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COMP3430 Assignment 4
Assignment Objectives:
To learn something about the implementation of basic free space management (which is
applicable to both RAM and persistent storage (such as hard disks) but which will be
limited to RAM in this assignment.
To improve your understanding of how paged virtual memory systems work.
To improve your understanding of i-node based disk space management.
Assignment Instructions:
In general, for written questions PDF (.pdf), Word (.docx), PowerPoint (.pptx), Rich Text
Format (.rtf) and Text (.txt) format files are acceptable. Handwritten then scanned PDF
files are
not acceptable. For any programming questions, please hand in all source files
and any requested results as well as a Makefile to generate the executable file(s) for the
target platform. You
must also include a text file named ‘README’ for each programming
question that briefly describes how to use your program.
Your programs must run on the Linux lab machines, but you can try to develop your code
on other platforms if you like though this is
not recommended. In some cases, this may
simply not be possible since your running programs will be making Linux system calls.
Should you choose to use different platforms for development, you
alone will be responsible
for dealing with any differences in the compilers, tools, etc. that you may encounter. In such
a case, be sure to leave plenty of time to port your code to Linux. Normally this takes less
than 30 minutes but sometimes it may take several hours if your code is still a little buggy.
(For example, the C compiler on some systems sets all uninitialized memory to zeroes.
Hence, bad pointers are treated as NULLs and thus, sometimes incorrect code works there
but not with other compilers and operating systems such as gcc on Linux.)
Note that assignments are to be done entirely independently unless otherwise explicitly
stated and that inclusion of materials from online sites is entirely forbidden.
Assignment Questions:
[10] 1. You are to extend the simple free list-based memory management system written in C provided
with this assignment on the course homepage. Your system should extend the basic first-fit
algorithm code to support free space
amalgamation (i.e. when an area is freed that is adjacent
to other free areas, they should be combined into a single larger free area). The provided
memory management system provides three functions:
void myinit(unsigned long areasz);
void *
mymalloc(unsigned long sz);
void
myfree(void * addr, unsigned long sz);
A call to the function myinit(areasz) initializes the memory management system. It begins
by using Linux’s
malloc() call to allocate areasz bytes of memory from which subsequent
calls to
mymalloc() are processed. It then builds an initial free list (consisting of a single
node with size
areasz) within the allocated space. (Each node on the free list is physically
stored
in the first part of the corresponding free area and contains an unsigned long
specifying the size of the area and a void * pointer to the next free area on the list.) Once the
system is initialized, it services allocation and freeing requests (via calls to
mymalloc() and
myfree()). Each time a call to mymalloc(sz) is made, the first-fit algorithm is applied to
the free list to find the first block large enough in size (i.e.
sz or more bytes) from which the
allocation is made. The code then updates the free list to reflect the allocation and returns the
address of the allocated region to the caller of
mymalloc(). Space is allocated from the end
of the selected free region to simplify the updating of the size field in the free list node. Each
time a call to
myfree(addr, sz) is made, the allocated region specified by addr and sz
must be returned to the free list and amalgamated with any adjacent free blocks. This is the
code you must extend! You may assume that arguments to
myfree() are always valid. Note
that you will need to
maintain the free list in order by address. (This makes free space
amalgamation easier and more efficient.)
To test your functions, a program that builds a linked list of positive integers typed at the
command line (until zero is entered) is provided. This program reads the integers and stores
them in a list of nodes created using
mymalloc() and myfree(). It then prints the list
nodes and deletes the nodes. Why is a minimum allocation size (
MINALLOCSIZE) required?
2. Answer the following questions about the paged virtual memory system described below.
a) A paged virtual memory system has a page (and frame) size of 512 bytes and a physical
memory consisting of 10 page
frames numbered 0 to 9. The logical address space of the
single resident process has 512 pages numbered 0 to 511. The current contents of physical
memory (RAM) are as shown below. Addresses are shown in hex (0x) and decimal.
PHYSICAL MEMORY (RAM)

Page 38
Page 9
Page Table
Page 61
Page 10

0x000 (0000)
0x800 (2048)
0x600 (1536)
0xE00 (3584)
0x1200 (4608)
0xC00 (3072)

[5] i. Draw a picture of the process’ page table with gaps shown using ellipses (“…”).
[1] ii. What happens when virtual address 0x7918(31000) is referenced? Explain briefly.
[4] iii. Draw a table showing which physical addresses (represented as page
frame numbers
and offsets) are referenced by the virtual addresses 0x1200(4608), 0x13FF(5119),
0x1400(5120), 0x7A08(31240)?
[10] b) Given the sequence of page references shown
below and assuming that the available 5
frames of memory initially contain the pages 15, 7, 1, 11, and 8 (in that order, page 5 being
most recently used though to page 8 being least recently used), illustrate the operation of
the LRU replacement algorithm as the pages in the sequence below are referenced. Your
answer should clearly show which pages are resident following
each reference and should
indicate
whenever page faults occur. Also, be sure to clearly show what the “LRU order”
of the resident pages is
before each reference. Some sort of table with ordered entries is
probably a good way to show the necessary information.
Page Reference Sequence: 7, 15, 10, 11, 14, 11, 9, 17, 9, 14
3. Answer the following questions about a filesystem similar to the Unix Fast File System that
uses inode structures containing 8 direct block pointers as well as single, double and triple
indirect pointers. Assume that disk
addresses are 8 bytes and that disk blocks are 512 bytes.
[3] a) What is the largest file that can be stored in such a file system? Explain how you arrived at
this maximum!
[5] b) How many disk blocks (data
and index) are needed to store a file of the following sizes
and how much
internal fragmentation occurs in each case? (You can ignore space occupied
by the inodes.) In each case, explain how you arrived at your answer.
[1] i. 512 bytes
[1] ii. 516 bytes
[1] iii.10752 bytes
[1] iv. 2134016 bytes
[1] v. 8401408 bytes
[2] c) How many disk operations will it take to sequentially read an entire file of 77824 bytes?
Explain how you arrived at your answer? You can ignore the access to the inode itself
Total: 40 marks
No Challenge Questions this time
– you are better off focusing on core material in this and other
courses you are taking and also starting to prepare for your final exams!