CS439: Principles of Computer Systems
Homework 6, Part 1
Due: 9:45a Thursday, October 16, 2014 Part 1 of the
homeworks must be submitted electronically. Please refer to the
homework turnin instructions.
What causes a page fault? What is the end result for the running
Sam P. Hacker is a Head Guru in a software company known for
operating systems with very sorry quality. Hacker suggested a trick to
reduce the pressure on the swap space. Instead of swapping out pages
that belong to code texts into the swap area, the operating system
could just take away the memory frames. If the code page is needed
later, it could be paged directly from its binary file. Hacker's
argument is that the operating system will save time by not storing
the code page into the swap space, and will save space in the swap
area. The code text exists on disk anyway, and could be fetched from
there. Would you approve of this design? Why or why not?
What is TLB (Translation Lookaside Buffer)? What issue it tries to solve? How does it solve that issue?
Consider a multi-level paging system with 128 pages and a page size
of 256 bytes. The system has 1024 bytes of physical memory and is
byte addressable. Assume the first-level page table holds 8
- How many bits are in a physical address?
- How many bits of the virtual address represent the
first-level page table?
- How many bits of the virtual address represent the page number?
- How many bits are in the complete virtual address?
- What size are the page frames?
What is the difference between global and local page replacement
policies? Name a disadvantage of each.
Belady's anomaly: Intuitively, it seems that the more frames the
memory has, the fewer page faults a program will get.
Surprisingly enough, this is not always true. Belady (1969)
discovered an example in which FIFO page replacement causes more
faults with four page frames than with three. This strange
situation has become known as Belady's anomaly. To illustrate, a
program with five virtual pages numbered from 0 to 4 references
its pages in the order:
0 1 2 3 0 1 4 0 1 2 3 4
- Using FIFO replacement and assuming demand paging, compute
the number of page faults with 3 frames. Repeat for 4 frames.
Compute the number of page faults under LRU, the clock
algorithm, and the optimal algorithm. What do you notice?