Memory Management
Jordan University of Science & Technology
CPE 746 Embedded Real-Time Systems
Prepared By: Salam Al-Mandil & Hala Obaidat
Supervised By: Dr. Lo’ai Tawalbeh
Outline
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Introduction
Common Memory Types
Composing Memory
Memory Hierarchy
Caches
Application Memory Management
Static memory management
Dynamic memory management
Memory Allocation
The problem of fragmentation
Memory Protection
Recycling techniques
Introduction
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An embedded system is a special-purpose
computer system designed to perform one or
a few dedicated functions, sometimes with
real-time computing constraints.
An embedded system is part of a larger
system.
Embedded systems often have small memory
and are required to run a long time, so
memory management is a major concern
when developing real-time applications.
Common Memory Types
RAM
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DRAM:
Volatile memory.
Address lines are multiplexed. The 1st half is
sent 1st & called the RAS. The 2nd half is sent
later & called CAS.
A capacitor and a single transistor / bit =>
better capacity.
Requires periodical refreshing every 10-100
ms => dynamic.
Cheaper / bit => lower cost.
Slower => used for main memory.
Reading DRAM Super cell (2,1)
Step 1(a): Row access strobe (RAS)
selects row 2.
Step 1(b): Row 2 copied from DRAM array to
row buffer.
16 x 8 DRAM chip
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cols
0
RAS = 2
2
/
1
2
3
0
addr
1
rows
memory
controller
2
8
/
3
data
internal row buffer
Reading DRAM Super cell (2,1)
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Step 2(a): Column access strobe (CAS)
selects column 1.
Step 2(b): Super cell (2,1) copied from buff
to data lines, and eventually back to the
16 x 8 DRAM chip
CPU.
cols
0
CAS = 1
2
/
2
3
0
addr
To CPU
1
rows
memory
controller
super cell
(2,1)
1
2
8
/
3
data
super cell
(2,1)
internal row buffer
RAM
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SRAM:
Volatile memory.
Six transistors / bit => lower capacity.
No refreshing required => faster & lower
power consumption.
More expensive / bit => higher cost.
Faster => used in caches.
Some Memory Types
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ROM:
Non-volatile memory.
Can be read from but not written to, by a
processor in an embedded system.
Traditionally written to, “programmed”,
before inserting to embedded system.
Stores constant data needed by system.
Horizontal lines = words, vertical lines =
data.
Some embedded systems work without
RAM, exclusively on ROM, because their
programs and data are rarely changed.
Some Memory Types
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Flash Memory:
Non-volatile memory.
Can be electrically erased & reprogrammed.
Used in memory cards, and USB flash drives.
It is erased and programmed in large blocks
at once, rather than one word at a time.
Examples of applications include PDAs and
laptop computers, digital audio players,
digital cameras and mobile phones.
Type
Volatile?
Writeable?
Erase Size
Max Erase
Cycles
Cost (per
Byte)
Speed
SRAM
Yes
Yes
Byte
Unlimited
Expensive
Fast
DRAM
Yes
Yes
Byte
Unlimited
Moderate
Moderate
Masked
ROM
No
No
n/a
n/a
Inexpensive
Fast
No
Once, with a
device
program
mer
n/a
n/a
Moderate
Fast
No
Yes, with a
device
program
mer
Moderate
Fast
Expensive
Fast to read,
slow to
erase/wri
te
Moderate
Fast to read,
slow to
erase/wri
te
PROM
EPROM
EEPROM
Flash
NVRAM
No
No
No
Yes
Yes
Yes
Entire Chip
Limited
(consult
datasheet
)
Byte
Limited
(consult
datasheet
)
Sector
Limited
(consult
datasheet
)
Byte
Unlimited
Expensive
(SRAM +
battery)
Fast
Composing Memory
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When available memory is larger, simply
ignore unneeded high-order address bits and
higher data lines.
When available memory is smaller, compose
several smaller memories into one larger
memory:
Connect side-by-side.
Connect top to bottom.
Combine techniques.
Connect side-by-side
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To increase width of words.
2m × 3n ROM
2m × n ROM
enable
2m × n ROM
2m × n ROM
Increase width of words
A0
…
…
…
Am
…
Q3n-1
…
Q2n-1
…
Q0
Connect top to bottom
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To increase number of words.
Increase number of words
2m+1 × n ROM
2m × n ROM
A0
Am-1
…
…
1 × 2 decoder
…
Am
2m × n ROM
enable
…
…
…
Qn-1
Q0
Combine techniques
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To increase number and width of words.
A
Increase number and width of words
enable
outputs
Memory Hierarchy
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Is an approach for organizing memory and
storage systems.
A memory hierarchy is organized into several
levels – each smaller, faster, & more
expensive / byte than the next lower level.
For each k, the faster, smaller device at level
k serves as a cache for the larger, slower
device at level k+1.
Programs tend to access the data at level k
more often than they access the data at level
k+1.
An Example Memory Hierarchy
L0:
registers
Smaller,
faster,
and
costlier
(per byte)
storage
devices
L1: on-chip L1
cache (SRAM)
L2:
L3:
Larger,
slower,
and
cheaper
(per byte)
storage
devices
L5:
L4:
CPU registers hold words retrieved
from L1 cache.
off-chip L2
cache (SRAM)
L1 cache holds cache lines retrieved
from the L2 cache memory.
L2 cache holds cache lines
retrieved from main memory.
main memory
(DRAM)
Main memory holds disk
blocks retrieved from local
disks.
local secondary storage
(local disks)
Local disks hold files
retrieved from disks on
remote network servers.
remote secondary storage
(distributed file systems, Web servers)
Caches
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Cache: The first level(s) of the memory
hierarchy encountered once the address
leaves the CPU.
The term is generally used whenever
buffering is employed to reuse commonly
occurring items such as webpage caches, file
caches, & name caches.
Caching in a Memory Hierarchy
Level k:
8
4
9
10
4
Level k+1:
14
10
3
Smaller, faster, more expensive
device at level k caches a
subset of the blocks from level k+1
Data is copied between
levels in block-sized transfer
units
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Larger, slower, cheaper storage
device at level k+1 is partitioned
into blocks.
General Caching Concepts
14
12
Level
k:
0
1
2
3
4*
12
9
14
3
12
4*
Level
k+1:
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Request
12
14
Request
12
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0
1
2
3
4
4*
5
6
7
8
9
10
11
12
13
14
15
Program needs object d, which is
stored in some block b.
Cache hit
 Program finds b in the cache
at level k. E.g., block 14.
Cache miss
 b is not at level k, so level k
cache must fetch it from level
k+1.
E.g., block 12.
 If level k cache is full, then
some current block must be
replaced (evicted). Which one
is the “victim”? We’ll see later.
Cache Placement
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1.
2.
3.
There are 3 categories of cache
organization:
Direct-mapped.
Fully-associative.
Set-associative.
Direct-Mapped
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The block can appear in 1 place only.
Fastest & simplest organization but highest
miss rate due to contention.
Mapping is usually: Block address % Number
of blocks in cache.
Tag
Index
Offset
V T D
Data
Valid
=
Fully-Associative
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The block can appear anywhere in the cache.
Slowest organization but lowest miss rate.
Tag
Offset
Data
V T D
V T D
V T D
…
Valid
=
=
=
Set-Associative
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The block can appear anywhere within a
single set. (n-way set associative)
The set number is usually: Block address %
Number of sets in the cache.
Tag
Index
V T D
Offset
V T D
Data
Valid
=
=
Cache Replacement
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In a direct-mapped cache, only 1 block is
checked for a hit, & only that block can be
replaced.
For set-associative or fully-associative caches,
the evicted block is chosen using three
strategies:
Random.
LRU.
FIFO.
Cache Replacement
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As the associativity increases => LRU harder
& more expensive to implement => LRU is
approximated.
LRU & random perform almost equally for
larger caches. But LRU outperforms others for
small caches.
Write Policies
1.
1.
Write Back: the information is only written
to the block in the cache.
Write Through: the information is written to
both the block in the cache & to the block in
lower levels.
Reducing the Miss Rate
1.
2.
Larger Block Sizes & Caches.
Higher Associativity.
Application Memory Management
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Allocation: to allocate portions of memory to
programs at their request.
Recycling: freeing it for reuse when no longer
needed.
Memory Management
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In many embedded systems, the kernel and
application programs execute in the same
space i.e., there is no memory protection.
The embedded operating systems thus make
large effort to reduce its memory occupation
size.
Memory Management
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An RTOS uses small memory size by
including only the necessary
functionality for an application.
We have two kinds of memory
management:
Static
Dynamic
Static memory management
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provides tasks with temporary data
space.
The system’s free memory is divided
into a pool of fixed sized memory
blocks.
When a task finishes using a memory
block it must return it to the pool.
Static memory management
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Another way is to provide temporary
space for tasks is via priorities:
A high priority pool : is sized to have
the worst-case memory demand of the
system
A low priority pool : is given the
remaining free memory.
Dynamic memory management
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employs memory swapping, overlays,
multiprogramming with a fixed number
of tasks (MFT), multiprogramming with
a variable number of tasks (MVT) and
demand paging.
Overlays allow programs larger than the
available memory to be executed by
partitioning the code and swapping
them from disk to memory.
Dynamic memory management
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MFT: a fixed number of equalized code
parts are in memory at the same time.
MVT: is like MFT except that the size of
the partition depends on the needs of
the program.
Demand paging : have fixed-size pages
that reside in non-contiguous memory,
unlike those in MFT and MVT
Memory Allocation
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is the process of assigning blocks of
memory on request .
Memory for user processes is divided
into multiple partitions of varying sizes.
Hole : is a block of available memory.
Static memory allocation
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means that all memory is allocated to each
process or thread when the system starts up.
In this case, you never have to ask for
memory while a process is being executed.
This is very costly.
The advantage of this in embedded systems
is that the whole issue of memory-related
bugs-due to leaks, failures, and dangling
pointers-simply does not exist .
Dynamic Storage-Allocation
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How to satisfy a request of size n from
a list of free holes. This means that
during runtime, a process is asking the
system for a memory block of a certain
size to hold a certain data structure.
Some RTOSs support a timeout function
on a memory request. You ask the OS
for memory within a prescribed time
limit.
Dynamic Storage-Allocation Schemes
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First-fit: Allocate the first hole that is
big enough, so it is fast
Best-fit: Allocate the smallest hole that
is big enough; must search entire list,
unless ordered by size.
Buddy: it divides memory into
partitions to try to satisfy a memory
request as suitably as possible.
Buddy memory allocation
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allocates memory in powers of 2
it only allocates blocks of certain sizes
has many free lists, one for each
permitted size
How buddy works?
If memory is to be allocated
1-Look for a memory slot of a suitable size (the
minimal 2k block that is larger then the
requested memory)
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If it is found, it is allocated to the program
If not, it tries to make a suitable memory slot. The
system does so by trying the following:
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Split a free memory slot larger than the requested
memory size into half
If the lower limit is reached, then allocate that amount of
memory
Go back to step 1 (look for a memory slot of a suitable
size)
Repeat this process until a suitable memory slot is found
How buddy works?
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1.
2.
3.
If memory is to be freed
Free the block of memory
Look at the neighboring block - is it
free too?
If it is, combine the two, and go back
to step 2 and repeat this process until
either the upper limit is reached (all
memory is freed), or until a non-free
neighbor block is encountered
Example: buddy system
64K
64K
64K
64K
64K
64K
64K
64K
64K
t=0
1024K
t=1
A64K
64K
128K
256K
512K
t=2
A64K
64K
B-128K
256K
512K
t=3
A64K
C64K
B-128K
256K
512K
t=4
A64K
C64K
B-128K
D-128K
128K
512K
t=5
A64K
64K
B-128K
D-128K
128K
512K
t=6
128K
B-128K
D-128K
128K
512K
t=7
256K
D-128K
128K
512K
t=8
1024K
64K
64K
64K
64K
64K
64K
64
K
The problem of fragmentation
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neither first fit nor best fit is clearly
better that the other in terms of storage
utilization, but first fit is generally faster.
All the previous schemes has external
fragmentation.
the buddy memory system has little
external fragmentation.
Fragmentation
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External Fragmentation–total
memory space exists to satisfy a
request, but it is not contiguous.
Internal Fragmentation–allocated
memory may be slightly larger than
requested memory; this size difference
is memory internal to a partition, but
not being used.
Example: Internal Fragmentation
Memory Protection
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it may not be acceptable for a hardware
failure to corrupt data in memory. So, use of
a hardware protection mechanism is
recommended.
This hardware protection mechanism can be
found in the processor or MMU.
MMUs also enable address translation, which
is not needed in RT because we use crosscompilers that generate PIC code (Position
Independent Code).
Hardware Memory Protection
Recycling techniques
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There are many ways for automatic
memory managers to determine what
memory is no longer required
garbage collection relies on determining
which blocks are not pointed to by any
program variables .
Recycling techniques
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Tracing collectors : Automatic
memory managers that follow pointers
to determine which blocks of memory
are reachable from program variables.
Reference counts : is a count of how
many references (that is, pointers)
there are to a particular memory block
from other blocks .
Example : Tracing collectors
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Mark-sweep collection:
Phase1: all blocks that can be reached by
the program are marked.
Phase2: the collector sweeps all allocated
memory, searching for blocks that have not
been marked. If it finds any, it returns them
to the allocator for reuse.
Mark-sweep collection
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The drawbacks :
It must scan the entire
memory in use before
any memory can be
freed.
It must run to completion
or, if interrupted, start
again from scratch.
Example : Reference counts
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Simple reference counting :
a reference count is kept for each object.
The count is incremented for each new reference,
decremented if a reference is overwritten, or if the
referring object is recycled.
If a reference count falls to zero, then the object is
no longer required and can be recycled.
it is hard to implement efficiently because of the cost
of updating the counts.
References:
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http://www.memorymanagement.org/articles
/recycle.html
http://www.dedicated-systems.com
http://www.Wikipedia.org
http://www.cs.utexas.edu
http://www.netrino.com
S. Baskiyar,Ph.D. and N.Meghanathan,A
Survey of Contemporary Real-time Operating
Systems.