Memory Management II:
Dynamic Storage Allocation
Oct 13, 1998
Topics
•other placement policies (cont)
•buddy systems
•implementation issues (sequential fits)
class15.ppt
15-213
Introduction to Computer Systems
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Memory Management II:
Dynamic Storage Allocation
Oct 13, 1998
Topics
other placement policies (cont)
buddy systems
implementation issues (sequential fits)
class15.ppt
Introduction to Computer Systems
CS 213 F’
class15.ppt
Basic allocator mechanisms
Sequential fits
best fit, first fit, or next fit placement
various splitting and coalescing options
- immediate or deferred coalescing – splitting thresholds
Segregated free lists
simple segregated storage
segregated fits
- buddy systems
CS 213 F’
class15.ppt
Segregated fits
To allocate a block of size n:Array of free lists, each one for some size class
search appropriate free list for block of size m > n
if an appropriate block is found:
- split block and place fragment on appropriate list (optional)
if no block is found, try next larger class
repeat until block is found
To free a block:
coalesce and place on appropriate list (optional)
Tradeoffs
classes)faster search than sequential fits (i.e., log time for power of two size
controls fragmentation of simple segregated storage
coalescing can increase search times
- deferred coalescing can help
CS 213 F’
class15.ppt
Buddy systems
Special case of segregated fits.
all blocks are power of two sizes
Basic idea:
Heap is 2
m
words^
Maintain separate free lists of each size 2
, 0 <= k <= m.k
Requested block sizes are rounded up to nearest power of 2.
Originally, one free block of size 2
m
CS 213 F’
class15.ppt
Buddy systems (cont)
To free a block of size 2
k
buddy
To free a block of size 2
k
if buddy free, coalesce with buddy and return new block to free list
To free a block of size 2
k
if buddy not free, just return block to free list
CS 213 F’
class15.ppt
Buddy systems (cont)
Key fact about buddy systems:
address of its buddygiven the address and size of a block, it is easy to compute the
e.g., block of size 32 with address
xxx...x00000 has buddy
xxx...x
Tradeoffs:
fast search and coalesce
subject to internal fragmentation
CS 213 F’
class15.ppt
Steele mail fraud case
March 6, 1993 (Pittsburgh, PA)
onto some diskettes, then resigns.copies school board billing records from firm's laptopsSteele & Hoffman, after watching the movie "The Firm",Phil McCalister, disgruntled associate at Pgh law firm
July 29, 1993
Charlie Steele, managing partner of Steele & Hoffman.evidence of systematic overbilling of school systems byMcCalister hands over 4 diskettes to postal instpectors as
September, 1996
originals from March 6, 1993 (they weren't).I'm asked by defense to determine if the 4 diskettes are the
December, 1996
years in federal pen and $80,000 fine.Charlie Steele convicted of mail fraud and sentenced to 3Despite brilliant testimony by the computer expert witness,
CS 213 F’
class15.ppt
Anatomy of a Computer
memory
disk buffer (temporary)
programs
dataand
keyboard
display
hard disk (fixed)
diskette (removeable)
CS 213 F’
class15.ppt^ Internal fragmentation in DOS files
Files allocated in fixed size logical sectors
cluster
data abc
slack (internal fragmentation)
CS 213 F’
class15.ppt
How slack takes a picture of a
disk when a file is copied (1)
abc
source disk
destination disk
disk buffer
1. read source directory ("DE" is directory entry)
DE
DE
DE
DE
CS 213 F’
class15.ppt
How slack takes a picture of a
disk when a file is copied (3)
abc
source disk
destination disk
disk buffer
snapshot of the files on the source disk when the file was copied.3. write file to destination disk. Notice that slack now contains a
DE
DE
DE
DE
abc
DE
DE
DE
DE
abc
CS 213 F’
class15.ppt
Federal diskette F1 is not a duplicate
Cluster
(^) Sector
[F1:1991-$.IN
C1638-1789]
Name
.Ext
Size
Date
Time
Cluster
(^) Arc
R/O
(^) Sys
(^) Hid
(^) Dir
Vol
YS
(^) am
R/O
(^) Sys
(^) Hid
MSDOS
SYS
(^) am
R/O
Sys (^) Hid
CONFIG
SYS
am
Arc
AUTOEXEC
BAT
(^) am
Arc
DOS
4:40 pm
Dir
WININST
4:41 pm
Dir
WINDOWS
4:43 pm
Dir
COMMAND
COM
(^) am
Arc
SCAN
4:50 pm
Dir
WINA
(^) am
HARCHLRD
REG
12:50 pm
Arc
ASP
(^) am
Dir
DO
12:01 pm
Dir
GOLF
12:01 pm
Dir
LOTUS
4:32 pm
Dir
NORTON
12:04 pm
Dir
Source: Norton Utilities Diskedit program
CS 213 F’
class15.ppt
Cluster^ Federal diskette F3 is not a duplicate
Sector
[F3:SUMMARY
C905-936]
Name
.Ext
Size
Date
Time
Cluster
Arc (^) R/O
Sys
Hid (^) Dir
Vol
LOVIS...-----------------------------------------------------------------------------
2:23 pm
Dir
BACKUP
(^) am
Arc
BURRELL
(^) am
Dir
MIFFLINB
(^) am
Dir
Source: Norton Utilities Diskedit program
CS 213 F’
class15.ppt
Files on the federal diskettes came
from the hard disk
Cluster
Sector
[F2:PRICE
C2020-2023]
Name
.Ext
Size
Date
Time
Cluster
Arc (^) R/O
Sys
Hid (^) Dir
Vol
DECEMBER
2:07 pm
Arc
EMWAUG
(^) am
Arc
EMWAUG
2:20 pm
Arc
EMWDEC
1:31 pm
Arc
EMWDEC
(^) am
Arc
EMWDEC
(^) am
Arc
EMWFEB
(^) am
Arc
EMWJAN
5:22 pm
Arc
EMWJUL
(^) am
Arc
EMWJUL
(^) am
Arc
EMWJUL
2:14 pm
Arc