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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 2 Korea Univ 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 3 Korea Univ 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 4 Korea Univ 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 5 Korea Univ HW/SW Stack of Embedded Systems • Identical to the general-computer systems Application Software OS / Device Drivers Hardware 6 Korea Univ Components of Embedded Systems • Hardware It is mainly composed of processor (1 or more), memory, I/O devices including network devices, timers, sensors etc. 7 Korea Univ 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 8 Korea Univ 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 9 Korea Univ 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 10 Korea Univ 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 11 Korea Univ 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 12 Korea Univ 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 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 13 Korea Univ 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/ 14 Korea Univ 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 trimmeddown 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. 15 Korea Univ 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 16 ASIC/SoC chip Final product Korea Univ 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! 17 Korea Univ 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 18 Korea Univ x86 History (as of 2008) 19 Korea Univ 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 20 Korea Univ 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 21 Korea Univ 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 22 Korea Univ 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/ 23 Korea Univ An Old GP Computer System Example 24 Korea Univ PCI Express Slots in GP Systems PCI express slot 25 Korea Univ GP Computer System in terms of PCIe North Bridge South Bridge 26 Korea Univ Core i7-based Systems • Core i7 860 (Lynnfield) – based system • Core i7 920 (Bloomfield) – based system 27 Korea Univ 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 ...) 28 Korea Univ 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 29 Korea Univ 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 30 Korea Univ 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 31 Korea Univ 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 32 Korea Univ 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, 2000h mov ds, ax Data segment starts from 20000h (2000h << 4) 33 Korea Univ 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 34 Korea Univ 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 35 Korea Univ 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 36 Korea Univ A20M (Cont) 37 Korea Univ 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? 38 Korea Univ 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 39 Main memory Segment Descriptor Segment Descriptor Segment Descriptor Segment Descriptor Segment Descriptor Segment Descriptor Segment Descriptor Korea Univ Segment Descriptor Format • Software (OS) creates descriptor tables (GDT, LDT) 40 Korea Univ Address Translation in Protected Mode 41 Korea Univ 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 42 Korea Univ