Home | Schedule | Policies and grading | Labs | Infrastructure | Exams | Reference materials | AnnouncementsCS202: Lab 4: WeensyOSIntroductionIn this lab, you will implement process memory isolation, virtual memory, and a system call (fork()) in a tiny(but real!) operating system, called WeensyOS.This will introduce you to virtual memory and reinforce some of the concepts that we have covered thissemester.
The WeensyOS kernel runs on x86-64 CPUs. Because the OS kernel runs on the “bare” hardware, debuggingkernel code can be tough: if a bugcausesmisconfigurationofthe hardware, the usual result is a crash of theentire kernel (and all the applications running on top of it). And because the kernel itself provides the mostbasic system services (for example, causing the display hardware to display error messages), deducing what ledto a kernel crash can be particularly challenging. In the old days, the usual way to develop code for an OS(whether as part of a class, in a research lab, or in industry) was to boot it on a physical CPU. The lives of kerneldevelopers have gotten much better since. You will run WeensyOS in QEMU.QEMU is a software-based x86-64 emulator: it “looks” to WeensyOS just like a physical x8664 CPU with aparticular hardware configuration. However, if your WeensyOS code-in-progress wedges the (virtual)hardware, QEMU itself and the whole OS that is running on the “real” hardware (that is, the “real” Linux OS thatQEMU is running on) survive unscathed (“real” is in quotation marks for reasons that will be unpacked in thenext paragraph). So, for example, your last few debugging printf()s before a kernel crash will still get loggedto disk (by QEMU running on Linux), and “rebooting” the kernel you’re developing amounts to re-running theQEMU emulator application.What is the actual software/hardware stack here? The answer is different for studentswith x86-64 computers(for example, Windows machines and older Macs) and ARMs. All students are running a host OS (on yourcomputer) on top of either x86-64 or ARM hardware (ARM being the architecture for so-called Apple silicon,namely M1 and M2 chips). Then, the Docker containerization environment runs on top of the host OS (as aprocess). That environment, loosely speaking, emulates either an x86 or an ARM CPU, and running on top ofthat emulated CPU is Ubuntu Linux, targeted to x86-64 or ARM. Running on top of Ubuntu is QEMU. QEMUpresents an emulated x86-64 interface, and QEMU itself is either an x86-64 or ARM binary, againdependingon the underlying hardware. Finally, WeensyOS is exclusively an x86-64 binary, and that of course runs onQEMU (though if you have some x86-64 hardware sitting around, you can try installing WeensyOS andrunning it “bare”). Taking that same progression, now top-down: if you have an ARM CPU, that means you are
running the WeensyOS kernel’s x86-64 instructions in QEMU, a software-emulated x86-64 CPU that is an ARMbinary, on top of Linux (targeted to ARM), running in the containerization environment (also itself an
ARM binary), on macOS, running on an ARM hardware CPU.Heads up. As always, it’s important to start on time. In this case, on time means 3 weeks beforetheassignment is due, as you will almost certainly need all of the allotted time to complete the lab. Kerneldevelopment is less forgiving than developing user-level applicationstiny deviations in the configurationof hardware (such as the MMU) by the OS tend to bring the whole (emulated) machine to a halt.o save yourself headaches later, read this lab writeup in its entirety before you begin.Resources.
2024/11/15 18:17CS202: Lab 4: WeensyOS1/19You may want to look at Chapter 9 of CS:APP3e (from which our x86-64 virtual memory handout isborrowed). The book is on reserve at the Courant library. Section 9.7 in particular describes the 64-bitvirtual memory architecture of the x86-64 CPU. Figure 9.23 and Section 9.7.1 show anddiscuss thePTE_P, PTE_W, and PTE_U bits; these are flags in the x86-64 hardware’s page table entries that play acentral role in this lab.You may find yourself during the lab wanting to understand particular assembly instructions. Here aretwo guides to x86-64 instructions, from Brown and CMU. The former is more digestible; the latter ismore comprehensive. The supplied code also uses certain assembly instructions like iret; see here fora reference.Getting StartedYou’ll be working in the Docker container as usual. We assume that you have set up the upstream as describedin the lab setup. Then run thefollowing on your local machine (Mac users can do this on their local machine orwithin the Docker container; Windows and CIMS users should do this from outside the container):$ cd ~/cs202$ git fetch upstream$ git merge upstream/maiThis lab’s files are located in the lab4 subdirectory.If you have any “conflicts” from lab 3, resolve them before continuing further. Run git push to save your workback to your personal repository.Another heads up. Given the complexity of this lab, and the possibility of breaking the functionality of thekernel if you code in some errors, make sure to commit and push your code often! It's very important thatyour commits have working versions of the code, so if something goes wrong, you can always go back to aprevious commit and get back a working copy! At the very least, for this lab, you should be committingonce per step (and probably more often), so you can go back to the last step if necessary.GoalYou will implement complete and correct memory isolation for WeensyOSprocesses. Then you'll implement
full virtual memory, which will improve utilization. You'll implement fork() (creating new processes at runtime)
and for extra credit, you’ll implement exit() (destroying processes at runtime)We’ve provided you with a lot of support code for this assignment; the code you will need to write is in factlimited in extent. Our complete solution (for all stages)consists of well under 300 lines of code beyond whatwe initially hand out to you. All the code you write will go in kernel.c (except for part of step 6).Testing, checking, and validationFor this assignment, your primary checking method will be to run your instance of Weensy OS and visuallyompare it to the images you see below in the assignment.2024/11/15 18:17CS202: Lab 4: WeensyOS2/19Studying these graphical memory maps carefully is the best way to determine whether your WeensyOS codefor each stage is working correctly. Therefore, you will definitely want to make sure you understand how to read these maps before you start to code.We supply some grading scripts, outlined at the end of the lab, but those will not be your principal source offeedback. For the most part, they indicate only whether a given step is passing or failing; look to the memorymaps to understand why.Initial state
the Docker environment:$ ./cs202-run-dockercs202-user@172b6e333e91:~/cs202-labs$ cd lab4/cs202-user@172b6e333e91:~/cs202-labs/lab4$ make run
The rest of these instructions presume that you are in the Docker environment. We omit the cs202-user@172b6e333e91:~/cs202-labs part of the prompt.make run should cause you to see something like the below, which shows four processes running in parallel,each running a version of the program in p-allocator:This image loops forever; inan actual run, the bars will move to the right and stay there. Don't worry if yourimage has different numbers of K's or otherwise has different details.
If your bars run painfully slowly, edit the p-allocator.c file and reduce the ALLOC_SLOWDOWN constant.Stop now to read and understand p-allocator.c. Here’s how to interpret the memory map display:WeensyOS displays the current state of physical and virtual memory. Each character represents 4 KB ofmemory: a single page. There are 2 MB of physical memory in total. (Ask yourself: how many pages isthis?)WeensyOS runs four processes, 1 through 4. Each process is compiled from the same source code (pallocator.c), but linked to use a different region of memory./11/15 18:17
CS202: Lab 4: WeensyOS3/19Each process asks the kernel for more heap memory, one page at a time, until it runs out of room. As, each process's heap begins just above its code and global data, and ends just below its stack.The processes allocate heap memory at different rates: compared to Process 1, Process 2 allocates
twice as quickly, Process 3 goes three times faster, and Process 4 goes four times faster. (A random
number generator is used, so the exact rat代写CS202 Weensy OS es may vary.) The marching rows of numbers show howquickly the heap spaces for processes 1, 2, 3, and 4 are allocated.Here are two labeled memory diagrams, showing what the characters mean and how memory is arranged.The virtual memory display is similar.The virtual memory display cycles successively among the four processes’ address spaces. In the baseversion of the WeensyOS code we give you to start from, all four processes’ address spaces are thesame (your job will be to change that!).Blank spaces in the virtual memory display correspond to unmapped addresses. If a process (or the
The character shown at address X in the virtual memory display identifies the owner of the
corresponding physical page.In the virtual memory display, a character is reverse video if an application process is allowed toaccess the corresponding address. Initially, any process can modify all of physical memory, includingthe kernel. Memory is not properly isolated.Running WeensyOSReadthe README-OS.md file for information on how to run WeensyOS.There are several ways to debug WeensyOS. We recommend adding log_printf statements to your code. Theoutput of log_printf is written to the file /tmp/log.txt outside QEMU. We also recommend that you useassertions (of which we saw a few in lab 1) to catch problems early. For example, call the helper functions we’veprovided, check_page_table_mappings and check_page_table_ownership to test a page table forobviouserrors.2024/11/15 18:17CS202: Lab 4: WeensyOS4/19Finally, you can and should use gdb, which we cover at the end of this section.Memory system layout WeensyOS memory system layout is defined by several constants:Constant
Meaning KERNEL_START_ADDRStart of kernel code.
KERNEL_STACK_TOPTop of kernel stack. The kernel stack is one page long.Address of CGA console memory.PROC_START_ADDRhe kernel and all processes in a single, sharedaddress space. This address space is defined by the kernel_pagetable page table. kernel_pagetable isnitialized to the identity mapping: virtual address X maps to physical address X.2024/11/15 18:17CS202: Lab 4: WeensyOS5/19As you work through the lab, you will shift processes to using their own independent address spaces, whereach process can access only a subset of physical memory.The kernel, though, must remain able to access any location in physical memory. Therefore, all kernel unctionsrun using the kernel_pagetable page table. Thus, in kernel functions, each virtual address maps to the physicaladdress with the same number. The exception()function explicitly installs kernel_pagetable when it begins.WeensyOS system calls are more expensive than they need to be, since every system call switches adressspaces twice (once to kernel_pagetable and once back to the process’s page table). Real-world operatingsystems avoid this overhead. To do so, real-world kernelsaccess memory using process page tables, rather thana kernel-specific kernel_pagetable. This makes a kernel’s code more complicated, since kernels can’t alwaysaccess all of physical memory directly under that design.Using tmuxIt will be handy to be able to “see” multiple sessions within Docker at the same time. A good tool for this iscalled tmux.We suggest reading, and typing along with, this excellent tmux tutorial. It should take no more than 10
minutes and will be well worth it. Our debugging instructions below will assume that you have done so. Othertmux resources:MIT’s missing semester: Search for the section called “Terminal Multiplexers”.Cheatsheet: This is a more comprehensive list of commands, though the formatting is not the best, beinginterspersed with ads.If you find yourself needing to exit tmux, either exit all of the panes in the current window, or do: C-b :killsession<return>. (The C-b is the usual Ctrl-b, and then you typekill-session and press return or enter.) gdbThe debugger that we have seen, gdb, can be used to debug an already running process, even one over anetwork. QEMU supports this facility (see here). As a result, you can use gdb to single-step the software that is
on top of the emulated processor created by QEMU.
Here are the steps. These steps assume that (1) you have taken the 10 minutes to work through the tmuxarrow keyIn that terminal, invoke gdb. Do this via the script gdb-wrapper.sh, which will invoke the correct in that directory tells gdb about these things for you. Soyou type:$ ./gdb-wrapper.shMake sure you see:...The target architecture sset to "i386:x86-64".add symbol table from file "obj/bootsector.full" at.text_addr = 0x7c00add symbol table from file "obj/p-allocator.full" at.text_addr = 0x100000
add symbol table from file "obj/p-allocator2.full" at
text_addr = 0x140000
add symbol table from file "obj/p-allocator3.full" at.text_addr = 0x180000add symbol table from file "obj/p-allocator4.full" at
text_addr = 0x1c0000add symbol table from file "obj/p-fork.full" at.text_addr = 0x100000add symbol table from file "obj/p-forkexit.full" at.text_addr = 0x100000If youdonot see something likethat, then it means that you did not load the appropriate debugginginformation into gdb; likely, you are not running from withinthelab4directory.Now, set a breakpoint, for example at the function kernel() (or whatever function you want to break at):
(gdb) break kernelBreakpoint 1 at 0x40167: file kernel.c, line 86.Now run the “remote” software (really, the WeensyOS kernel in the left-hand pane). Do this by telling gdb tocontinue, via the c command:
gdb) cYou should see in the right-hand pane:2024/11/15 18:17CS202: Lab 4: WeensyOS7/19Continuing.Breakpoint 1, kernel (command=0x0) at kernel.c:8686 voidkernel(const char* command) {1: x/5i $pc=> 0x40167<kernel>:endbr640x4016b <kernel+4>: push %rbp0x4016c <kernel+5>: mov %rsp,%rbp
0x4016f <kernel+8>: sub $0x20,%rsp0x40173 <kernel+12>: mov %rdi,-0x18(%rbp)(gdb)You will also see the kernel begin to execute in the left-hand pane.Now, you can and should use the existing facilities of gdb to poke around. gdb understands the hardware verywell. So can, for example, ask it to print out the value of %cr3:(gdb) info cr3cr3 0x8000 [ PDBR=8 PCID=0 ]You are encouraged to use gdb’s facilities. Type help at the (gdb) prompt to get a menu.See the tmux section above for how to exit tmux.Step 1: Kernel isolationn the starting code we’ve given you, WeensyOS processes could stomp all over the kernel’s memory if theywanted to. Better preventthat. Change kernel(), thefunction,so that kernel memory isinaccessible to applications, except for the memory holding the CGA console (the single page at (uintptr_t)console == 0xB8000).When you are done, WeensyOS should look like the below. In the virtual map, kernel memory is no longerreverse-video, since the user can’t access it. Note the lonely CGA console memory block in reverse video in the
virtual address space.Hints:1
2024/11/15 18:17CS202: Lab 4: WeensyOS
8/19Use virtual_memory_map. A description of this function is in kernel.h. You will benefit from reading all the
function descriptions in kernel.h. You can supply NULL for the allocator argument for now.
If you really want to look at the code for virtual_memory_map, it is in k-hardware.c, along with many
other hardware-related functions.
The perm argument to virtual_memory_map is a bitwise-or of zero or more PTE flags, PTE_P, PTE_W, and
PTE_U. PTE_P marks Present pages (pages that are mapped). PTE_W marks Writable pages. PTE_U marks
User-accessible pages—pages accessible to applications. You want kernel memory to be mapped with
permissions PTE_P|PTE_W, which will prevent applications from reading or writing the memory, while
allowing the kernel to both read and write.
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.
When you're done with this step, make sure to commit and push your code!
Step 2: Isolated address spaces
Implement process isolation by giving each process its own independent page table. Your OS memory map
should look something like this when you’re done:
(Yours won’t look exactly like that; in the first line of physical and virtual memory, instead of having the pattern
R11223344, yours will probably have a pattern like R1111222233334444. This is because the gif is from a 32-bit
architecture; recall that on a 64-bit architecture, there are four levels of page table required.)
That is, each process only has permission to access its own pages. You can tell this because only its own pages
are shown in reverse video.
What goes in per-process page tables:
The initial mappings for addresses less than PROC_START_ADDR should be copied from those in
kernel_pagetable. You can use a loop with virtual_memory_lookup and virtual_memory_map to copy
them. Alternately, you can copy the mappings from the kernel’s page table into the new page tables; this
is faster, but make sure you copy the right data!
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CS202: Lab 4: WeensyOS
9/19The initial mappings for the user area—addresses greater than or equal to PROC_START_ADDR—should be
inaccessible to user processes (that is, PTE_U should not be set for these PTEs). In our solution (shown
above), these addresses are totally inaccessible (so they show as blank), but you can also change this so
that the mappings are still there, but accessible only to the kernel, as in this diagram:
The reverse video shows that this OS also implements process isolation correctly.
[Note: This second approach will pass the automated tests for step 2 but not for steps 3 and beyond. Thus, we
recommend taking the first approach, namely total inaccessibility.]
How to implement per-process page tables:
Change process_setup to create per-process page tables.
We suggest you write a copy_pagetable(x86_64_pagetable* pagetable, int8_t owner) function that
allocates and returns a new page table, initialized as a full copy of pagetable (including all mappings
from pagetable). This function will be useful in Step 5. In process_setup you can modify the page table
returned by copy_pagetable according to the requirements above. Your function can use pageinfo to find
free pages to use for page tables. Read about pageinfo at the top of kernel.c.
Remember that the x86-64 architecture uses four-level page tables.
The easiest way to copy page tables involves an allocator function suitable for passing to
virtual_memory_map.
You’ll need at least to allocate a level-1 page table and initialize it to zero. You can also set up the whole
four-level page table skeleton (for addresses 0…MEMSIZE_VIRTUAL - 1) yourself; then you don’t need an
allocator function.
A physical page is free if pageinfo[PAGENUMBER].refcount == 0. Look at the other code in kernel.c for
some hints on how to examine the pageinfo[] array.All of process P’s page table pages must have pageinfo[...].owner == P or WeensyOS’s consistencychecking functions will fail. This will affect your allocator function. (Hint: Don’t forget that global variablesare allowed in your code!)2024/11/15 18:17CS202: Lab 4: WeensyOS10/19If you create an incorrect page table, WeensyOS might crazily reboot. Don’t panic! Add log_printfstatements. Anotheruseful technique that may at first seem counterintuitive: add infinite loops to your kernel to track down exactly where a fault occurs. (If the OS hangs push!
Step 3: Virtual page allocationp to this point in the lab, WeensyOS processes have used physical page allocation: the page with physical address X is used to satisfy the sys_page_alloc(X) allocatiorequestfor virtual address X. This strategy isinflexible and limits utilization. Change the implementation of the INT_SYS_PAGE_ALLOC system call so that it
can use any free physical page to satisfy a sys_page_alloc(X) request.Your new INT_SYS_PAGE_ALLOC code must perform the following tasks.
Find a free physical page using the pageinfo[] array. Return -1 to the application if you can’t find one.Use any algorithm you like to find a free physical page; our solution just returns the first one we find.
Record the physical page’s allocation in pageinfo[].Map that physical page at the requested virtual address.
Don’t modify the assign_physical_page helper function, which is also used by the program loader. You canwrite a new function if you need to.Here’s how our OS looks after this step.Now commit and push your code before moving on to step 4!
Step 4: Overlapping address spacesNow the processes are isolated, which is awesome. But they’re still not taking full advantage of virtual memory.Isolated address spacescan use the same virtual addresses for different physical memory. There’s no need to the four process address spaces disjoint.
2024/11/15 18:17CS202: Lab 4: WeensyOS11/19In this step, change each process’s stack to start from address 0x300000 == MEMSIZE_VIRTUAL. Now the
after you’ve done it successfully:
Notice the single reverse video page in the bottom right, for all processes. This is their stack page: each process
has the same virtual address for its stack page, but (if you’ve implemented it correctly) different physical pages.
If there’s no physical memory available, sys_page_alloc should return an error to the caller (by returning -1).
Our solution additionally prints “Out of physical memory!” to the console when this happens; you don’t need
As always, make sure to commit and push after finishing this step!Step 5: ForkThe fork() system call is one of Unix’s great ideas. It starts a new process as a copy of an existing one. Thefork() system call appears to return twice, once to each process. To the child process, it returns 0. To theparent process, it returns the child’s process ID.
Run WeensyOS with make run or make run-console. At any time, press the ‘f’ key. This will soft-rebootWeensyOS and ask it to run a single process from the p-fork application, rather than the gang of allocatorprocesses. You should see something like this in the memory map:
2024/11/15 18:17CS202: Lab 4: WeensyOS
12/19That’s because you haven’t implemented fork() yet.How to implement fork():
When a process calls fork(), look for a free process slot in the processes[] array. Don’t use slot 0. If no
free slot exists, return -1 to the caller.If a free slot is found, make a copy of current->p_pagetable, the forking process’s page table, using your
function from earlier.But you must also copy the process data in every application page shared by the two processes. The
processes should not share any writable memory except the console (otherwise they wouldn’t be isolated).So fork() must examine every virtual address in the old page table. Whenever the parent process has anapplication-writable page at virtual address V, then fork() must allocate a new physical page P; copy thedata from the parent’s into P, using memcpy(); and finally map page P at address V in the childprocess’s page table. (memcpy() works like the one installed on your Linux dev box; use the man pages forreference.)child process’s registers are initialized as a copy of the parent process’s registers, except for reg_raxUse virtual_memory_lookup to query the mapping between virtual and physical addresses in a page
table.
When you’re done, you should see something like the below after pressing ‘f’.
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CS202: Lab 4: WeensyOS
13/19An image like the below, however, means that you forgot to copy the data for some pages, so the processes
are actually sharing stack and/or data pages when they should not:
Other hints.
Make sure you’re setting the owner correctly when allocating new page tables.ailing this step of the lab does not mean that the bug is actually in this step. It’s very common that a
student’s step 5 code fails because of errors made in any of the earlier stepsDon't forget to commit and push after finishing fork!
(Extra credit) Step 6: Shared read-only memory
This extra credit and the next are challenging—and the point values will not be commensurate to the extraeffort. We supply these for completeness, and for those who want to go deeper into the material.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 processisolation, as long as neither process could write the code.2024/11/15 18:17CS202: Lab 4: 14/19Step A: change the process loader in k-loader.c to detect read-only program segments and map them as
read-only for applications (PTE_P|PTE_U). A program segment ph is read-only iff (ph->p_flags &
ELF_PFLAG_WRITE) == 0.
Step B: From step 5, your fork() code already shouldn’t copy shareable pages. But make sure in this step that
your code keeps track accurately of the number of active references to each user page. Specifically, if
pageinfo[pn].refcount > 0 and pageinfo[pn].owner > 0, then pageinfo[pn].refcount should equal thenumber of times pn is mapped in process page tables.When you’re done, running p-fork should look like this:Hint:gain, commit and push!Extra credit) Step 7: Freeing memorySo far none of your test programs have ever freedmemory or exited. Memory allocation’s pretty easy untilyou add free! So let’s do that, by allowing applications to exit. In this exercise you’ll implement the sys_exit()system call, which exits the current process.This exercise is challenging: freeing memory will tend to expose weaknesses andproblems in your other code.To test your work, use make run and then type ‘e’. This reboots WeensyOSto run the p-forkexit program.(Initially it’ll crash because sys_exit() isn’t implemented yet.)p-forkexit combines two types of behavior:Process 1 forks children indefinitely.The child processes, #2 and up, are memory allocators, as in the previous parts of the lab. But with smallprobability at each step, each child process either exitsor attempts to fork a new child.The result is that once your code is correct, p-forkexit makes crazy patterns forever. An example:2024/11/15 18:17CS202: Lab 4: WeensyOS15/19Your picture might look a little different; for example, thanks to Step 6, your processes should share a codepage, whichwould appear as a darker-colored “1”.Here’s your task.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 pagesused for its pagingstructures.In p-forkexit, unlike in previous parts of the lab, sys_fork() can run when there isn’t quite enough
memory to create a new process. Your code should handle this case. If there isn’t enough free memory toallocate a process, fork() should clean up after itself (i.e., free anymemory that was allocated for the newprocess before memory ran out, including pages that were allocated as part of the paging structures), andthen return -1 to the caller. There should be no memory leaks.The check_virtual_memory function, which runs periodically, should help catch some errors. Feel free to addchecks of your own.Further study (extra-extra credit)If you are finished and can't wait to do more of this type of work, try the following. These will receive onlytoken points, and are for you to explore, if you’re interested:Copy-on-write page allocation!Faster system calls, for instance using the syscall and sysexit instructions!Running the grading testsAs stated at the start of this lab, the visual memory map displayed by QEMU as your WeensyOS kernel runs isthe best way to determine how your code is behaving.However, we provide automated tests, to help us grade, and for you to confirm that you’ve completed a step.The tests are not dispositive: there will be cases where yourcode passes the tests but is not ultimately correct(and will lose points on manual inspection during grading).We have not seen the reverse, however: caseswhere your code fails the tests but is correct. Thus, if the tests are failing, you almost certainly have a bug.2024/11/1518:17CS202: Lab 4: WeensyOS16/19The bottom line: run with make run or make run-console (or make run-gdb) to visualize how memory is beingused while you are coding and validating your design. Then, switch to the automated tests described belowwhen you think you’ve completed a step and want to double-check.There are five tests, one for each step. You can run each of them with the shell commands make grade-onethrough grade-five. Note that the step numbers are written out in text and not using digits. Each step’s resultis all-or-nothing.There are three invariants in all five steps’ tests that your code must satisfy. These invariants are:The CGA console must be accessible by all processes (thisrequirement isdiscussed in the text above onstep 1).If we consider process P, there should be no virtual page in P’s page table at a user-space address (that is,whose address is above the kernel/user virtual address split point) that is owned by P but not accessibleby P.When we run our tests, we configure the WeensyOS kernel to exit after 10 seconds of execution (1000WeensyOS kernel “ticks”). If a bug in your code makes the kernel crash before 1000 WeensyOS kernelticks, you’ll fail the test on which that happens. Alternatively, if your kernel enters an infinite loop, and thusnever reaches our exit at 1000 ticks, the VM will get stuck in the nfinite loop and never exit, so the testswill hang and you’ll need to terminate your hung kernel. Do so by opening another terminal window andissuing the command make kill, which will kill all QEMUprocesses you have running.These invariants are reasonable: regardless of what your memory map display lookslike, a good solutionshould neither crash nor enter an infinite loop.Our tests are cumulative: each step’s test runs all prior steps’ tests. If any of the prior steps’ tests fail, the“current” step’s test is deemed to have failed. As a consequence, if you have a regression bug—for example,code in a current step re-introduces a bug in an earlier ste—you can lose points not only for the current stepyou are working on, but also for prior steps. If you need tosubmit and find this has happened, don’t despair:simply revert your code to the last good version before your regression (using the history provided by GitHub—so, once again, make sure you commit and push often!).Usinggdb in “grading” modeThe make grade-X scripts pass slightly different options to QEMU than make run. If you find that you’re failingthe grading tests, you can run with the grading options, under gdb. To do so, follow the gdb instructionsabove, except where it says make run-gdb, type instead make grade-gdb.
Miscellaneous tipsThe kernel defines a constant, HZ, which determines how many times per second the kernel’s clock ticks. Don’tchange this value—there is absolutely no need to do so while solving the lab, and doing so will likely causeyour code to fail our tests!After you run any of our per-stage make grade-N tests, if you happen to examine the /tmp/log.txt file, you’llsee a vast amount of output therein that we generate for use in the automated tests. You can ignore it (and itwill be absent when you run with make run while you are developing, so it won’t clutter your own debuggingogprintf()s in those runs).
- Executing this checklist:Make sure your code builds, with no compiler warnings.Make sure you’ve used git add to add any files that you’ve created.Fill out the top of the answers.txt file, including your name and NYU IdMake sure you’ve answered every question in answers.txtake sure you have answered all code exercises in the files.reate a file called slack.txt noting how many slack days you have used for this assignment. (This isto help us agreeon the number that you have used.) Include this file even if you didn’t use any slackdays.Push your code to GitHub, so we have it (from outside thecontainer or, if on Mac, this will also workfrom within the container):7337116..ceed758 main -> main
- Actually submit, by timestamping and identifying your pushed code:Decide which git commit you want us to grade, and copy its id (you will paste it in the next sub-step).commit id is a 40-character hexadecimal string. Usually the commit id that you want will be the onethat you created last. The easiest way to obtain the commit id for thelast commit is by running thecommand git log -1 --format=oneline. This prints both the commit id and the initial line of thecopied.You can submit as many times as you want; we will grade the last commit id submitted to Brightspace.NOTE: Ground truth is what and when you submitted to Brightspace. Thus, a non-existent commit id in Brightspace means that you have not submitted the lab, regardlessof what you have pushed to GitHub. And, the time of your submission for the purposes of tracking lateness is the time when you upload the id to
Brightspace, not the time when you executed git commit.
This completes the lab. /11/15 18:17CS202: Lab 4: WeensyOS18/19AcknowledgmentsThis lab is due to Eddie Kohler, with modifications and some infrastructure due to Brad Karp and NikolGvozdiev.
- We also disable ALLOC SLOWDOWN in p-allocator.c and p-fork.c during our tests, so that memoryallocation proceeds much more quickly, at machine speed rather than human-vision speed. Thus 1000are plenty of time for the workload to run and exhibit how your virtual memory systembehaves.↩︎2024/11/15 18:17CS202: Lab 4: Wee