Tutorial Week 8

<li>  
What is the difference between internal and external fragmentation?

Ans: 
Internal fragmentation is when a process is given memory and is not using all 
of it, as in the buddy system. External fragmentation is when their are lots 
of small entries on the free list that cannot easily be used.

<li>  
Consider a variable size partition memory management scheme where the free 
list consists of the following partitions, ordered as given:
  10k, 4k, 20k, 18k, 7k, 9k, 12k and 15k
Which hole is taken for successive segment requests of
  12k, 10k, 9k and 8k
for first fit, best fit, worst fit. What holes are left?

Ans: 
First fit: 20k, 10k, 18k, 8k hole left in 20k hole; left- 4k, 9k, 7k, 9k, 12k, 
15k
Best fit: 12k, 10k, 9k, 15k; left - 4k, 20k, 18k, 7k, 7k
Worst fit: 20k, 18k, 15k; left - 10k, 4k, 8k, 8k, 7k, 9k, 8k, 6k

<li>  
Which of first fit, best fit or worst fit is used by MSDOS? Why? (See the textbook.)

Ans: 
All of them. Because of the horrible memory model. You may need a different 
strategy for badly fragmented upper memory to ordinary memory.

<li>  
Discuss an algorithm to implement ``realloc''. What assumptions does this make 
about a memory allocation scheme?

Ans: 

if new_size == old_size
    return old_ptr
if new_size < old_size
    free spare blocks at the top
    return old_ptr
if there is a free block size (new_size - old_size)
    above and contiguous to the old one,
    remove this from free list
    return old_ptr
get a block from free list of new_size
set new_ptr to it
copy old to new
return new_ptr

<li>  
Consider a virtual memory paging scheme for a 6 page frame memory and a 12 
page virtual address space with 512 (01000) pages. Using the following page 
table, what are the actual physical addresses of these virtual addresses?
    020, 04100, 010300, 05376

Virtual
page
Page
index
0
2
1
2
3
3
4
5
5
6
7
0
8
4
9
1



Ans: 
LA 20 ! PA 02020; LA 4100 ! PA 05100; LA 010300 ! PA 04300; LA 05376 ! page 
frame fault.

Laboratory Week 8

There is no formal laboratory this week. Use it to catch up on outstanding 
tutorial or laboratory material.