Transcript Topics in Embedded Systems

COM609 Topics in Embedded Systems
Lecture 1. Embedded Systems vs GeneralPurpose Systems
Prof. Taeweon Suh
Computer Science Education
Korea University
Embedded Systems
• Embedded System is a
special-purpose computer
system designed to perform
one or a few dedicated
functions -- Wikipedia
 In general, it does not provide
programmability to users, as
opposed to general purpose
computer systems like PC
 Embedded systems are virtually
everywhere in your daily life
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Embedded Systems (Cont)
• Even though embedded systems cover a
wide range of special-purpose systems,
there are common characteristics
 Low cost
• Should be cheap to be competitive
 Memory is typically very small compared to a
general purpose computer system
 Lightweight processors are used in embedded
systems
 Low power
• Should consume low power especially in case
of portable devices
• Low-power processors are used in embedded
systems
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Embedded Systems (Cont)
 High performance
• Should meet the computing requirements of
applications
 Users want to watch video on portable
devices
• Audio should be in sync with video
 Gaming gadgets like playstation should
provide high performance
 Real-time property
• Job should be done within a time limit
 Aerospace applications, Car control systems,
Medical gadgets are critical in terms of time
constraint – Otherwise, it could lead to
catastrophe such as loss of life
• Will talk more about this
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Embedded Systems (Cont)
• It is challenging to satisfy the characteristics
 You may not be able to achieve high performance
while maintaining low power consumption and
making use of cheap components
 So, you got to do your best in a given circumstance
to be competitive in the market
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HW/SW Stack of Embedded Systems
• Identical to the general-computer systems
Application Software
OS / Device Drivers
Hardware
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Components of Embedded Systems
• Hardware
 It is mainly composed of processor (1 or more), memory,
I/O devices including network devices, timers, sensors etc.
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Components of Embedded Systems
• Software
 System software
• Operating systems
 Many times, a multitasking (multithreaded) OS is required, as
embedded applications become complicated
• Networking, GUI, Audio, Video
• Processor is context-switched to process multiple jobs
 Operating system footprint should be small enough to fit into
memory of an embedded system
• In the past and even now, real-time operating systems (RTOS)
such as VxWorks or uC/OS-II have been used because they are
light-weighted in terms of memory requirement
• Nowadays, little heavy-weighted OSs such as Windows-CE or
embedded Linux (uClinux) are used, as embedded processors
support computing power and advanced capabilities such as MMU
(Memory Management Unit)
• Device drivers for I/O devices
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Components of Embedded Systems (Cont)
• Software (cont.)
 Application software
• Run on top of operating system
• Execute tasks that users wish to perform
 Web surfing, Audio, Video playback
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Real-Time System
• Real-time operating system (RTOS)
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Multitasking operating system intended for real-time applications
RTOS facilitates the creation of real-time systems
RTOS does not necessarily have a high throughput
RTOS is valued more for how quickly and/or predictably it can
respond to a particular event
• Hard real-time systems are required to complete a critical task
within a guaranteed amount of time
• Soft real-time systems are less restrictive
 Implementing real-time system requires a careful design of
scheduler
• System must have the priority-based scheduling
 Real-time processes must have the highest priority
 Priority inheritance (next slide)
• Solve the priority inversion problem
• Process dispatch latency must be small
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Priority Inversion Problem
• Pathfinder mission on Mars in 1997
 Used VxWorks, an RTOS kernel, from WindRiver
 Software problems caused the total system resets of the
Pathfinder spacecraft in mission
• Watchdog timer goes off, informing that something has gone
dramatically wrong and initiating the system reset
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Priority Inversion Problem
• VxWorks provides preemptive priority scheduling of threads
 Tasks on the Pathfinder spacecraft were executed as threads with
priorities that were assigned in the usual manner reflecting the
relative urgency of these tasks.
Task 1 tries to get the semaphore
Task 1 preempts Task3
Task 1 gets the semaphore
and execute
Priority Inversion
Task 1
(highest priority)
Task 2
(medium priority)
Task 2 preempts task 3
Task 3
(lowest priority)
Task 3 is resumed
Task 3 gets semaphore
Task 3 is resumed
Time
Task 3 releases the semaphore
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Priority Inheritance
•
A chain of processes could all be accessing resources that the high-priority
process needs
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
All these processes inherit the high priority until they are done with the resource
When they are finished, their priority reverts to its original value
Task 1 tries to get the semaphore
(Priority of Task 3 is raised to Task 1’s)
Task 1 preempts Task3
Priority Inversion
Task 1 completes
Task 1
(highest priority)
Task 2
(medium priority)
Task 3
(lowest priority)
Task 3 gets semaphore
Task 3 is resumed
with the highest priority
Time
Task 3 releases the semaphore
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Operating Systems for Embedded Systems
• RTOSs
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pSOS
VxWorks
VRTX (Versatile Real-Time Executive)
uC/OS-II
• Palm OS (source: Wikipedia)
 Embedded operating system initially developed by U.S. Robotics-owned
Palm Computing, Inc. for personal digital assistants (PDAs) in 1996
• Symbian OS (source: Wikipedia)
 Proprietary operating system designed for mobile devices by Symbian Ltd.
 A descendant of Psion's EPOC and runs exclusively on ARM processors
• Android
 Open Handset Alliance Project
 Based on Linux 2.6 kernel
 http://code.google.com/android/
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Operating Systems for Embedded Systems
•
Windows CE (WinCE) (source: Wikipedia)
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•
Microsoft's operating system for minimalistic computers and embedded systems
WinCE is a distinctly different operating system and kernel, rather than a trimmeddown version of desktop Windows
Embedded Linux (uClinux, ELKS, ThinLinux) (source: Wikipedia)
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The use of a Linux operating system in embedded computer systems
According to survey conducted by Venture Development Corporation, Linux was used
by 18% of embedded engineers
Embedded versions of Linux are designed for devices with relatively limited resources,
such as cell phones and set-top boxes
Due to concerns such as cost and size, embedded devices usually have much less
RAM and secondary storage than desktop computers, and are likely to use flash
memory instead of a hard drive
Since embedded devices are used for specific purposes rather than general purposes,
developers optimize their embedded Linux distributions to target specific hardware
configurations and usage situations
• These optimizations can include reducing the number of device drivers and software
applications, and modifying the Linux kernel to be a real-time operating system

Instead of a full suite of desktop software applications, embedded Linux systems often
use a small set of free software utilities such as busybox, and replace the glibc C
standard library with a more compact alternative such as dietlibc, uClibc, or Newlib.
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Embedded System Design Flow
ASIC/SoC design
Planning & Architect
(modeling &
simulation)
System prototype
board
Hardware Design
with CAD tools
Hardware debugging &
Software development
 ASIC: Application-Specific Integrated Circuit
 SoC: System-on-Chip
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ASIC/SoC chip
Final product
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A General-Purpose Computer System
(till 2008)
CPU
Main
Memory
(DDR2)
FSB
(Front-Side Bus)
North
Bridge
Graphics
card
Peripheral
devices
DMI
(Direct Media I/F)
Hard disk
USB
South
Bridge
PCIe card
But, don’t forget the big picture!
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Present, Near Future and More…
•
•
Core 2 Duo – based Systems
CPU
CPU
FSB
(Front-Side Bus)
Main
Memory
(DDR2)
Quickpath (Intel) or
Hypertransport (AMD)
IOH
(Input/Output
Hub)
North
Bridge
DMI
(Direct Media I/F)
Core i7– based Systems
South
Bridge
DMI
(Direct Media I/F)
Main
Memory
(DDR3)
South
Bridge
Keep in mind that CPU and computer systems are evolving at a fast pace
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x86 History (as of 2008)
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x86 History (Cont.)
4-bit
32-bit
(i586)
8-bit
32-bit
(i386)
16-bit
32-bit
(i686)
64-bit
(x86_64)
2009
Core i7
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x86?
• What is x86?
 Generic term referring to processors from Intel, AMD and VIA
 Derived from the model numbers of the first few generations of
processors:
• 8086, 80286, 80386, 80486  x86
 Now it generally refers to processors from Intel, AMD, and VIA
• x86-16: 16-bit processor
• x86-32 (aka IA32): 32-bit processor
• x86-64: 64-bit processor
* IA: Intel Architecture
• Intel takes about 80% of the PC market and AMD takes
about 20%
 Apple also have been introducing Intel-based Mac from Nov. 2006
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Chipset
•
We call North and South Bridges as Chipset
•
Chipset has many PCIe devices inside
•
North Bridge
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Memory controller
PCI express ports to connect Graphics card
http://www.intel.com/Assets/PDF/datasheet/316966.pdf
South Bridge
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HDD (Hard-disk) controller
USB controller
Various peripherals connected
• Keyboard, mouse, timer etc
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PCI express ports
http://www.intel.com/Assets/PDF/datasheet/316972.pdf
Note that the landscape is being changed!

For example, memory controller is integrated into CPU
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PCI, PCI Express Devices
•
PCI (Peripheral Component Interconnect)
•
PCIe (PCI Express)
 Computer bus connecting all the peripheral devices to the computer
motherboard
 Replaced PCI in 2004
 Point-to-point connection
PCI express slots
PCI slot
PCI express slot
x16
http://www.pcisig.com/specifications/pciexpress/
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An Old GP Computer System Example
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PCI Express Slots in GP Systems
PCI express
slot
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GP Computer System in terms of PCIe
North Bridge
South Bridge
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Core i7-based Systems
• Core i7 860 (Lynnfield)
– based system
• Core i7 920 (Bloomfield)
– based system
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Software Stack
Applications
(MS-office, Google Earth…)
API
(Application Program I/F)
Operating System
(Linux, Vista, Mac OS …)
BIOS provides
common I/Fs
BIOS
(AMI, Phoenix Technologies …)
Computer Hardware
(CPU, Chipset, PCIe cards ...)
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How the GP Computer System Works?
• x86-based system starts to execute from the reset address
0xFFFF_FFF0
 The first instruction is “jmp xxx” off from BIOS ROM
• BIOS (Basic Input/Output System)
 Detect and initialize all the devices (including PCI devices via PCI
enumeration) on the system
 Provide common interfaces to OS
 Hand over the control to OS
• OS
 Manage the system resources including main memory
• Control and coordinate the use of the hardware among various
application programs for the various users
 Provide APIs for system and application programming
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So… What?
• How is it different from embedded systems?
 General-purpose computer systems provide
programmability to end-users
• You can do any kinds of programming on your PC
 C, C++, C#, Java etc
 General-purpose systems should provide backward
compatibility
• A new system should be able to run legacy software, which
could be in the form of binaries with no source codes written
30 years ago
 So, general purpose computer system becomes messy and
complicated, still containing all legacy hardware functionalities
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x86 Operation Modes
• Real Mode (= real address mode)
 Programming environment of the 8086 processor
 8086 is a 16-bit processor from Intel
• Protected Mode
 Native state of the 32-bit Intel processor
• For example, Windows is running in protected mode
 32-bit mode
• IA-32e mode (64-bit mode)
 There are 2 sub modes
•
•
Compatibility mode
64-bit mode
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Registers in 8086
• Registers inside the 8086
 16-bit segment registers
• CS, DS, SS, ES
 General-purpose registers
• all 16-bits
• AX, BX, CX, DX, SP, BP, SI, DI
• Registers in x86-32
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Real Mode Addressing
•
In real mode (8086), general purpose registers are all 16-bit wide
•
Real model
 Segment registers specify the base address of each segment
 Segment registers
•
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CS: Code Segment -> used to store instructions
DS: Data Segment -> used to store data
SS: Stack Segment -> stack
ES: Extra Segment -> could be used to store more data
 Addressing method
• Segment << 4 + offset = physical address
• Example:
mov ax, 2000h
mov ds, ax
 Data segment starts from 20000h (2000h << 4)
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Data Segment in Real Mode
• Memory addressing in real mode (8086)
0xFFFFF
mov ax, 2000h
mov ds, ax
mov al, [100h]
offset
DS
Main
Memory
(1MB)
100h
20100h
20000h = 2000h << 4
2000h
0x0
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A20M
• 8088/8086 allowed only 1MB memory access since
they have only 20-bit physical address lines
 220 = 1MB
• Memory is accessed with segment:offset in
8086/8088 (still the same though)
 What if CS=0xFFFF, IP=0x0020?
• CS << 4 + IP = 0x100010
• But, we have only 20 address lines. So, 8088 ends up accessing
0x00010 ignoring the “1” in A21
• Some (weird?) programmers took advantage of this
mechanism
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A20M (Cont)
• How about now?
 Your Core 2 Duo has 48-bit physical address lines
 What happens if there is no protection in the previous case
• Processor will access 0x100010, breaking the legacy code
 So, x86 provides a mechanism called A20M (A20 Mask)
to make it compatible with the old generations
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A20M (Cont)
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Another Example
• Protected mode addressing (32-bit)
 As application programs become larger, 1MB main
memory is too small
 Intel introduced protected mode to address a larger
memory (up to 4GB)
 But, Intel still wants to use 16-bit segment registers
for the backward compatability
 How to access a 4GB space with a 16-bit register?
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Protected Mode Addressing
15
3
Segment Selector Index
2
T
I
10
R
P
L
TI = 1
TI = 0
GDT
LDT
Segment
Descriptor
Segment
Descriptor
Visible to software
Hardware
Inside the CPU
(Registers)
Segment
Descriptor
Invisible to software
31
0 19
Base
0
Limit
Access
info
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Main
memory
Segment
Descriptor
Segment
Descriptor
Segment
Descriptor
Segment
Descriptor
Segment
Descriptor
Segment
Descriptor
Segment
Descriptor
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Segment Descriptor Format
• Software (OS) creates descriptor tables (GDT, LDT)
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Address Translation in Protected Mode
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One More Example
• 8259 Interrupt Controller
CPU
Main
Memory
(DDR)
FSB
(Front-Side Bus)
Still in South Bridge
North
Bridge
82C59A
(Master)
DMI
(Direct Media I/F)
South
Bridge
82C59A
(Slave)
IR0
IR1
IR2
IR3
IR4
IR5
IR6
IR7
INTR
IR0
IR1
IR2
IR3
IR4
IR5
IR6
IR7
CPU (8086)
INTR
INTR
INTA
INTA
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