## Sample questions on the second part of the course

The final exam will take place on Thursday May 9, 2:00 - 3:50 in room 703, Main Building. It will be closed book and closed notes. It will be cumulative, covering material from the entire course, but emphasizing more the material from the second half (since the first half was already tested in the midterm.)

The following are some sample questions that might be asked about the material in the second half of the course. This is NOT a sample exam; the real exam will have some questions from the first half, and fewer questions from the second half.

### Short problems

#### Problem 1:

An OS uses the elevator algorithm to schedule the disk-arm. I/O requests are currently pending for blocks on tracks 1, 3, 8, 11, 15, and 16. The disk arm is currently at track 9 and moving upwards. In what order will these requests be handled?

#### Problem 2:

An OS uses a paging system with 1Kbyte pages. A given process uses a virtual address space of 128K and is assigned 16K of physical memory. How many entries does its page table contain?

#### Problem 3:

A user is typing input to a program at a keyboard, and hits the following sequence of twelve keys:
`T', `h', `r', `e', `e', Delete key, Delete key, Delete key, `e', `r', `e', Carriage return.
If keyboard input is in raw mode, how many characters are passed to the user program? What if it is in cooked mode?

#### Problem 4:

Consider a file system that uses an FAT. The state of the directory is
```
File name | First block | Size in bytes
---------------------------------------
AA               5           1350
BB               7           2200
```
(The other information in the directory is omitted.) The current state of the FAT is
``` 0  -- FREE
1  -- FREE
2 --  -1
3 --   4
4 --  -1
5 --   2
6 --  FREE
7 --   3
8 --  FREE
```
The system uses a block size of 1 Kbyte. What is the last block of file BB? How many bytes of that block are used? (Useful fact: 1K=1024).

#### Problem 5:

A particular system uses a page size of 1K bytes. A page table for a particular process begins as follows: [ 3, 4, *, 1, *, 8 ...]
• A. What physical address corresponds to the virtual address of 50?
• B. Name a virtual address that will generate a page fault.
(Note: everything uses 0-based indexing.)

### Long Problems

#### Problem 6:

The linker, which converts an object file into an executable file, has the task of choosing the placement of the code for different subroutines. Ordinarily, subroutines are simply placed in order in contiguous sections of virtual address space. However, linkers in a paging system sometimes take the alternative approach of placing subroutines so that each subroutine lies within as few pages as possible.

For example, suppose that the page size is 1Kbyte, and the program contains routines ``main'' of 750 bytes; ``a'' of 1000 bytes; ``b'' of 1200 bytes; and ``c'' of 200 bytes. Ordinarily, the linker would place ``main'' in bytes 0-749; ``a'' in 750-1749; ``b'' in 1750-2949; and ``c'' in 2950-3150. However, a linker sensitive to paging might put ``main'' in 0-749; ``c'' in 750-950; ``a'' in 1024-2023; and ``b'' in 2048-3247. (Note: 1K = 1024; 2K = 2048; 3K = 3072.)

#### Problem 7:

A user program has opened a file for reading and been reading sequentially through the program for some while. It now attempts to read the next byte, and finds that this byte belongs to a new block, and that this block is not in memory. Describe the major steps that are involved in the passage of control from the user program, which wants to read the byte, to the disk, which must carry out the read, and in the passage of information from the disk to the user program. In particular, your answer should discuss the operations of the device driver, the device-independent software, the actual disk, and the disk controller, in both directions. You may assume that the program is running on a single-process system. At any point, if there are different options for implementation, you can pick one to describe; you need not describe all the options.

#### Problem 8:

In many operating systems, every process occupies a contiguous section of core memory. There are few if any operating systems in which each file occupies a contiguous section of disk. Give two reasons for this difference between these two storage media.

#### Problem 9:

What is the "Translation Lookahead Buffer" (TLB) and how is it used?

#### Problem 10:

Consider a paging system that uses the "Not Recently Used" (NRU) page replacement algorithm.
• What is the significance of the "R bit" and the "M bit"?
• When are the R bit and M bit set and reset?
• How is a page chosen for replacement?