Basic Memory Management Concepts
Address spaces
Physical address space — The address space
supported by the hardware
MAXsys
Ø  Starting at address 0, going to address MAXsys
Logical/virtual address space — A process’s
view of its own memory
Memory Management Basics
MAXprog
Program
P
Ø  Starting at address 0, going to address MAXprog
0
But where do addresses come from?
MOV r0, @0xfffa620e
0
1
2
Basic Concepts
Address generation
The compilation pipeline
Which is bigger, physical or virtual address
space?
0
Ø A. Physical address space
Ø B. Virtual address space
Ø C. It depends on the system.
1000
Library
Routines
Library
Routines
prog P
:
:
foo()
:
:
end P
0
P:
:
push ...
inc SP, 4
jmp 75
:
...
75
:
push ...
inc SP, x
jmp _foo
:
foo: ...
Compilation
100
Assembly
175
1100
:
:
:
jmp 175
:
...
Linking
1175
:
:
:
jmp 1175
:
...
Loading
3
4
Basic Concepts (Cont’d.)
Program Relocation
Address Translation
Program issues virtual addresses
Machine has physical addresses.
If virtual == physical, then how can we have multiple
programs resident concurrently?
Instead, relocate virtual addresses to physical at run
time.
MAXsys
MEMORY
EXCEPTION
CPU
Ø  While we are relocating, also bounds check addresses for
safety.
Logical
Addresses
Instructions
MAXprog
I can relocate that program (safely) in two registers…
Program
P’s
logical
address
space
0
5
no
≤
500
yes
+
Physical
Addresses
1000
1500 Program
P’s
physical
address
1000 space
Limit
Base
Register Register
0
6
Evaluating Dynamic Allocation Techniques
The fragmentation problem
External fragmentation
With base and bounds registers, the OS needs a hole
in physical memory at least as big as the process.
Ø  Unused memory between units of
allocation
Ø  E.g, two fixed tables for 2, but a party of 4
Ø  A. True
Ø  B. False
MAX
Internal fragmentation
Ø  Unused memory within a unit of allocation
Ø  E.g., a party of 3 at
a table for 4
Program Code
(“text”)
Program
Q’s
PAS
Execution
Data Stack
Execution Stack
Program
0 R’s PAS
7
Simple Memory Management Schemes
First Fit Allocation
Dynamic allocation of partitions
Simple approach:
Ø  Allocate a partition when a process is admitted
into the system
Ø  Allocate a contiguous memory partition to the
process
OS keeps track of...
Full-blocks
Empty-blocks (“holes”)
8
MAX Program
P1
To allocate n bytes, use the
first available free block such
that the block size is larger
than n.
Program
P2
P5
1K bytes
2K bytes
2K bytes
500 bytes
500 bytes
Program
P3
Allocation strategies
First-fit
Best-fit
Worst-fit
To allocate 400 bytes,
we use the 1st free block
available
Program
P4
0
9
Rationale & Implementation
Best Fit Allocation
Simplicity of implementation
To allocate n bytes, use the
smallest available free block
such that the block size is
larger than n.
Requires:
Ø  Free block list sorted by address
Ø  Allocation requires a search for a suitable partition
Ø  De-allocation requires a check to see if the freed partition could be
merged with adjacent free partitions (if any)
Advantages
◆ 
◆ 
Simple
Tends to produce larger
free blocks toward the end
of the address space
10
1K bytes
1K bytes
2K bytes
2K bytes
Disadvantages
◆ 
◆ 
Slow allocation
External fragmentation
To allocate 400 bytes,
we use the 3rd free block
available (smallest)
11
500 bytes
12
Rationale & Implementation
Worst Fit Allocation
To avoid fragmenting big free blocks
To allocate n bytes, use the
largest available free block
such that the block size is
larger than n.
To minimize the size of external fragments produced
Requires:
Ø  Free block list sorted by size
Ø  Allocation requires search for a suitable partition
Ø  De-allocation requires search + merge with adjacent free partitions,
if any
◆ 
1K bytes
2K bytes
Disadvantages
Advantages
◆ 
1K bytes
Works well when most
allocations are of small size
Relatively simple
◆ 
◆ 
◆ 
External fragmentation
Slow de-allocation
Tends to produce many
useless tiny fragments (not
really great)
To allocate 400 bytes,
we use the 2nd free block
available (largest)
500 bytes
Doug Lea’s malloc “In most ways this malloc is a best-fit
allocator”
13
Rationale & Implementation
Allocation strategies
To avoid having too many tiny fragments
First fit, best fit and worst fit all suffer from
external fragmentation.
Requires:
Ø A. True
Ø B. False
Ø  Free block list sorted by size
Ø  Allocation is fast (get the largest partition)
Ø  De-allocation requires merge with adjacent free partitions, if any,
and then adjusting the free block list
Advantages
◆ 
14
Disadvantages
Works best if allocations
are of medium sizes
◆ 
◆ 
◆ 
Slow de-allocation
External fragmentation
Tends to break large free
blocks such that large
partitions cannot be allocated
15
Dynamic Allocation of Partitions
Memory Management
Eliminating Fragmentation
Sharing Between Processes
Compaction
Ø  Relocate programs to coalesce holes
MAX Program
P1
  Swapping
Ø  Preempt processes & reclaim their memory
Ready
Suspended
suspended
queue
ready
queue
?
16
Ø  A single name space per process
Ø  No sharing
Run-Time
Program
Stack
P’s
VAS
Program
Data
How can one share code and data between
programs without paging?
Program
P3
Waiting
Heap
Schemes so far have considered only a single
address space per process
Program
P2
Running
semaphore/condition queues
2n-1
Program
Text
Program
P4
0
0
17
Program
P’s
VAS
18
Supporting Multiple Name Spaces
Multiple Name Spaces
Segmentation
Example — Protection/Fault isolation & sharing
2n-1
New concept: A segment — a memory “object”
2n -1
4
Heap
2n -1
Run-Time
Stack
A process now addresses objects —a pair (s, addr)
0
3
Ø  s — segment number
Ø  addr — an offset within an object
Run-Time
Stack
2n -1
Program
Data
2n -1
Program
Program 0
Text
Text
❖  Don’t
know size of object, so 32 bits for offset?
0
2
2n -1
Program
Data
6
0
1
0
Ø  A virtual address space
Heap
Libraries
n2
2 -1
0
0
0
n1
s
n5
User
Code
0
n
addr
Segment + Address register scheme
0
s
addr
Single address scheme
19
Implementing Segmentation
Memory Management Basics
Physical
Memory
Base + Limit register scheme
Are We Done?
Add a segment table containing base &
limit register values
Program
P
20
Segmentation allows sharing
… but leads to poor memory utilization
CPU
s
n
o
0 32
no
0
≤
Logical
Addresses
base
Limit
Register
500
yes
+
Ø  We might not use much of a large segment, but we must keep the
whole thing in memory (bad memory utilization).
Ø  Suffers from external fragmentation
Ø  Allocation/deallocation of arbitrary size segments is complex
Program
P’s
Segment
How can we improve memory management?
1000
Ø  Paging
Base
1000 Register
limit
s
STBR
1500
MEMORY
EXCEPTION
0
Segment Table
21
22