* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Operating Systems Concept
?What is an operating system?
?Operating system architecture
?Process concept
?CPU scheduling
?Memory management
?File and I/O systems
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Memory Management
?Logical address vs Physical Address
* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 245-246.
CPU
Logical Address for Instructions or Data
Memory Management
Unit
Physical Address
Process Image Memory Process
Image
PC
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Memory Management
?Contiguous Allocation
?Single Partition
* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 249-264.
The User Process Memory
OS
CPU
Memory Management
Unit
If logical -address < Limit-Register Then
physical-address = logical address + Relocation -Register Else
Signal the user process!
Limit-Register Relocation-Register
345
10000
10345
5000
Memory Management
?Contiguous Allocation
?Multiple Partitions
P1 OS
400K
1000K
P2
2000K P3
2300K 2560K
P1 OS
400K
1000K
P4
2000K P3
2300K 2560K 1700K -P2+P4
P5 OS
400K
1000K
P4
2000K P3
2300K 2560K 1700K -P1+P5
900K
3holes = 660KB!!!
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Memory Management
?Contiguous Allocation
?Multiple Partitions
?First-Fit
?Best-Fit
?Worst-Fit
?Fragmentation
?External
?Internal P5
OS
400K
1000K
P4
2000K P3
2300K 2560K 1700K
900K P5
OS
400K
P6
2300K 2560K 1700K 900K
Next request is for 819000B, i.e., 800KB-200B
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Memory Management
?Solutions to Fragmentation
?Compaction
P5
400K OS
1300K
P4
2000K P3
2300K 2700K 1700K 1000K
P5
400K OS
1400K P4
P3
2700K 1700K 1000K
Move 700KB!
P5
400K OS
P4
2000K P3
2300K 2700K 1000K
P5
400K OS
1300K
P4 P3
2700K 1700K 1000K
Move 400KB! Move 300KB!
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Memory Management
?Paging – Another solution to external fragmentation!
Page 0 Page 1 Page 2 Page 3
Frame #
Page 0
Page 2 Page 1
Page 3
7 3
3 2
4 1
1 0
Frame # Page #
0 1 2 3 4 5 6
logical 7
memory
physical memory A page
table for each process
Paging
? Address Translation
page number offset
p bits d bits
frame number offset
f bits d bits
F
P D
Logical Addr.
P
F D
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Paging
Page 0 Page 1 Page 2 Page 3
physical address
Page 0
Page 2 Page 1
Page 3
111 11
011 10
100 01
001 00
F P
00000 00100 01000 01100 10000 10100 11000 11100
logical memory
physical memory page
table
P D
2 bits 2 bits
F D
3 bits 2 bits
0000 0100 1000 1100
01 10 100 10
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Paging
Page 0 Page 1 Page 2 Page 3
physical address
Page 0
Page 2 Page 1
Page 3
111 11
011 10
100 01
001 00
F P
00000 00100 01000 01100 10000 10100 11000 11100
logical memory
physical memory page
table
0000 0100 1000 1100
1100 A 1101 B 1110 C 1111 D
Page 3
11100 A 11101 B 11110 C 11111 D
Frame 7
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Paging
?Hardware Support for Paging
?Registers as Page Tables
?Memory-Resident Page Tables
?Translation Look-aside Buffer (TLB)
F
P D
P
F D
P F
TLB
Paging
? Paging-TLB
?TLB Hit
?Access time = TLB-access-time + Inst/Data- Memory -Access
?E.g., 20ns + 100ns
?TLB Miss
?Access Time = TLB-access-time + Page- Table-Memory-Access + Inst/Data-Memory- Access
?E.g., 20ns + 100ns + 100ns
? Hit Ratio 80%
?Effective access time = 20ns + 100ns +
0.2 *100ns = 140ns
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Virtual Memory
?Definition
?A technique that allows the execution of
processes that may not be completed in memory.
?Swapping
?Process image may reside in the backing store rather than swap the entire image in.
?Page fault: occurs when program references a non-memory-resident page.
?Thrashing
?A process is spending more time in page faults than executing.
* “Operating system concept”, Silberschatz and Galvin, Addison Wesley, pp. 289,291-293,317.
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
Context Switching-Revisiting
?Def.
?Switch the CPU from one process to another process
?Save the state (or called context) of the running process
?Reload the state of the ready process
?Context Switching Time
?Hardware-dependent!
?Multiple register sets!
?Special hardware instructions!
* All rights reserved, Tei-Wei Kuo, National Taiwan University, 2001.
