Lecture 1. x86 General-purpose vs Embedded Systems

Prof. Taeweon SuhComputer Science Education

Korea University

COM509 Computer Systems

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Embedded Systems

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• 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 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

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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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OS / Device Drivers

Hardware

Application Software

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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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A General-Purpose Computer System (as of 2008)

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CPU

North Bridge

South Bridg

e

Main Memor

y(DDR2)

FSB (Front-Side Bus)

DMI (Direct Media I/F)

Hard disk

USB

PCIe card

Peripheral devices

Graphics card

But, don’t forget the big picture!

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Present and More…

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Keep in mind that CPU and computer systems are evolving at a fast pace

CPU

North Bridge

South Bridge

Main Memor

y(DDR2)

FSB (Front-Side Bus)

DMI (Direct Media I/F)

• Core 2 Duo – based Systems

• Core i7– based Systems

CPU

IOH (Input/Outpu

t Hub)

South Bridge

Main Memor

y(DDR3)

DMI (Direct Media I/F)

Quickpath (Intel) orHypertransport (AMD)

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x86 History (as of 2008)

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x86 History (Cont.)

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32-bit (i386)

32-bit (i586)

64-bit (x86_64)

32-bit (i686)

8-bit 16-bit 4-bit

Core i7 (Nehalem

)

2009

Sandy Bridge

2011

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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.pdf

• 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.pdf

• Note that the landscape is being changed! For example, memory controller is integrated into CPU

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PCI, PCI Express Devices

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• 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 slotPCI express slots 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

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PCI express slot

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GP Computer System in terms of PCIe

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North Bridge

South Bridge

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Core i7-based Systems

• Core i7 860 (Lynnfield) – based system

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• Core i7 920 (Bloomfield) – based system

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Software Stack

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Computer Hardware(CPU, Chipset, PCIe cards ...)

BIOS (AMI, Phoenix Technologies

…)

Operating System(Linux, Vista, Mac OS …)

Applications(MS-office, Google Earth…)

API (Application Program I/F)

BIOS provides common I/Fs

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How the GP Computer System Works?

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• 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 -> 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, 2000hmov ds, ax Data segment starts from 20000h (2000h << 4)

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Data Segment in Real Mode

• Memory addressing in real mode (8086)

Main Memory(1MB)

2000hDS

mov ax, 2000hmov ds, axmov al, [100h]

100hoffset

0x0

0xFFFFF

20000h = 2000h << 4

20100h

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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

IndexSegment Selector TI

RPL

15 3 2 1 0

GDT LDT

TI = 0 TI = 1

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

Segment Descriptor

BaseAccess

info

31 0 19 0

Limit

HardwareInside the CPU

(Registers)

Mainmemory

Visible to software

Invisible to software

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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

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IR0IR1IR2IR3IR4IR5IR6IR7

INTR

IR0IR1IR2IR3IR4IR5IR6IR7

INTR

INTA

INTA

INTR82C59A(Master)

82C59A(Slave)

CPU (8086)

CPU

North Bridge

South Bridge

Main Memory(DDR)

FSB (Front-Side Bus)

DMI (Direct Media I/F)

Still in South Bridge

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Backup Slides

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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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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

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Planning & Architect

(modeling & simulation)

Hardware Design with CAD tools

ASIC/SoC designASIC/SoC

chip

System prototype board

Hardware debugging &

Software development

Final product

ASIC: Application-Specific Integrated Circuit SoC: System-on-Chip

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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.

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Task 3(lowest priority)

Task 2(medium priority)

Task 1(highest priority)

TimeTask 3 gets semaphore

Task 1 preempts Task3

Task 1 tries to get the semaphore

Task 3 is resumed

Task 2 preempts task 3

Task 3 is resumed

Task 3 releases the semaphore

Task 1 gets the semaphore and execute

Priority Inversion

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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

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Task 3(lowest priority)

Task 2(medium priority)

Task 1(highest priority)

TimeTask 3 gets semaphore

Task 1 preempts Task3

Task 1 tries to get the semaphore(Priority of Task 3 is raised to Task 1’s)

Task 3 is resumed with the highest priority

Task 1 completesPriority Inversion

Task 3 releases the semaphore