map read-only program segments using read-only memory. Use the seg.writable() function to detect whether a program segment can use read-only memory. Then, in fork, share read-only pages between processes rather than copying them; and, in exit, ensure that shared pages are correctly accounted for when a process exits.

When you’re done, running p-fork should look like this:

Shared read-only memory

Each process’s virtual address space begins with an “S”, indicating that the corresponding physical page is Shared by multiple processes.

Extra credit

If you are finished and can't wait to do more of this type of work, try:


You will turn in your code by pushing your git repository to and updating the grading server.