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ASST3: Virtual Memory
Table of Contents
Checklist
Due Dates and Mark Distribution
Introduction
The System/161 TLB
The System/161 Virtual Address Space Map
Setting Up Assignment 3
Configure OS/161 for Assignment 3
Building for ASST3
Coding Assignment
Address Space Management
Address Translation
Testing and Debugging Your Assignment
Hints
FAQ, Gotchas and Video
Basic Assignment Submission
Advanced Assignment
Advanced Assignment Submission
Checklist
Read this spec.
Set up the assignment.
Watch the assignment overview video.
Do the week 9 tutorial exercises yourself.
Attend the tutorial.
Read the FAQ for assignment 3 on the wiki.
Do the assignment.
Don't forget to commit your most recent changes.
Submit your assignment.
Due Dates and Mark Distribution
Due Date & Time: 16:00 Friday 22nd April
Update: There is a 48hr grace period before applying any late penalty. The deadline remains the same, but
the late penalty is zero for submissions less than 48 hours late.
Marks: The base assignment is worth 30 marks (of the 100 available for the class mark component of the
course)
The 2% per day bonus for each day early applies, capped at 10%, as per course outline.
Students can do the advanced part with the permission of the lecturer, and only if basic assignment is
completed a week prior to the deadline. Obtained bonus marks can make up for any shortfall in the class
mark component, up to a maximum of 5 marks for this assignment.
Update: A substantial submission of the basic assignment 5 days prior to the deadline will qualify for the
advanced part. No need to explicitly ask for permission, we'll run some basic tests of the submissions and
record the permission for you.
There are familiarisation questions in your week 9 tutorial. Please answer the questions and bring them to
your tutorial. Feel free to ask any assignment related questions need to clarify your understanding.
Introduction
In this assignment you will implement the virtual memory sub-system of OS/161. The existing VM
implementation in OS/161, dumbvm, is a minimal implementation with a number of shortcomings. In this
assignment you will adapt OS/161 to take full advantage of the simulated hardware by implementing
address space management, page table management, and management of the MIPS software-managed
Translation Lookaside Buffer (TLB).
The System/161 TLB
This section provides a summary of the R3000 (System/161) virtual memory mechanisms. Further info is
provided in the lectures and in the R3000 Reference Manual and Hardware Guide on the course website.
The R3000 TLB entry includes a 20-bit virtual page number and a 20-bit physical frame number as well as
the following five fields:
global: 1 bit; if set, ignore the PID bits in the TLB.
valid: 1 bit; set if the TLB entry contains a valid translation.
dirty: 1 bit; enables writing to the page referenced by the entry; if this bit is 0, the page is only
accessible for reading.
nocache: 1 bit; unused in System/161. In a real processor, indicates that the hardware cache will be
disabled when accessing this page.
asid: 6 bits; a context or address space ID that can be used to allow entries to remain in the TLB after
a context switch.
All these bits/values are maintained by the operating system (i.e. your code). When the valid bit is set, the
TLB entry contains a valid translation. This implies that the virtual page is present in physical memory. A
TLB miss occurs when no TLB entry can be found with a matching virtual page and address space ID
(unless the global bit is set in which case the address space ID is ignored) and a valid bit that is set.
For this assignment, you may largely ignore the ASID field and set it to zero in your TLB entries. Note: In
OS/161, as_activate() is called whenever a new address space becomes active in the TLB, so
as_activate() is typically programmed to flush the TLB (why?).
The System/161 Virtual Address Space Map
The MIPS divides its address space into several regions that have hardwired properties. These are:
kseg2, TLB-mapped cacheable kernel space
kseg1, direct-mapped uncached kernel space
kseg0, direct-mapped cached kernel space
kuseg, TLB-mapped cacheable user space
Both direct-mapped segments map to the first 512 megabytes of the physical address space.
The top of kuseg is 0x80000000. The top of kseg0 is 0xa0000000, and the top of kseg1 is 0xc0000000.
The memory map thus looks like this:
Address Segment Special Properties
0xffffffff kseg2
0xc0000000
0xbfffffff kseg1
0xbfc00180 Exception address if BEV set.
0xbfc00100 UTLB exception address if BEV set.
0xbfc00000 Execution begins here after processor reset.
0xa0000000
0x9fffffff kseg0
0x80000080 Exception address if BEV not set.
0x80000000 UTLB exception address if BEV not set.
0x80000000
0x7fffffff kuseg
0x00000000
Setting Up Assignment 3
We assume after ASST0, ASST1, and ASST2 that you now have some familiarity with setting up for
OS/161 development. If you need more detail, refer back to ASST0.
Clone the ASST3 source repository from gitlab.cse.unsw.edu.au. Note: replace XXX with your 3 digit group
number.
Note: The gitlab repository is shared between you and your partner. You can both push and pull changes to
and from the repository to cooperate on the assignment.
Configure OS/161 for Assignment 3
Remember to set your PATH environment variable as in previous assignments (or run the 3231 command).
Before proceeding further, configure your new sources, and build and install the user-level libraries and
binaries.
% cd ~/cs3231/asst3-src
% ./configure
% bmake
% bmake install
You have to reconfigure your kernel before you can use the framework provided to do this assignment. The
procedure for configuring a kernel is the same as before, except you will use the ASST3 configuration file:
% cd ~/cs3231/asst3-src/kern/conf
% ./config ASST3
You should now see an ASST3 directory in the compile directory.
Building for ASST3
When you built OS/161 for ASST0, you ran bmake from compile/ASST0. When you built for ASST1, you
ran bmake from compile/ASST1 ... you can probably see where this is heading:
% cd ../compile/ASST3
% bmake depend
% bmake
% bmake install
If you now run the kernel as you did for previous assignments, you should get to the menu prompt. If you
try and run a program, it will fail with a message about an unimplemented feature (the failure is due to the
unimplemented as_* functions that you must write). For example, run p /bin/true at the OS/161 prompt to
run the program /bin/true in ~/cs3231/root.
OS/161 kernel [? for menu]: p /bin/true
Running program /bin/true failed: Function not implemented
Program (pid 2) exited with status 1
Operation took 0.173469806 seconds
OS/161 kernel [? for menu]:
Note: If you don't have a sys161.conf file, you can use the one from ASST1.
Coding Assignment
This assignment involves designing and implementing a number of data-structures and the functions that
manipulate them. Before you start, you should work out what data you need to keep track of, and what
operations are required.
Address Space Management
OS/161 has an address space data type that encapsulates the book-keeping needed to describe an address
space: the struct addrspace. To enable OS/161 to interact with your VM implementation, you will need to
implement the functions in kern/vm/addrspace.c and potentialy modify the data type. The semantics of these
functions is documented in kern/include/addrspace.h.
Note: You may use a fixed-size stack region (say 16 pages) for each process.
Address Translation
The main goal for this assignment is to provide virtual memory translation for user programs. To do this,
you will need add a extend OS/161 address spaces with a page table, and implement a TLB refill handler for
the page table.
For this assignment, you will implement a 2-level hierarchical page table. The first level of the page table
is to be indexed using the 11 most significant bits of the page number, the second-level nodes of the page
tabe are to be indexed using the 9 least significant bits of the page number. Thus the first-level node will
have 2048 (2^11) entries and the second-level nodes will have 512 (2^9) entries.
Note that a hierarchical page table is a lazy data-structure. This means that the contents of the page table,
including the second-level nodes in the hierarchy, are only allocated when they are needed. You may find
allocating the required pages at load time helps you start your assignment, however, your final solution
should allocate pages (and level-2 nodes if needed) only when a page-fault occurs.
The following questions may assist you in designing the contents of your page table
What information do you need to store for each page?
How does the page table get populated?
Is the data structure global or a per-process data structure?
Note: Applications expect pages to contain zeros when first used. This implies that newly allocated
frames that are used to back pages should be zero-filled prior to mapping
Testing and Debugging Your Assignment
To test this assignment, you should run a process that requires more virtual memory than the TLB can map
at any one time. You should also ensure that touching memory not in a valid region will raise an exception.
The huge and faulter tests in testbin may be useful. See the Wiki for more options.
Apart from GDB, you may also find the trace161 command useful. trace161 will run the simulator with
tracing, for example
% trace161 -t t -f outfile kernel
will record all TLB accesses in outfile.
Don't use kprintf() for vm_fault() debugging. See the Wiki for more info.
Hints
One approach to implementing the assignment is in the following order:
Review how the specified page table works from the lectures, and understand its relationship with the
TLB.
Review the assignment specification and its relationship with the supplied code.
dumbvm is not longer compiled into the OS/161 kernel for this assignment
(kern/arch/mips/vm/dumbvm.c), but you can review it as an example implementation within the
interface/framework you will be working within.
The only candidate for code re-use is the TLB flush in as_activate().
Note: Your implementation of TLB refill in vm_fault() should use tlb_random().
Work out a basic design for your page table implementation.
Modify kern/vm/vm.c to insert , lookup, and update page table entries in your page table structure.
Implement the TLB exception handler vm_fault() in vm.c to refill the TLB and keep it consistent with
your page table.
Implement the functions in kern/vm/addrspace.c that are required for basic functionality (e.g.
as_create(), as_prepare_load(), etc.). Allocating user pages in as_define_region() may also
simplify your assignment, however good solution allocate pages in vm_fault().
E.g. as_create() should initialise your page table, as_destroy() should clean it up.
Test and debug this. Use the debugger or trace161!
Note: Interrupts should be disabled when writing to the TLB, see dumbvm for an example. Otherwise,
unexpected concurrency issues can occur.
as_activate() and as_deactivate() can be copied from dumbvm.
FAQ, Gotchas and Video
Basic Assignment Submission
The submission instructions are available on the Wiki. Like previous assignments, you will be submitting
the git repository bundle via CSE's give system. For ASST3, the submission system will do a test build and
run a simple test to confirm your bundle at least compiles.
Warning! Don't ignore the submission system! If your submission fails the submission process, you
may not receive any marks.
Warning! Don't forget to commit your changes prior to generating your bundle.
To submit your bundle:
% cd ~
% give cs3231 asst3 asst3.bundle
You're now done.
Even though the generated bundle should represent all the changes you have made to the supplied code,
occasionally students do something "ingenious". So always keep your git repository so that you may recover
your assignment should something go wrong.
Advanced Assignment
The advanced assignment consists of a student-chosen subset of the problems below. The total marks
available are capped at 5 marks.
Students can do the advanced part with the permission of the lecturer, and only if basic assignment is
completed a week prior to the deadline.
(easy) 2 marks Shared pages and copy-on-write.
(easy) 2 marks Implement sbrk() to enable user-level malloc() to function with more memory than its
initially allocated pool.
(hard) 3 marks.
Implement a simplified mmap() and munmap(). Note: you only need to support the simplified case
of mapping a file, and munmap() the entire region that was mapped.
The prototypes are expected to be
void *mmap(size_t length, int prot, int fd, off_t offset);
int munmap(void *addr);
Where prot can be PROT_READ and/or PROT_WRITE. Compared to traditional mmap, there are no
flags, and the OS chooses the virtual address to locate the region. You must ensure that
applications can open a file, updated it, and have the updated file propagate to the filesystem.
And, implement demand-loading. You should load pages only when they are referenced by the
user process, as opposed to at process creation.
(seriously hard) 5 marks Implement paging. You should implement some page replacement algorithm
and demonstrate your solution running under memory pressure.
Given you're doing the advanced version of the assignment, I'm assuming you are competent with managing
your git repository and don't need detailed directions. We expect you to work on a specific branch in your
repository to both build upon your existing assignment, while keeping your advanced assignment separate at
the same time.
Here are some git commands that will be helpful.
One member of your group should create the branch and push it back to gitlab.
git checkout -b asst3_adv
git push --set-upstream origin asst3_adv
To switch back to the basic assignment at some point.
git checkout master
To switch to the advanced assignment at another point.
git checkout asst3_adv
Advanced Assignment Submission
Submission for the advanced assignment is similar to the basic assignment, except the advance component
is given to a separate assignment name: asst3_adv. Again, you need to generate a bundle based on your
repository. Note: Our marking scripts will switch to the asst3_adv branch prior to testing the advanced
assignment.
Submit your solution
% cd ~
% give cs3231 asst3_adv asst3_adv.bundle
You're now done.