Chapter 9 Virtual Memory

458 Chapter 9 Chapter 10 in 10E Virtual Memory
Programming Projects
Designing a Virtual Memory Manager
This project consists of writing a program that translates logical to physical
addresses for a virtual address space of size 216 = 65,536 bytes. Your program
will read from a file containing logical addresses and, using a TLB as well as
a page table, will translate each logical address to its corresponding physical
address and output the value of the byte stored at the translated physical
address. The goal behind this project is to simulate the steps involved in
translating logical to physical addresses.
Specifics
Your program will read a file containing several 32-bit integer numbers that
represent logical addresses. However, you need only be concerned with 16-bit
addresses, so you must mask the rightmost 16 bits of each logical address.
These 16 bits are divided into (1) an 8-bit page number and (2) 8-bit page offset.
Hence, the addresses are structured as shown in Figure 9.33.
Other specifics include the following:
• 28 entries in the page table
• Page size of 28 bytes
• 16 entries in the TLB
• Frame size of 28 bytes
• 256 frames
• Physical memory of 65,536 bytes (256 frames × 256-byte frame size)
Additionally, your program need only be concerned with reading logical
addresses and translating them to their corresponding physical addresses. You
do not need to support writing to the logical address space.

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Address Translation
Your program will translate logical to physical addresses using a TLB and page
table as outlined in Section 9.3. First, the page number is extracted from the
logical address, and the TLB is consulted. In the case of a TLB-hit, the frame
number is obtained from the TLB. In the case of a TLB-miss, the page table
must be consulted. In the latter case, either the frame number is obtained
offset
31 1516 78 0
page
number
Figure 9.33 Address structure.
Programming Projects 459
page
number
0
1
2
15
0
1
2
255
TLB
page
table
TLB hit
TLB miss
page 0
page 255
page 1
page 2
frame
number
.
.
.
.
.
.
.
.
0
1
2
255
physical
memory
frame 0
frame 255
frame 1
frame 2
.
.
.
.
page
number offset
frame
number offset
Figure 9.34 A representation of the address-translation process.
from the page table or a page fault occurs. A visual representation of the
address-translation process appears in Figure 9.34.
Handling Page Faults
Your program will implement demand paging as described in Section 10.2. The
backing store is represented by the file BACKING STORE.bin, a binary file of size
65,536 bytes.When a page fault occurs, you willread in a 256-byte page from the
file BACKING STORE and store it in an available page frame in physical memory.
For example, if a logical address with page number 15 resulted in a page fault,
your program would read in page 15 from BACKING STORE (remember that
pages begin at 0 and are 256 bytes in size) and store it in a page frame in
physical memory. Once this frame is stored (and the page table and TLB are
updated), subsequent accesses to page 15 will be resolved by either the TLB or
the page table.

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You will need to treat BACKING STORE.bin as a random-access file so that
you can randomly seek to certain positions of the file for reading. We suggest
using the standard C library functions for performing I/O, including fopen(),
fread(), fseek(), and fclose().
The size of physical memory is the same as the size of the virtual
address space—65,536 bytes—so you do not need to be concerned about
page replacements during a page fault. Later, we describe a modification
to this project using a smaller amount of physical memory; at that point, a
page-replacement strategy will be required.
460 Chapter 9 Virtual Memory
Test File
We provide the file addresses.txt, which contains integer values representing logical addresses ranging from 0 − 65535 (the size of the virtual address
space). Your program will open this file,read each logical address and translate
it to its corresponding physical address, and output the value of the signed byte
at the physical address.
How to Begin
First, write a simple program that extracts the page number and offset (based
on Figure 9.33) from the following integer numbers:
1, 256, 32768, 32769, 128, 65534, 33153
Perhaps the easiest way to do this is by using the operators for bit-masking
and bit-shifting. Once you can correctly establish the page number and offset
from an integer number, you are ready to begin.
Initially, we suggest that you bypass the TLB and use only a page table. You
can integrate the TLB once your page table is working properly. Remember,
address translation can work without a TLB; the TLB just makes it faster. When
you are ready to implement the TLB, recall that it has only 16 entries, so you
will need to use a replacement strategy when you update a full TLB. You may
use either a FIFO or an LRU policy for updating your TLB.
How to Run Your Program
Your program should run as follows:
./a.out addresses.txt
Your program will read in the file addresses.txt, which contains 1,000 logical
addresses ranging from 0 to 65535. Your program is to translate each logical
address to a physical address and determine the contents of the signed byte
stored at the correct physical address. (Recall that in the C language, the char
data type occupies a byte of storage, so we suggest using char values.)
Your program is to output the following values:
1. The logical address being translated (the integer value being read from
addresses.txt).
2. The corresponding physical address (what your program translates the
logical address to).
3. The signed byte value stored at the translated physical address.
We also provide the file correct.txt, which contains the correct output
values for the file addresses.txt. You should use this file to determine if your
program is correctly translating logical to physical addresses.
Statistics
After completion, your program is to report the following statistics:
Bibliographical Notes 461
1. Page-fault rate—The percentage of address references that resulted in
page faults.
2. TLB hit rate—The percentage of address references that were resolved in
the TLB.
Since the logical addresses in addresses.txt were generated randomly
and do not reflect any memory access locality, do not expect to have a high TLB
hit rate.
Modifications
This project assumes that physical memory is the same size as the virtual
address space. In practice, physical memory is typically much smaller than a
virtual address space. A suggested modification is to use a smaller physical
address space. We recommend using 128 page frames rather than 256. This
change will require modifying your program so that it keeps track of free page
frames as well as implementing a page-replacement policy using either FIFO
or LRU (Section 10.4).
Bibliographical Notes
Demand paging was first used in the Atlas system, implemented on the
Manchester University MUSE computer around 1960 ([Kilburn et al. (1961)]).
Another early demand-paging system was MULTICS, implemented on the GE
645 system ([Organick (1972)]). Virtual memory was added to Unix in 1979
[Babaoglu and Joy (1981)]
[Belady et al. (1969)] were the first researchers to observe that the FIFO
replacement strategy may produce the anomaly that bears Belady’s name.
[Mattson et al. (1970)] demonstrated that stack algorithms are not subject to
Belady’s anomaly.
The optimal replacement algorithm was presented by [Belady (1966)]
and was proved to be optimal by [Mattson et al. (1970)]. Belady’s optimal
algorithm is for a fixed allocation; [Prieve and Fabry (1976)] presented an
optimal algorithm for situations in which the allocation can vary.
The enhanced clock algorithm was discussed by [Carr and Hennessy
(1981)].
The working-set model was developed by [Denning (1968)]. Discussions
concerning the working-set model were presented by [Denning (1980)].
The scheme for monitoring the page-fault rate was developed by [Wulf
(1969)], who successfully applied this technique to the Burroughs B5500
computer system.
Buddy system memory allocators were described in [Knowlton (1965)],
[Peterson and Norman (1977)], and [Purdom, Jr. and Stigler (1970)]. [Bonwick
(1994)] discussed the slab allocator, and [Bonwick and Adams (2001)] extended
the discussion to multiple processors. Other memory-fitting algorithms can be
found in [Stephenson (1983)], [Bays (1977)], and [Brent (1989)]. A survey of
memory-allocation strategies can be found in [Wilson et al. (1995)].
[Solomon and Russinovich (2000)] and [Russinovich and Solomon (2005)]
described how Windows implements virtual memory. [McDougall and Mauro