Chapter 7: Main Memory

Question 1

Absolute code can be generated for \_\_\_\_.
A) compile time binding 
B) load time binding 
C) execution time binding
D) interrupt binding

Answer 1

A- COMPILE TIME BINDING

Question 2

Which of the following methods of binding instructions and data to memory is performed by most general-purpose operating systems?
A) interrupt binding 
B) compile time binding 
C) execution time binding
D) load time binding

Answer 2

C- EXECUTION TIME BINDING

Question 3

An address generated by a CPU is referred to as a ____.
A) physical address
B) logical address
C) post relocation register address
D) Memory-Management Unit (MMU) generated address

Answer 3

B- LOGICAL ADDRESS

Question 4

In a dynamically linked library, ____.
A) loading is postponed until execution time
B) system language libraries are treated like any other object module
C) more disk space is used than the option of using a statically-linked library
D) a stub is included in the image for each library-routine reference

Answer 4

D- A STUB IS INCLUDED IN THE IMAGE FOR EACH LIBRARY-ROUTINE REFERENCE

Question 5

Which of the following binding schemes facilitates swapping?
A) interrupt time 
B) load time 
C) assembly time 
D) execution time

Answer 5

D- EXECUTION TIME

Question 6

The roll out, roll in variant of swapping is used ____.
A) when a backing store is not necessary
B) for the round-robin scheduling algorithm
C) for priority-based scheduling algorithms
D) when the load on the system has temporarily been reduced

Answer 6

C- FOR PRIORITY-BASED SCHEDULING ALGORITHMS

Question 7

Which of the following dynamic storage-allocation algorithms results in the smallest leftover hole in memory?
A) first fit 
B) best fit 
C) worst fit 
D) None of the above

Answer 7

B- BEST FIT

Question 8

Which of the following is true of compaction?
A) It can be done at assembly, load, or execution time.
B) It is used to solve the problem of internal fragmentation.
C) It cannot shuffle memory contents.
D) It is possible only if relocation is dynamic and done at execution time

Answer 8

D- IT IS POSSIBLE ONLY IF RELOCATION IS DYNAMIC AND DONE AT EXECUTION TIME

Question 9

A(n) \_\_\_\_ page table has one page entry for each real page (or frame) of memory.
A) inverted 
B) clustered 
C) forward-mapped 
D) virtual

Answer 9

A- INVERTED

Question 10

How is a limit register used for protecting main memory?

Answer 10

When the CPU is executing a process, it generates a logical memory address that is added to a relocation register in order to arrive at the physical memory address actually used by main memory. A limit register holds the maximum logical address that the CPU should be able to access. If any logical address is greater than or equal to the value in the limit register, then the logical address is a dangerous address and an error results.

Question 11

What is the advantage of using dynamic loading?

Answer 11

Dynamic loading enables a program to not have to store itself, in its entirety, in main memory. This allows the system to obtain better memory-space utilization. This also allows unused routines to stay out of main memory so that memory can be used more effectively. For example, code used to handle an obscure error would not always use up main memory.

Question 12

What is the context switch time, associated with swapping, if a disk drive with a transfer rate of 2 MB/s is used to swap out part of a program that is 200 KB in size? Assume that no seeks are necessary and that the average latency is 15 ms. The time should reflect only the amount of time necessary to swap out the process.

Answer 12

200KB / 2048 KB per second + 15 ms = 113 ms

Question 13

When does external fragmentation occur?

Answer 13

As processes are loaded and removed from memory, the free memory space is broken into little pieces. External fragmentation exists when there is enough total memory space to satisfy a request, but the available spaces are not contiguous; storage is fragmented into a large number of small holes. Both the first-fit and best-fit strategies for memory allocation suffer from external fragmentation.

Question 14

Explain the basic method for implementing paging.

Answer 14

Physical memory is broken up into fixed-sized blocks called frames while logical memory is broken up into equal-sized blocks called pages. Whenever the CPU generates a logical address, the page number and offset into that page is used, in conjunction with a page table, to map the request to a location in physical memory.

Question 15

How does a transaction look-aside buffer (TLB) assist in the hardware implementation of a page table?

Answer 15

Typically, large page tables are stored in main memory, and a page-table base register points to the page table. Therefore, two memory accesses are needed to access a byte (one for the page-table entry, one for the byte), causing memory access to be slowed by a factor of 2. The standard solution to this problem is to use a TLB, a special, small fast-lookup hardware cache. The TLB is associative, high speed memory. Each entry consists of a key and value. An item is compared with all keys simultaneously, and if the item is found, the corresponding value is returned.

Question 16

How are illegal page addresses trapped by the operating system?

Answer 16

Illegal addresses are trapped by the use of a valid-invalid bit, which is generally attached to each entry in the page table. When this bit is set to “valid”, the associated page is in the process’s logical address space and is thus a legal (or valid) page. When the bit is set to “invalid”, the page is not in the process’s logical address space. The operating system sets this bit for each page to allow or disallow access to the page.

Question 17

Describe the elements of a hashed page table.

Answer 17

A hashed page table contains hash values which correspond to a virtual page number. Each entry in the hash table contains a linked list of elements that hash to the same location (to handle collisions). Each element consists of three fields: (1) the virtual page number, (2) the value of the mapped page frame, and (3) a pointer to the next element in the linked list.

Question 18

Briefly describe the segmentation memory management scheme. How does it differ from the paging memory management scheme in terms of the user’s view of memory?

Answer 18

Segmentation views a logical address as a collection of segments. Each segment has a name and length. The addresses specify both the segment name and the offset within the segment. The user therefore specifies each address by two quantities: a segment name and an offset. In contrast, in a paging scheme, the user specifies a single address, which is partitioned by the hardware into a page number and an offset, all invisible to the programmer

Question 19

A relocation register is used to check for invalid memory addresses generated by a CPU (T/F)

Answer 19

FALSE

Question 20

Pure code cannot be shared (T/F)

Answer 20

FALSE

Question 21

In current systems, pages are typically between 4 KB and 8 KB in size. (T/F)

Answer 21

TRUE

Question 22

Hierarchical page tables are appropriate for 64-bit architectures. (T/F)

Answer 22

FALSE

Question 23

The Linux operating system does not rely on segmentation and uses it minimally (T/F)

Answer 23

TRUE

Question 24

A word is a

Answer 24

fixed-size unit of data. A typical word size is 1, 2, or 4 bytes.

Question 25

A word is a

Answer 25

fixed-size unit of data. Typical word size is 1, 2, or 4 bytes.

Question 26

first fit vs next fit

Answer 26

First fit looks in taken places trying to fill up. Next fit doesn’t attempt used holes.

Question 27

The role of the linker is to

Answer 27

combine all object files into a single executable file such as a.out on Linux systems or a .exe file on Windows systems. The loader can then load the executable file into memory and make it available to run on a CPU core.

Question 28

External memory fragmentation is the

Answer 28

loss of usable memory space due to holes between allocated blocks of variable sizes.

Question 29

The 50% rule states that, if the

Answer 29

probability of finding an exact match for a request approaches 0, one third of all memory partitions are holes and two thirds are occupied blocks.

Question 30

The 50% rule talks only about the number of holes and blocks but not the amount of

Answer 30

memory occupied by each. Memory utilization depends on the average sizes of holes and occupied blocks.

Question 31

A page table is an

Answer 31

array that keeps track of which pages of a given logical address space reside in which page frames.

Question 32

A page frame is a

Answer 32

fixed-size contiguous block of physical memory identified by a single number, the page frame number.

Question 33

A segment is a

Answer 33

variable-size block of a logical address space identified by a single number, the segment number.

Question 34

A segment table is an

Answer 34

array that keeps track of which segment resides in which area of physical memory.

Question 35

A translation lookaside buffer (TLB) is a

Answer 35

fast associative memory buffer that maintains recent translations of logical addresses to frames in physical memory for faster retrieval.

Question 36

The principle of locality states that

Answer 36

locations accessed recently are more likely to be accessed again than locations accessed in the distant past.