Topics in Embedded Systems

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COM609 Topics in Embedded Systems
Lecture 1. Embedded Systems vs General-Purpose
Systems
Prof. Taeweon Suh Computer Science
Education Korea University
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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

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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)
• 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 gets the semaphore and execute
Task 1 preempts Task3
Priority Inversion
Task 1 (highest priority)
Task 2 (medium priority)
Task 2 preempts task 3
Task 3 (lowest priority)
Task 3 is resumed
Time
Task 3 gets semaphore
Task 3 is resumed
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
• 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 1s)
Task 1 preempts Task3
Task 1 completes
Priority Inversion
Task 1 (highest priority)
Task 2 (medium priority)
Task 3 (lowest priority)
Time
Task 3 gets semaphore
Task 3 is resumed with the highest priority
Task 3 releases the semaphore
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Operating Systems for Embedded Systems

• RTOSs
• 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)
• Microsoft's operating system for minimalistic
  computers and embedded systems
• WinCE is a distinctly different operating system
  and kernel, rather than a trimmed-down version of
  desktop Windows
• Embedded Linux (uClinux, ELKS, ThinLinux)
  (source Wikipedia)
• 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 Application-Specific Integrated Circuit
• SoC System-on-Chip

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A General-Purpose Computer System (till 2008)
CPU
Main Memory (DDR2)
FSB (Front-Side Bus)
North Bridge
Graphics card
DMI (Direct Media I/F)
Peripheral devices
South Bridge
Hard disk
USB
PCIe card
But, dont forget the big picture!
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Present, Near Future and More

• Core i7 based Systems

• Core 2 Duo based Systems

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.)
32-bit (i386)
8-bit
16-bit
4-bit
32-bit (i586)
64-bit (x86_64)
32-bit (i686)
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 IA
  Intel Architecture
• x86-64 64-bit processor
• 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
• Memory controller
• PCI express ports to connect Graphics card
• http//www.intel.com/Assets/PDF/datasheet/316966.p
  df
• South Bridge
• HDD (Hard-disk) controller
• USB controller
• Various peripherals connected
• Keyboard, mouse, timer etc
• PCI express ports
• http//www.intel.com/Assets/PDF/datasheet/316972.p
  df
• 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)
• Computer bus connecting all the peripheral
  devices to the computer motherboard
• PCIe (PCI Express)
• Replaced PCI in 2004
• Point-to-point connection

PCI express slot x16
PCI express slots
PCI slot
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
• CS Code Segment -gt used to store instructions
• DS Data Segment -gt used to store data
• SS Stack Segment -gt stack
• ES Extra Segment -gt could be used to store more
  data
• Addressing method
• Segment ltlt 4 offset physical address
• Example
• mov ax, 2000h
• mov ds, ax
• ? Data segment starts from 20000h (2000h ltlt 4)

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Data Segment in Real Mode

• Memory addressing in real mode (8086)

0xFFFFF
Main Memory (1MB)
mov ax, 2000h mov ds, ax mov al, 100h
20100h
100h
offset
2000h
DS
20000h 2000h ltlt 4
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 segmentoffset in
  8086/8088 (still the same though)
• What if CS0xFFFF, IP0x0020?
• CS ltlt 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
Hardware Inside the CPU (Registers)
Main memory
GDT
LDT
Segment Descriptor
Segment Descriptor
Segment Descriptor
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

Still in South Bridge