Porting Operating Systems Phan Duy Hùng (PhD) ES Lecturer – Hanoi FPT University

Bootloader Kernel Filesystem Bootloader (again) 2 Content

Embedded Linux Introduction Linux Support CPU: x86, ARM, MIPS, SH, i960, PowerPC, …etc Open Source Powerful Network Functions Easy Software Porting GNU Complier and Debugger

4 Cross-Development Environment Target has limited resource (memory, storage, low speed processor). Host and Target are different architecture Cross-platform Development Environment Host Bootloader Kernel Root Filesystem Target

Ways of Hardware Connection

Linux Architecture Applications Bash Shell Linux Kernel hardware

Linux Architecture Hardware BootloaderDrivers Kernel File System Protocol Stacks Kernel System Call Interface GLIBCOther Libraries Java GUI Applications

8 All the things we need Cross-Platform development toolchain Bootloader Usually provided by the vendor Linux kernel Linux kernel + some patches if needed Filesystem Busybox Device node Configuration

9 Bootloader Concepts (cont`) Typical space allocation

10 Bootloader Concepts What is bootloader ? Load kernel and filesystem Initialize hardware elements Many bootloaders for each architecture LILO and GRUB for x86 LILO: GRUB: U-Boot for x86, ARM, PowerPC U-Boot:

11 Bootloader Concepts (cont`) Bootloader also depends on various hardware All type CPU will execute the instructions from the defined address when power on Normally, there exists some solid storage devices mapped to defined address ROM, EEPROM, FLASH, etc

12 The Bootloader's Job One main mission: load the operating system(s). Tasks: Initialize the machine properly (the kernel can do this later too). Access the kernel and initrd files in their storage medium (need to support the corresponding filesystem too) Because of the above 2 tasks, bootloaders may platform specific! Load the kernel and initrd files Execute the kernel file with the right command line

bootloader

14 Some Bootloaders LILO: LInux LOad. Original Linux bootloader. Still in use! Supports: x86 GRUB: GRand Unified Bootloader from GNU. More powerful. Supports: x86 LinuxBIOS: Linux based BIOS replacement Supports: x86

15 Some Bootloaders (cont.) UBoot: Universal Bootloader. The most used on arm. Supports: arm, ppc, mips, x86 RedBoot: eCos based bootloader from RedHat Supports: x86, arm, ppc, mips, sh, m68k...

16 Why Use Linux Open Source Reliability Salability Secure Supports Virtually All Network Communication Protocols

17 GCC Cross Compiler creates executables for another platform (such as ARM), the output form can be ELF (Executable and Linking Format) If you build a Linux Kernel, you have to use Cross Compiler, i.e. GCC for most cases

18 GCC = GNU Compiler Collection Features Portable Cross-compiling Support different language C, C++, Objective-C, Fortran, Java, and Ada Free Software Compiler: arm-linux-gcc compiling source codes to run on arm platforms $ arm-linux-gcc hello.c –o hello /usr/local/arm/include/asm,linux

System Boot Flow

20 System Storage The storage devices used in embedded systems are often quite different from those used in workstations and servers. Embedded systems tend to use solid-state storage devices such as flash chips and flash disks. Need the MTD subsystem to support bootloader boot parameters kernel root filesystem

Kernel Considerations Kernel Configuration Kernel Compiling

22 Root File System Content

Root File System / /bin /boot /grub /dev /etc /home /lib /mnt /proc /sbin /tmp /usr /src /bin /sbin /var /log /spool

24 Building Root Filesystem Download the BusyBox source code from Building BusyBox make clean make all make install After make complete, the busybox will be at./path/to/busybox_src/_install/

25 Graphic User Interface Window System X Window (TinyX) QPE (Qt Plamtop Environment) / Qtopia GPE (GPE Palmtop Environment) Microwindows MiniGUI

Development Tools Editor (VIM) Toolchains

27 Bootloader Concepts What is bootloader ? Load kernel and filesystem Initialize hardware elements Many bootloaders for each architecture LILO and GRUB for x86 LILO: GRUB: U-Boot for x86, ARM, PowerPC U-Boot:

28 Linux-capable open source bootloaders

29 Bootloader Concepts (cont`) Bootloader also depends on various hardware All type CPU will execute the instructions from the defined address when power on Normally, there exists some solid storage devices mapped to defined address ROM, EEPROM, FLASH, etc

30 Bootloader Concepts (cont`) Typical space allocation

31 Bootloader Concepts (cont`) Single stage or Multi-stage ? Multi-stage bootloader can provide more complex capability and better ability to cross platform Bootloader booted from solid storage device can be divided into two stages Stage1 –Written by assembly language –Fast and simple Stage 2 –Written by C –more complex capability –Easy to read and modify

32 Bootloader Concepts (cont`) Two operation modes Boot loading mode (autonomous mode) Normal working mode Downloading mode Bootloader can download images from host end through UART or network

33 Bootloader Concepts (cont`) Communication equipment and protocol UART (Universal Asynchronous Receiver/Transmitter) and network UART protocol: xmodem, ymodem, zmodem slower Network protocol: TFTP protocol TFTP service required

34 Bootloader’s task and structure Stage1 and stage2 Stage1 Execute from flash Copy stage2 to RAM Stage2 Execute from RAM Copy kernel & Root file system to RAM Load kernel & Root file system

35 Bootloader’s task and structure (cont`) Steps of stage1 (according to implement order) Hardware initialization Prepare memory space for loading stage2 Copy stage2 to memory Set stack Go to starting address of stage2

36 Bootloader’s task and structure (cont`) Fundamental hardware initialization Mask all interrupts through CPU’s interrupt mask register or state register Set CPU speed and clock Initialize memory Initialize LED Ensure hardware are initialized correctly Initialize CPU internal instruction/data cache

37 Bootloader’s task and structure (cont`) Prepare memory space for loading stage2 Determine start and end address for stage2 in RAM Stage2_start, stage2_end Take stack space into consideration –Stage2 is written in C Stage2 is placed in upper part of bootloader memory space

38 Bootloader’s task and structure (cont`) Copy stage2 to memory Set stack pointer (sp) In order to execute C code of stage2 sp points to the location before stage2 end (sp=stage2_end – 4) Jump to the start address of stage2

39 Bootloader’s task and structure (cont`) mem_st art

40 Bootloader’s task and structure (cont`) Steps of stage2 (according to implement order) Initialize the hardware devices used in stage2 Check system memory mapping Copy kernel image and root filesystem to memory Set boot parameters Load kernel and root filesystem

41 Bootloader’s task and structure (cont`) Initialize the hardware devices used in stage2 Initialize one UART at least Initialize timer Check system memory mapping We should know which virtual memory space is really mapped to physical memory

42 Bootloader’s task and structure (cont`) Copy kernel image and root filesystem Memory allocation mapping Memory range of kernel image –Normally, locate in memory_start + 0x8000 0x8000 (32k) is used to store boot parameter and kernel page table Memory range of root filesystem –Normally, locate in memory start + 0x Copy from flash

43 Bootloader’s task and structure (cont`) Set boot parameters In U-boot, boot parameters are defined in the header file of target board Boot parameters: boot command root device root filesystem type console memory size network configuration etc.

44 Bootloader’s task and structure (cont`) Load kernel and root filesystem Jump to memory_start + 0x8000 CPU mode Disable interrupts (IRQs and FIQs) In SVC mode (supervisor)

45 Bootloader’s task and structure (cont`) After loading kernel Bootloader is replaced with operating system