UM0844 datasheet(12/43 Pages) STMICROELECTRONICS

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The pre-flashed software

UM0844

12/43

Doc ID 16544 Rev 3

When not interrupted after board power-on, U-Boot loads the Linux kernel to DDR memory

and then starts its execution. The kernel, after further preliminary initialization, mounts the

so-called root filesystem. This is the binary image containing all software and data that

comprise the operating system complementing the kernel. The root filesystem is the

hierarchical file structure that end users see from the familiar Linux shell prompt. For pre-

flashed software, it contains a generic subset of initialization scripts, system commands and

runtime libraries.

There is a wide choice of file system standards for Flash memories in the Linux world (for

example, CRAMFS, JFFS2, YAFFS2, LogFS, UBIFS).

The pre-flashed software is based on the most commonly used standard, namely JFFS2.

The main advantage of JFFS2 is the built-in support for compression. It is also the most

consolidated filesystem, while generally not providing the best performance in terms of

booting time.

By using the SPEAr Linux Support Package (LSP2.3), evaluation boards with NAND Flash

can be also configured to use YAFFS2. YAFFS2 aims at providing better support for NAND

Flash devices and faster boot time. However, YAFFS2 is not yet part of official open source

Linux kernels and requires a dedicated patch, however this patch is already applied to

SPEAr-ported Linux kernels.

2.3

Booting up to the Linux prompt

To boot Linux just power up the board. You then see the bootloader messages and prompt.

If you press a key in this phase, you stop the bootloader execution and display the U-Boot

command prompt (see next section). Instead, wait for a few seconds while the Linux kernel

is loaded and launched. Progress is displayed by a sequence of kernel boot messages. The

root filesystem is mounted automatically, the system shell is run and a prompt appears

when it is ready to accept commands.

The pre-flashed software provides a default set of system commands and runtime libraries.

System commands are the familiar basic utilities mainly provided to support the

development and debugging stages. They are all stored under standard Linux paths. They

may be not strictly needed in a final product, however their small size allows them to be kept

in production devices without a significant penalty in terms of memory footprint.

As is typical for embedded Linux environments, STLinux uses BusyBox, an open source

program combining tiny versions of many common user-space Linux utilities into a single

small executable, an important factor when minimized Flash footprint is required.

BusyBox has been developed with size-optimization and limited resources in mind, for this

reason the available commands typically have fewer options than their full-featured GNU

counterparts. However, the most important options are still available with all the functions

needed for developing and testing embedded products.

As well as the features of BusyBox, a number of additional executable programs are also

included.

In addition to playing with commands, it can be also helpful to explore the Linux standard

/proc pseudo-filesystem. This subtree contains user-accessible entries that pertain to the

runtime state of the kernel and, by extension, the executing processes that run on top of it.

The "pseudo" term is used because the proc filesystem exists only as a reflection of the in-

memory kernel data structures it displays. This is why most files and directories within /proc

are zero bytes in size.

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