Project 5
Matt Levan
Nolan Thompson
Due May 13th, 2016
CSC341 Operating Systems
Dr. Bin Tang
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 2^16 = 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.
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:
- An 8-bit page number and
- 8-bit page offset.
Hence, the addresses are structured as show in Figure 9.33.
Other specifics include the following:
- 2^8 entries in the page table
- Page size of 2^8 bytes
- 16 entries in the TLB
- Frame size of 2^8 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.
Adress Translation
Your program will translate logical to physical addresses using a TLB and page table as outlined in Section 8.5. 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 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 9.2. 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 will read 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.
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 yo 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.
Test File
We provide the file addresses.txt, which contains integer values
representing logical addresses ranging from 0-65536 (the size of the virutal
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 The Program
Compile with:
gcc virtual_mem.c -o virtual_mem.o -std=c99 -lm
Run with:
./virtual_mem.o addresses.txt output.txt BACKING_STORE.bin
Your program will read in the fileaddresses.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:
- The logical address being translated (the integer value being read
from
addresses.txt). - The corresponding physical address (what your program translates the logical address to).
- 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:
- Page-fault rate-- The percentage of address references that resulted in page faults.
- 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 recomend using 128 page frame rather 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 9.4).