1. CS 286: Memory Paging and Virtual Memory
Virtual Memory – Concepts and Implementation
Department of Computer Science
Southern Illinois University Edwardsville
Fall, 2017
Dr. Hiroshi Fujinoki
Virtual_Memory/000

2. CS 286: Memory Paging and Virtual Memory
The problem in memory system
Throughput
“An improvement rate of 60%/year in microprocessor performance,
while the access time to DRAM has been improving at less than 10%/year”
“The Gap between Processor and Memory Speeds” by Carlos Carvalho

3. CS 286: Memory Paging and Virtual Memory
Memory Hierarchy
CPU Core (ALU)
Registers
L1 Cache
L2 Cache
Main Memory
Virtual Memory
Disk
Virtual_Memory/002

4. CS 286: Memory Paging and Virtual Memory
Memory Hierarchy
Fastest
CPU registers
Cache memory within a CPU (Level-1 Cache)
Cache memory on a motherboard (Level-2 Cache)
Main memory
Hard drive
Slowest
MemoryBasics/006

5. CS 286: Memory Paging and Virtual Memory
We have a trade-off problem
Memory Hierarchy
Fastest
Highest Cost
CPU registers
Cache memory within a CPU (Level-1 Cache)
Cache memory on a motherboard (Level-2 Cache)
Main memory
Hard drive
Slowest
Lowest Cost
“Cost” = $$$ amount for each byte
MemoryBasics/007

6. Level-1 Cache (Part 2 – Intel’s Pentium 4)
CS 286: Memory Paging and Virtual Memory
Level-1 Cache (Part 2 – Intel’s Pentium 4)
Level-1 (L1) Cache
MemoryBasics/009

7. “External Fragmentation”
CS 286: Memory Paging and Virtual Memory
Memory External Fragmentation
170MB Physical Memory A
40MB
Memory F
30MB
A lot of small
free-memory
spots, but none
is big enough A
40MB A
40MB
10MB
“External Fragmentation”
(unused gap in memory
between two programs) B
40MB E
30MB B
40MB E
30MB
10MB
10MB C
40MB C
40MB C
40MB D
40MB D
40MB D
40MB
10MB
10MB
10MB
10MB free
20MB free
30MB free G
30MB
Virtual_Memory/001

8. Solution to these problems
CS 286: Memory Paging and Virtual Memory
Memory External Fragmentation
Problems
A lot of small
free-memory
spots, but none
is big enough
 The programs must be stopped
during compaction
 For multi-tasking computers
that have a large number of
processes, this is VERY expensive
 Tight compaction will not allow
programs to grow in size
This process is
called “compaction”
Solution to these problems
Paging
Virtual_Memory/001

9. CS 286: Memory Paging and Virtual Memory
What is virtual memory?
1. It is a mechanism that expands memory space using hard drive
2. It is NOT memory
CPU
Main Memory
64M Bytes
(16)
FFFFFFFFFF(16)
Memory
Address Space
Without virtual memory, memory address space is always
same as the amount of main memory
Virtual_Memory/003

10. CS 286: Memory Paging and Virtual Memory
What is virtual memory (continued):
Disk
Virtual Memory
Physical
Memory
Address
Space
Memory
Address Space
CPU
(16)
FFFFFFFFFF(16)
Logical
Memory
Address
Space
64MB
Page
Main Memory
Virtual_Memory/004

11. CS 286: Memory Paging and Virtual Memory
What is virtual memory (summary):
1. It is a mechanism that expands memory space by mapping some of
the virtual memory address space to hard drive
2. This implies that a CPU can execute a program that is larger than
actual main memory
3. Virtual memory is not memory. It’s just an array of pointers
that map pages in virtual address space to the pages in
physical address space
Virtual_Memory/005

12. CS 286: Memory Paging and Virtual Memory
Implementation of Virtual Memory:
Disk
CPU
Page#
Offset 1 2 3 4 5 6 7 8 9 10
Page numbers
Valid Flag 1 2 3 4 5 6 3 1
Main Memory
Virtual Memory Table
Virtual_Memory/006

13. CS 286: Memory Paging and Virtual Memory
What is “offset”?
Target memory address
CPU
Page#
Offset
Offset 1 2 3 4 5 6 7 8 9 10
Page numbers
Valid Flag 1 2 3 4 5 6 3 1
Main Memory
Virtual Memory Table
Virtual_Memory/006

14. CS 286: Memory Paging and Virtual Memory
If the requested page is in the main memory:
1. Fetch the page# field from the requested address
2. Check the valid flag in the virtual memory table
3. If the valid flag is one (valid),
use the page number stored in this slot as the main memory page#
4. Use the offset field as the offset from the beginning of the page
Virtual_Memory/007

15. CS 286: Memory Paging and Virtual Memory
Implementation of Virtual Memory:
Disk
CPU
Page#
Offset #2 #7 1 2 3 4 5 6 7 8 9 10
Valid Flag 3 1 1 2 3 4 5 6 3 1
This operation is
called “page fault”
Main Memory
Virtual Memory Table
Virtual_Memory/008

16. CS 286: Memory Paging and Virtual Memory
If the requested page is not in the main memory:
1. Fetch the page# field from the requested address
2. Check the valid flag in the virtual memory table
Called “page fault”
3. If the valid flag of the requested page is zero (invalid),
save the destination page in the main memory to hard drive
4. Change the valid flag of the virtual memory that was previously
using the memory page to zero (invalid)
5. Load the requested page from HDD to main memory
6. Change the valid field of the requested page to one (valid)
Virtual_Memory/009

17. CS 286: Memory Paging and Virtual Memory
Dirty Flag
Disk
CPU
Page#
Offset 1 2 3 4 5 6 7 8 9 10
Dirty Flag 3 1 1 2 3 4 5 6 3
Main Memory
Valid Flag
Virtual Memory Table
Virtual_Memory/010

18. CS 286: Memory Paging and Virtual Memory
Dirty Flag
 When a new page is loaded to a physical memory page,
set its “dirty flag” to ‘0’.
 When a physical memory page is updated, set its “dirty flag” to ‘1’.
When a physical memory page is being replaced by another virtual
memory page, perform one of the followings:
If the page’s (the one currently in the physical memory)
dirty flag is ‘0 (clean)’:
Let a new page to replace the previous one
in the physical memory
(b) If the page’s (the one currently in the physical memory)
dirty flag is ‘1 (dirty)’:
Save the current page back to the hard drive, then let
a new page to replace the previous one
in the physical memory
Virtual_Memory/011

19. CS 286: Memory Paging and Virtual Memory
Implementation of Virtual Memory (Continued):
Disk
CPU
CPU
Logical Address
Page#
Offset
Translation Lookaside Buffer (TLB)
Main Memory 1 2 3 4 5 6
High speed cache
within CPU
Physical Address
Virtual_Memory/012
Virtual Memory Table

20. CS 286: Memory Paging and Virtual Memory
Implementation of Virtual Memory with L2 Cache:
Disk
CPU
Page#
Offset
Memory
Controller
Translation Lookaside Buffer
Main Memory 1 2 3 4 5 6
CPU
L2 Cache
Virtual_Memory/013
Virtual Memory Table