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Project 4 Solution: Implementing Virtual Memory
Project 4: Implementing Virtual Memory
Virtual memory is a powerful construction that allows programs to believe they have access to a larger
amount of memory resources than is present in the physical RAM on a computer. In this project you will
simulate a virtual memory system with paging and swapping.
Part 1
Your objective is to create a memory manager that takes instructions from simulated processes and
modifies physical memory accordingly. In part 1 of this project, you must implement the basics of paging,
including a per-process page table, address translation, and support for multiple processes residing in memory
concurrently.
Memory Implementation. To keep the implementation simple, you will simulate memory as an array
of bytes, e.g., unsigned char memory[SIZE]). It is the responsibility of your memory manager to logically
partition those bytes into pages, perform translations from virtual to physical addresses, load and store
values in memory, and update the page tables. Hint: given our simple representation of memory, a physical
address is simply an index into the memory array.
Page Table Implementation. You are free to use whatever format you would like for the page table and
its entries (hint: the OSTEP book has plenty of good examples). However, each process’s page table
must be stored in the memory array. That is, each page table will require a page of physical memory.
Further, we’ve set the virtual address space size such that the page table should require at most a single
page of memory.
Input. Instructions will be supplied to your memory manager via stdin and will always a 4-tuple in the
format described below:
process_id,instruction_type,virtual_address,value
The process id is an integer value in the range [0, 3] used to simulate instructions from different processes. The instruction type specifies the desired memory operation—see the instruction list below. The
virtual address is an integer value in the range [0, 63] specifying the virtual memory location for given
process. The meaning of value depends on the instruction, but must be an integer value in the range [0, 255].
Instructions. Your memory manager must support the following instructions:
• map tells the memory manager to allocate a physical page, i.e., it creates a mapping in the page table
between a virtual and physical address. The manager must determine the appropriate virtual page
number using the virtual address. For example, virtual address of 3 corresponds to virtual page
0. value argument represents the write permission for the page. If value=1 then the page is writeable
and readable. If value=0, then the page is only readable, i.e., all mapped pages are readable. These
permissions can be modified by using a second map instruction for the target page.
1• store instructs the memory manager to write the supplied value into the physical memory location
associated with the provided virtual address, performing translation and page swapping as necessary.
Note, page swapping is a requirement for part 2 only.
• load instructs the memory manager to return the byte stored at the memory location specified by
virtual address. Like the store instruction, it is the memory manager’s responsibility to translate
and swap pages as needed. Note, the value parameter is not used for this instruction, but a dummy
value (e.g., 0) should always be provided.
Output. The memory manager should be output the results of its actions by printing messages to stdout.
See below for examples of printed output. If the process attempts an illegal instruction (e.g., writing to a
read-only page) or provides invalid arguments, the manager should print a warning and ignore the instruction.
Simulation parameters. You may make the following simplifying assumptions.
• Physical memory consists of 64 bytes divided into pages of 16 bytes. Likewise, the virtual address
space is also 64 bytes.
• For the purposes of this simulation, a process is simply a PID provided in the input command. Each
process is given its own, isolated, virtual address space.
• Your memory manager should be single-threaded.
• All pages are readable, but only some are writeable—as specified by the value argument for the map
instruction. We do not consider other protection bits for this project (e.g., executable).
• The there can be at most 4 different processes issuing instructions. These processes will have PIDs in
the range [0, 3].
• The manager will create page tables on-demand. That is, the manager will create the page table for a
process when it receives the first command for that process. Each page table will be placed in its own
page in the simulated memory array.
• The manager will simply print an error if it cannot allocate a physical page to satisfy a request. In
part 2 of the project, you will relax this assumption using swapping.
• Each process will have a simulated hardware register pointing to the start of their respective page
tables. You can simulate these registers with an array indexed by process id. This array does not
need to be stored in your simulated physical memory array.
• You may store limited information outside of your simulated physical memory array, e.g., local variables,
a free list, and the page table registers. Yes, technically, these objects could also be put in the simulated
memory, but it would complicate the implementation. However, as mentioned above, all page tables
must be placed in the simulated physical memory as well as all loads and stores from the
simulated processes.
Example output. An example run of the memory manager might look like the following. Note, Instruction?
is the prompt printed by the manager.
Instruction? 0,map,0,1
Put page table for PID 0 into physical frame 0
Mapped virtual address 0 (page 0) into physical frame 1
Instruction? 0,store,12,24
Stored value 24 at virtual address 12 (physical address 28)
Instruction? 0,load,12,0
The value 24 is virtual address 12 (physical address 28)
2Part 2
In part 2, you will extend your memory manager to support swapping memory pages to disk. The manager
must ensure that the address spaces remain isolated. That is, memory from one process should not be
readable from another process. The manager is expected to swap out page tables as necessary to free up
memory. Hint: things get interesting when you swap out page table frames, especially if it is the page table
for the process making the request.
Specifically, when the manager cannot satisfy a request because it does not have a free page it will do
the following.
• First, the manager will pick a page to evict. You get to pick the eviction strategy, but a simple roundrobin should be sufficient. Note, we do not consider “always evict page 1” to be an acceptable eviction
strategy. The manager must be able to swap out all pages, even those containing a page table.
• Second, the manager will write the evicted page to disk. The swap space will be modeled by reading
from and writing to a file. You can assume your swap space is very large.
• Third, the manager will update the appropriate page table to record the swap-file location for the
evicted page. Hint: you need a way to specify in the page table whether the page is in memory or
swapped to disk.
• Finally, the manager will use the newly-freed page to satisfy the request.
Example output. An example run of the memory manager might look like the following. Note, we
manually added annotations to help clarify the example.
Instruction? 0,map,0,0
Put page table for PID 0 into physical frame 0
Mapped virtual address 0 (page 0) into physical frame 1
#this should error
Instruction? 0,store,7,255
Error: writes are not allowed to this page
#this should update the permissions
Instruction? 0,map,0,1
Updating permissions for virtual page 0 (frame 1)
Instruction? 0,store,7,255
Stored value 255 at virtual address 7 (physical address 23)
Instruction? 0,load,7,0
The value 255 is virtual address 7 (physical address 23)
#this should print an error
Instruction? 0,map,10,1
Error: virtual page 0 is already mapped with rw_bit=1
#let’s map a couple other pages
Instruction? 0,map,16,1
Mapped virtual address 16 (page 1) into physical frame 2
Instruction? 0,map,32,1
Mapped virtual address 32 (page 2) into physical frame 3
#Our physical memory should be full at this point, now we need to swap
Instruction? 1,map,0,0
Swapped frame 2 to disk at swap slot 0
Put page table for PID 1 into physical frame 2
Swapped frame 1 to disk at swap slot 1
Mapped virtual address 0 (page 0) into physical frame 1
Instruction? 0,load,7,0
3Swapped frame 3 to disk at swap slot 2
Swapped disk slot 1 into frame 3
The value 10 is virtual address 7 (physical address 55)
Instruction? End of file
Checkpoint Contributions
Students must submit work that demonstrates substantial progress towards completing the project on the
checkpoint date. Substantial progress is judged at the discretion of the grader to allow students flexibility
in prioritizing their efforts. For this project, substantial progress might be defined as supporting the map,
load, store commands for a single simulated process. Projects that fail to submit a checkpoint
demonstrating significant progress will incur a 10% penalty during final project grading.
Deliverables and Grading
When submitting your project, please include the following:
• The source code for the memory manager,
• a set of testing files demonstrating the correct operation of your manager,
• output from your tests,
• a Makefile that compiles your code, and
• a document called README.txt explaining your project and anything that you feel the instructor
should know when grading the project. Only plaintext write-ups are accepted.
Please compress all the files together as a single .zip archive for submission. As with all projects, please
only standard zip files for compression; .rar, .7z, and other custom file formats will not be accepted.
The project programming is only a portion of the project. Students should use the following checklist in
turning in their projects to avoid forgetting any deliverables:
1. Sign up for a project partner or have one assigned (URL: https://ia.wpi.edu/cs3013/request_
teammate.php),
2. Submit the project code and documentation via InstructAssist (URL: https://ia.wpi.edu/cs3013/
files.php),
3. Complete your Partner Evaluation (URL: https://ia.wpi.edu/cs3013/evals.php), and
4. Schedule your Project Demonstration (URL: https://ia.wpi.edu/cs3013/demos.php), which may
be posted slightly after the submission deadline.
A grading rubric has been provided at the end of this specification to give you a guide for how the project
will be graded. No points can be earned for a task that has a prerequisite unless that prerequisite is working
well enough to support the dependent task. Students will receive a scanned markup of this rubric as part of
their project grading feedback.
Groups must schedule an appointment to demonstrate their project to the teaching assistants. Groups
that fail to demonstrate their project will not receive credit for the project. If a group member fails to attend
his or her scheduled demonstration time slot, he or she will receive a 10 point reduction on his or her project
grade.
During the demonstrations, the TAs will be evaluating the contributions of group members. We will use
this evaluation, along with partner evaluations, to determine contributions. If contributions are not equal,
under-contributing students may be penalized.
4Project 4 – Virtual Memory – Grading Sheet/Rubric
Evaluation?
Grader: Student
Name:
Date/Time: Student
Name:
Team ID: Student
Name:
Late?:
Checkpoint?: Project Score:
Earned Weight Task ID Description
_____ 5% 0 Correct parsing of input instructions and parameters.
_____ 20% 1 Correct implementation of the map, load, and store commands.
_____ 25% 2 Correct implementation of the page table data structure, and supporting
functions, which stores all the needed information for paging.
_____ 25% 3 Pages are correctly swapped to and from disk. Manager handles any
cascading faults or evictions that may result.
_____ 15% 4 Memory manager supports requests from multiple concurrent processes.
_____ 5% 5 Appropriate user testing methodology with example input files. Should
include scenarios that result in page faults and scenarios with illegal
instructions.
_____ 5% 6 Appropriately verbose output (via stdout) that demonstrates the operation
of the implemented memory manager.
Grader Notes:
Virtual memory is a powerful construction that allows programs to believe they have access to a larger
amount of memory resources than is present in the physical RAM on a computer. In this project you will
simulate a virtual memory system with paging and swapping.
Part 1
Your objective is to create a memory manager that takes instructions from simulated processes and
modifies physical memory accordingly. In part 1 of this project, you must implement the basics of paging,
including a per-process page table, address translation, and support for multiple processes residing in memory
concurrently.
Memory Implementation. To keep the implementation simple, you will simulate memory as an array
of bytes, e.g., unsigned char memory[SIZE]). It is the responsibility of your memory manager to logically
partition those bytes into pages, perform translations from virtual to physical addresses, load and store
values in memory, and update the page tables. Hint: given our simple representation of memory, a physical
address is simply an index into the memory array.
Page Table Implementation. You are free to use whatever format you would like for the page table and
its entries (hint: the OSTEP book has plenty of good examples). However, each process’s page table
must be stored in the memory array. That is, each page table will require a page of physical memory.
Further, we’ve set the virtual address space size such that the page table should require at most a single
page of memory.
Input. Instructions will be supplied to your memory manager via stdin and will always a 4-tuple in the
format described below:
process_id,instruction_type,virtual_address,value
The process id is an integer value in the range [0, 3] used to simulate instructions from different processes. The instruction type specifies the desired memory operation—see the instruction list below. The
virtual address is an integer value in the range [0, 63] specifying the virtual memory location for given
process. The meaning of value depends on the instruction, but must be an integer value in the range [0, 255].
Instructions. Your memory manager must support the following instructions:
• map tells the memory manager to allocate a physical page, i.e., it creates a mapping in the page table
between a virtual and physical address. The manager must determine the appropriate virtual page
number using the virtual address. For example, virtual address of 3 corresponds to virtual page
0. value argument represents the write permission for the page. If value=1 then the page is writeable
and readable. If value=0, then the page is only readable, i.e., all mapped pages are readable. These
permissions can be modified by using a second map instruction for the target page.
1• store instructs the memory manager to write the supplied value into the physical memory location
associated with the provided virtual address, performing translation and page swapping as necessary.
Note, page swapping is a requirement for part 2 only.
• load instructs the memory manager to return the byte stored at the memory location specified by
virtual address. Like the store instruction, it is the memory manager’s responsibility to translate
and swap pages as needed. Note, the value parameter is not used for this instruction, but a dummy
value (e.g., 0) should always be provided.
Output. The memory manager should be output the results of its actions by printing messages to stdout.
See below for examples of printed output. If the process attempts an illegal instruction (e.g., writing to a
read-only page) or provides invalid arguments, the manager should print a warning and ignore the instruction.
Simulation parameters. You may make the following simplifying assumptions.
• Physical memory consists of 64 bytes divided into pages of 16 bytes. Likewise, the virtual address
space is also 64 bytes.
• For the purposes of this simulation, a process is simply a PID provided in the input command. Each
process is given its own, isolated, virtual address space.
• Your memory manager should be single-threaded.
• All pages are readable, but only some are writeable—as specified by the value argument for the map
instruction. We do not consider other protection bits for this project (e.g., executable).
• The there can be at most 4 different processes issuing instructions. These processes will have PIDs in
the range [0, 3].
• The manager will create page tables on-demand. That is, the manager will create the page table for a
process when it receives the first command for that process. Each page table will be placed in its own
page in the simulated memory array.
• The manager will simply print an error if it cannot allocate a physical page to satisfy a request. In
part 2 of the project, you will relax this assumption using swapping.
• Each process will have a simulated hardware register pointing to the start of their respective page
tables. You can simulate these registers with an array indexed by process id. This array does not
need to be stored in your simulated physical memory array.
• You may store limited information outside of your simulated physical memory array, e.g., local variables,
a free list, and the page table registers. Yes, technically, these objects could also be put in the simulated
memory, but it would complicate the implementation. However, as mentioned above, all page tables
must be placed in the simulated physical memory as well as all loads and stores from the
simulated processes.
Example output. An example run of the memory manager might look like the following. Note, Instruction?
is the prompt printed by the manager.
Instruction? 0,map,0,1
Put page table for PID 0 into physical frame 0
Mapped virtual address 0 (page 0) into physical frame 1
Instruction? 0,store,12,24
Stored value 24 at virtual address 12 (physical address 28)
Instruction? 0,load,12,0
The value 24 is virtual address 12 (physical address 28)
2Part 2
In part 2, you will extend your memory manager to support swapping memory pages to disk. The manager
must ensure that the address spaces remain isolated. That is, memory from one process should not be
readable from another process. The manager is expected to swap out page tables as necessary to free up
memory. Hint: things get interesting when you swap out page table frames, especially if it is the page table
for the process making the request.
Specifically, when the manager cannot satisfy a request because it does not have a free page it will do
the following.
• First, the manager will pick a page to evict. You get to pick the eviction strategy, but a simple roundrobin should be sufficient. Note, we do not consider “always evict page 1” to be an acceptable eviction
strategy. The manager must be able to swap out all pages, even those containing a page table.
• Second, the manager will write the evicted page to disk. The swap space will be modeled by reading
from and writing to a file. You can assume your swap space is very large.
• Third, the manager will update the appropriate page table to record the swap-file location for the
evicted page. Hint: you need a way to specify in the page table whether the page is in memory or
swapped to disk.
• Finally, the manager will use the newly-freed page to satisfy the request.
Example output. An example run of the memory manager might look like the following. Note, we
manually added annotations to help clarify the example.
Instruction? 0,map,0,0
Put page table for PID 0 into physical frame 0
Mapped virtual address 0 (page 0) into physical frame 1
#this should error
Instruction? 0,store,7,255
Error: writes are not allowed to this page
#this should update the permissions
Instruction? 0,map,0,1
Updating permissions for virtual page 0 (frame 1)
Instruction? 0,store,7,255
Stored value 255 at virtual address 7 (physical address 23)
Instruction? 0,load,7,0
The value 255 is virtual address 7 (physical address 23)
#this should print an error
Instruction? 0,map,10,1
Error: virtual page 0 is already mapped with rw_bit=1
#let’s map a couple other pages
Instruction? 0,map,16,1
Mapped virtual address 16 (page 1) into physical frame 2
Instruction? 0,map,32,1
Mapped virtual address 32 (page 2) into physical frame 3
#Our physical memory should be full at this point, now we need to swap
Instruction? 1,map,0,0
Swapped frame 2 to disk at swap slot 0
Put page table for PID 1 into physical frame 2
Swapped frame 1 to disk at swap slot 1
Mapped virtual address 0 (page 0) into physical frame 1
Instruction? 0,load,7,0
3Swapped frame 3 to disk at swap slot 2
Swapped disk slot 1 into frame 3
The value 10 is virtual address 7 (physical address 55)
Instruction? End of file
Checkpoint Contributions
Students must submit work that demonstrates substantial progress towards completing the project on the
checkpoint date. Substantial progress is judged at the discretion of the grader to allow students flexibility
in prioritizing their efforts. For this project, substantial progress might be defined as supporting the map,
load, store commands for a single simulated process. Projects that fail to submit a checkpoint
demonstrating significant progress will incur a 10% penalty during final project grading.
Deliverables and Grading
When submitting your project, please include the following:
• The source code for the memory manager,
• a set of testing files demonstrating the correct operation of your manager,
• output from your tests,
• a Makefile that compiles your code, and
• a document called README.txt explaining your project and anything that you feel the instructor
should know when grading the project. Only plaintext write-ups are accepted.
Please compress all the files together as a single .zip archive for submission. As with all projects, please
only standard zip files for compression; .rar, .7z, and other custom file formats will not be accepted.
The project programming is only a portion of the project. Students should use the following checklist in
turning in their projects to avoid forgetting any deliverables:
1. Sign up for a project partner or have one assigned (URL: https://ia.wpi.edu/cs3013/request_
teammate.php),
2. Submit the project code and documentation via InstructAssist (URL: https://ia.wpi.edu/cs3013/
files.php),
3. Complete your Partner Evaluation (URL: https://ia.wpi.edu/cs3013/evals.php), and
4. Schedule your Project Demonstration (URL: https://ia.wpi.edu/cs3013/demos.php), which may
be posted slightly after the submission deadline.
A grading rubric has been provided at the end of this specification to give you a guide for how the project
will be graded. No points can be earned for a task that has a prerequisite unless that prerequisite is working
well enough to support the dependent task. Students will receive a scanned markup of this rubric as part of
their project grading feedback.
Groups must schedule an appointment to demonstrate their project to the teaching assistants. Groups
that fail to demonstrate their project will not receive credit for the project. If a group member fails to attend
his or her scheduled demonstration time slot, he or she will receive a 10 point reduction on his or her project
grade.
During the demonstrations, the TAs will be evaluating the contributions of group members. We will use
this evaluation, along with partner evaluations, to determine contributions. If contributions are not equal,
under-contributing students may be penalized.
4Project 4 – Virtual Memory – Grading Sheet/Rubric
Evaluation?
Grader: Student
Name:
Date/Time: Student
Name:
Team ID: Student
Name:
Late?:
Checkpoint?: Project Score:
Earned Weight Task ID Description
_____ 5% 0 Correct parsing of input instructions and parameters.
_____ 20% 1 Correct implementation of the map, load, and store commands.
_____ 25% 2 Correct implementation of the page table data structure, and supporting
functions, which stores all the needed information for paging.
_____ 25% 3 Pages are correctly swapped to and from disk. Manager handles any
cascading faults or evictions that may result.
_____ 15% 4 Memory manager supports requests from multiple concurrent processes.
_____ 5% 5 Appropriate user testing methodology with example input files. Should
include scenarios that result in page faults and scenarios with illegal
instructions.
_____ 5% 6 Appropriately verbose output (via stdout) that demonstrates the operation
of the implemented memory manager.
Grader Notes:
1 file (247.6KB)
