CS620 Spring 2008 Assignment 4

CS 620

Assignment 4 (Total 50 points)

Due Date: 04/03/2008 (Thursday)

The assignment is to be handed in class. No late assignments will be accepted.

1. (2 points)

What is reentrant code? Explain why sharing a reentrant module is easier when segmentation is used than when pure paging is used.

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2. (7 points)

Consider a paging system with the page table stored in memory.

a. (1 points) If a memory reference takes 100 nanoseconds, how long does a paged memory reference take?

b. (2 points) If we add associative registers, and 75 percent of all page-table references are found in the associative registers, what is the effective memory reference time? (Assume that finding a page-table entry in the associative registers takes zero time, if the entry is there.)

c. (4 points) Now assume the page table is held in registers. It takes 10 milliseconds to service a page fault if an empty frame is available or if the replaced page is not modified, and 25 milliseconds if the replaced page is modified. Assume that the page to be replaced is modified 60 percent of the time. What is the maximum acceptable page-fault rate for an effective access time of no more than 200 nanoseconds.

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3. (3 points)

a. (1 points) What is the cause of thrashing?

b. (1 points) How does the system detect thrashing?

c. (1 points) Once it detects thrashing, what can the system do to eliminate this problem?

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4. (10 points)

Given memory partitions of 300K, 350K, 190K, 600K, 250K and 700K(in order), how would each of the
a. First-fit
b. Next-fit
c. Best-fit
d. Worst-fit
algorithms place processes of 255K, 450K, 185K, 310K, 650K(in order)? For first-fit algorithm, searching starts at the beginning of the set of holes every time. For next-fit algorithm, searching starts at the beginning of the set of holes the first time.

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5. (5 points)

Consider the two-dimensional array A:

               var A: array[1..200]of array [1..200] of integer;

Where A[1][1] is at location 200, in a paged memory system with pages of size 200. A small process is in page 0(location 0 to 199) for manipulating the matrix; thus, every instruction fetch will be from page 0. For three page frames, how many page faults are generated by the following array-initialization loops, using LRU repacement, and assuming page frame 1 has the process in it, and the other two are initially empty:

     a.            for j:= 1 to 200 do
                       for i:= 1 to 200 do 
                           A[i][j]:=0;

     b.            for i:= 1 to 200 do  
                       for j:= 1 to 200 do
                           A[i][j]:=0;

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6. (9 points)

Consider the following page reference string:

           A B C D B A E F C D B C G E B A B D F G

How many page faults would occur for the following replacement algorithms, assuming four frames? Remember all frames are initially empty, so your first unique pages will all cost one fault each. Please show complete working and how you have arrived at the answer.

a. Least Recently Used (LRU) replacement
b. Not Used Recently (NUR) replacement
c. Optimal (OPT) replacement

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7. (6 points)

For the following reference string:

          A C F B C E B C F D A E F F C D A A D E

How many page faults would occur for the Working set replacement algorithm with a window size of 3 assuming three and four frames respectively?
All frames are initially empty, so your first unique pages will all cost one fault each. Assume that the stack is ordered first-in-first-out. Please show complete working and how you have arrived at the answer.

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8. (8 points)

In the following problem, main memory consists of 64 10-bit words. The contents of main memory are as follows:

Address	Contents	Address	Contents	Address	Contents	Address	Contents
0	778	16	681	32	315	48	75
1	488	17	842	33	733	49	468
2	50	18	147	34	713	50	538
3	64	19	611	35	423	51	173
4	411	20	508	36	23	52	14
5	389	21	333	37	667	53	915
6	121	22	795	38	191	54	877
7	793	23	285	39	590	55	25
8	470	24	234	40	17	56	911
9	293	25	238	41	866	57	505
10	29	26	318	42	639	58	0
11	152	27	687	43	594	59	65
12	480	28	801	44	172	60	408
13	830	29	192	45	88	61	268
14	16	30	611	46	741	62	654
15	168	31	45	47	816	63	237

Note: the addresses in the problems below are virtual addresses. The normal convention is to place the most significant portion of the address in the most significant bits of the address. Thus, the virtual addresses below will be formatted as follows:

paging

page # | word #

Consider an operating system using paging. Main memory is divided into 4-word page frames. In a page table descriptor, the low order bits contain the frame in which the page resides. To the left of the frame number is the residency bit and the remaining bits are used by the operating system (protection, whether the page has been modified, etc.).
Thus the page descriptor looks like:

OS ResidencyBit Frame#

The page table pointer for a process points to address 27. Give the results of the following memory references by that process:

a. 60

b. 18

c. 47

d. 25

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