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Saturday, February 12, 2011

Developing Linux OS for ARM - Part I

The first thing that you possibly need to do after getting your hardware ready is to install the necessary software packages and toolchain on the host system (PC).These packages would be required to compile and download/burn packages on to your target platform (ARM9/any embedded controller). We would be talking specifically about the AT91SAM9260 from ATMEL, which is an ARM9 based architecture giving 200MIPS at 180MHz. Although AT91SAM9260 would be our target controller, most of the software components that we discuss would apply to all target platforms. The lower level softwares like the primary level bootloader would be architecture specific. The AT91SAM9260, also known as Smart ARM based controllers (aka SAM) is based on the integration of an ARM926EJS processor with fast ROM and RAM memories and a wide range of peripherals. The AT91SAM9260 embeds an Ethernet MAC, one USB Device Port, and a USB Host controller. It also integrates several standard peripherals, such as the USART, SPI, TWI, Timer, Counters, Synchronous Serial Controller, ADC and MultiMedia Card Interface. Primarily for porting the conventional Linux, we would need MMU support in hardware. ARM9 based MCUs feature an MMU (unlike most ARM7 MCU) and hence we do not need to use some other stripped down Linux variants like uCLinux which can be ported to MMU less architectures.

The following would be the software components required to get Linux running on the target system : 

Root filesystem

Linux Kernel
Hardware (MCU)

Bootloader :

The bootloader can be further classified into:

1. Primary bootloader: This might typically reside onchip (internal flash). In case of AT91SAM9260, a component called ROMboot enables us to dump the primary bootloader to the on chip flash. The primary bootloader is called AT91Bootstrap. A primary bootloader is needed for configuration of CPU parameters like clock frequency, incharge of hardware configuration, download UBoot (secondary bootloader) from FLASH to SDRAM and start the bootloader.

2. Secondary bootloader: In most cases, UBoot is the secondary bootloader, which is in charge of downloading kernel binaries from FLASH, SD/MMC, USB or ethernet and starting the kernel.

Once the bootloader has loaded the kernel in the memory, the operating system kernel will then takeover control of the hardware. The kernel comprises of the scheduler and the device drivers. The other utilities provided by any standard OS distribution are not a part of the kernel space. The Root filesystem contains applications which are executed on the target, using the OS kernel services. It forms the user space. Its primarily an archive of utilities like cat, ftp, vi, gedit, ls etc

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