1. Computer Architecture and Operating Systems CS 3230: Operating System Section Lecture OS-8 Memory Management (2)
Department of Computer Science and Software Engineering
University of Wisconsin-Platteville

2. Outlines Paging Address Translation Page Table Page Table Structure
Shared Pages

3. Paging
Each process is divided into a number of small, fixed-size partitions called pages
Physical memory is divided into a large number of small, fixed-size partitions called frames
Page size = frame size
Usually 512 bytes to 16K bytes ,lately tends to be 4K
To run a program of size n pages, need to find n free frames and load program

4. Page Table
Page Table : per process data structure that provides mapping from page to frame
Logical address space: continuous
Physical address space: fragmented

5. Page Table: Protection
Process may not use the whole page table
Unused portions of the page table are protected by valid/invalid bit
Try to address these pages will result in a trap with “memory protection violation”
Problem: size of page table
Solution: Page-Table Length Register (PRLR) indicates size of the page table (discussed later)

6. Paging: Address Translation
Logical address (Address generated by CPU) consists of page number and offset from beginning of the page
Physical address consists of frame base address and offset from beginning of the frame
Address Translation Architecture:

7. Page Table: Implementation
Page table is kept in main memory.
Problem: each page table must fit in one frame /page
Page-Table Base Register (PTBR) points to the page table (physical address of the page table)
Page-Table Length Register (PRLR) indicates size of the page table (# of current valid pages in the table)
Problem: every data/instruction access requires two memory accesses
One for the page table and one for the data/instruction
Solution:
fast-lookup hardware cache called associative memory or Translation Look-aside Buffer (TLB)
What happens on context switch?
clear TLB (destroying all info they hold)

8. TLB On page access: page # is first looked in TLB
if found (cache hit) can immediately access page
if not found (cache miss) have to look up the frame in the page table

9. Paging: Effective Access Time
Associative TLB Lookup =  time unit
Assume memory cycle time is 1 microsecond
Hit ratio – percentage of times that a page number is found in the TLB
Hit ratio = 
Effective Access Time (EAT):
EAT = (1 + )  + (2 + )(1 – )
= 2 +  – 

10. Page Table Structure Problem: Solution:
Address space of modern computer system is 232 to 264
Each page table must fit in one frame /page
However , size of page table is potentially large
As a result , page table searching time is potentially large
Solution:
Hierarchical Paging
Hashed Page Tables
Inverted Page Tables

11. Page Table Structure Hierarchical Paging Hashed Page Tables
Inverted Page Tables

12. Hierarchical Page Tables
Break up the logical address space into multiple page tables
A simple technique is a two-level page table

13. Hierarchical Page Tables
A logical address (on 32-bit machine with 4K page size) is divided into:
a page number consisting of 20 bits
a page offset consisting of 12 bits
Since the page table is paged, the page number is further divided into:
a 10-bit page number
a 10-bit page offset
Thus, a logical address is as follows:

14. Hierarchical Page Tables

15. Hashed Page Tables Used with address spaces > 32 bits and < 64
Hashes logical page number. In case of hash collision, page references are linked structure

16. Inverted Page Table Used with address spaces > 64 bits
Only one (global) page table for all processes
One entry for each frame
Need information about the process that owns that page
Search table to find the frame that logical page refers to
Use hash table to decrease the searching time

17. Inverted Page Table

18. Shared Pages Paging helps code reusing
One copy of read-only (reentrant) code shared among processes
(i.e., text editors, compilers, window systems)
Shared code must appear in same location in the logical address space of all processes

19. Segmentation with Paging
Problems of external fragmentation and lengthy search times can be solved by paging the segments