Memory Management

Question 1

Why is memory management relevant to to Operating System?

Answer 1

OS responsible for loading programs into memory, physical addresses are HW dependent.

Question 2

What is the purpose of memory protection?

Answer 2

Processes share physical memory, need to restrict access to prevent errors and improper access.

Question 3

What types of variables are there?

Answer 3

Locally declared (part of stored program), loaded into memory by OS

Dynamically allocated, Malloc - process asks for some memory.

Question 4

What is a logical address?

Answer 4

Memory address generated by the CPU and is in the virtual address space.

Question 5

what is a physical address?

Answer 5

Memory address generated by the Memory Management Unit (MMU)

Passed to memory hardware and is in the physical address space.

Question 6

What is address binding?

Answer 6

Mapping symbolic address to physical addresses.

Question 7

What are the attributes of compiled code?

Answer 7

Contains Physical memory addresses

Inflexible, have to re-compile to move process.

Question 8

What happens at load time?

Answer 8

Compiler generates relative addresses (re-locatable code)

Process must occupy contiguous region of memory.

Question 9

What do load time and compile time both result in?

Answer 9

Identical logical and physical memory addresses. (not used)

Question 10

What is run-time binding?

Answer 10

Yields different logical and physical addresses.

Logical addresses dynamically mapped to physical addresses

Question 11

What are the advantages of run-time binding?

Answer 11

It’s felxible, process can occupy non-contigous regions of physical memory and can be moved in memory during execution.

Question 12

What are the disadvantages of run-time binding?

Answer 12

Runtime overheads means HW support required, MMU carries out the mapping.

Question 13

What is static linking?

Answer 13

Linker combines object code of application modules with object code containing system library routines.

Question 14

What are the disadvantages of static linking?

Answer 14

Wastes space as each program has a copy of system library

If a library routine is changed program must be re-linked.

Question 15

What is dynamic linking?

Answer 15

Linker generates executable with stub (code specifying how to locate routine) for each library routine

Stub overwritten with address of routine on first execution. (subsequent execution incurs no overhead)

Question 16

What are the advantages of dynamic linking?

Answer 16

All executing programs use same copy.

If library routine is changed, new version will be used next time program is run.

Question 17

What types of allocation are there?

Answer 17

• Single contiguous
• Partitioned
• Paging
• Segmentation

Question 18

What is single contiguous memory allocation?

Answer 18

Everything goes to one process

Question 19

What is partitioned memory allocation

Answer 19

Contiguous partition for each program.

HW support for memory protection.

Question 20

How does segmentation find a memory address

Answer 20

Segment address + offset address

Question 21

How is memory protected?

Answer 21

Dynamic binding and checking, relocatable regions and resizable contiguous memory space.

Question 22

How does the OS allow variable size regions of memory?

Answer 22

OS holds a free list.

Search for memory hole large enough for new process assign the memory space and remove the address from the free list.

On termination, region is returned to the free list.

Question 23

How does virtual memory work?

Answer 23

Logical address space of process, each logical address is bound to a physical address dynamically.

Split into 4kb blocks known as pages.

Question 24

How is a linux process’s virtual memory structured?

Answer 24

Split into segments:

Code segment
Data segment
BSS (block started by symbol)
Stack Segment

Question 25

What’s the difference between Mmap and malloc

Answer 25

MMAP: Sys call, asks OS for additional memory and updates page table and protection bits

Malloc is a lib functions, may call mmap or gives a pointer to existing memory. Also does book keeping.

Question 26

What does the Malloc Data Structure consist of and enable?

Answer 26

Records block size and allocated/free status. Enables coalescing of adjacent free blocks.

Coalascing: Join together two free blocks of memory.