ECM 1413 memory management

Question 1

Virtual memory segmentation pros and cons

Answer 1

+can fit processes larger than memory
+Segment Sharing and protection
+No internal fragmentation
+Efficient compared to static partitioning
-more complex memory management

Question 2

Paging

Answer 2

Memory divided into equal fixed-size frames
A process is divided into same-size pages
Internal fragmentation is small as
- Only affects the last frame of each process
- The frame/page-size is fairly small

Question 3

Paging pros and cons

Answer 3

+no external fragmentation
+Simple memory allocation
-small internal fragmentation
-page tables can become large

Question 4

Internal fragmentation

Answer 4

the memory is split into mounted-sized blocks. Whenever a method is requested for the memory, the mounted-sized block is allotted to the method. In the case where the memory allotted to the method is somewhat larger than the memory requested, then the difference between allotted and requested memory is called internal fragmentation.

We fixed the sizes of the memory blocks, which has caused this issue. If we use dynamic partitioning to allot space to the process, this issue can be solved.

Question 5

External Fragmentation

Answer 5

there’s a sufficient quantity of area within the memory to satisfy the memory request of a method. However, the process’s memory request cannot be fulfilled because the memory offered is in a non-contiguous manner. Whether you apply a first-fit or best-fit memory allocation strategy it’ll cause external fragmentation.

(Space is wasted between partitions)

Question 6

Virtual memory paging

Answer 6

Paging + virtual memory
Pages are loaded into frames like simple paging, except not all pages need to be loaded at the same time.

Question 7

Pros and cons of virtual memory paging

Answer 7

+Can fit processes larger than memory
+No external fragmentation
+Simple memory allocation
-Virtual memory overhead
-Small internal fragmentation
-Page tables can become large

Question 8

Protection and sharing with segmentation and paging
Goals:

Answer 8

• Protection: Each process can only access its own pages/segments
• Sharing: Multiple processes can access the same page/segment

Question 9

Protection and sharing with segmentation and paging

Answer 9

Implementation:
- Each frame/segment has a number of protection bits specifying
○ Read, write, execute permissions, which processes have these permissions

Question 10

Memory management techniques

Answer 10

Fixed Partitioning
Dynamic Partitioning
Simple Segmentation
Virtual Memory Segmentation
Simple Paging
Virtual Memory Paging

Question 11

Fixed/Static Partitioning

Answer 11

Memory divided into partitions of fixed boundaries (equal or unequal sizes)
1-to-1 mapping of partitions and processes
Process loaded into partitions ≥ its size

Question 12

Fixed Partitioning Pros and Cons

Answer 12

+Easy to understand and implement
+No external fragmentation

-Internal Fragmentation
-Limited by number & size of partitions

Question 13

Dynamic Partitioning

Answer 13

Exact memory allocation – only what is needed
- Partitions of variable length and number
- 1-to-1 mapping
Starts well, then leads to external fragmentation

Question 14

Compaction

Answer 14

Shift processes for unified free memory space

Question 15

Dynamic partitioning Pros and Cons

Answer 15

+No internal fragmentation
+Efficient than static partitioning

-External fragmentation
-Overhead of compaction

Question 16

Simple segmentation

Answer 16

Process divided into segments (mirror semantic organization - text and data)
Segments are loaded into memory as in dynamic partitioning
Segments do not need to be contiguous

Question 17

Simple Segmentation – Pros and Cons

Answer 17

+Sharing and protection
+Better handling of larger programs
+No internal fragmentation
+Efficient than static partitioning

-External fragmentation (still less than dynamic)
-Overhead of compaction

Question 18

Virtual Memory Segmentation

Answer 18

Segmentation + Virtual Memory
- Virtual memory use disk space to extend main memory; illusion of larger memory
Segments can be swapped between memory and disk storage
OS loads segments into memory as needed