Getting Started Boot from ROM

There are several components which are needed to allow Linux to boot from ROM. Together these make for a very flexible and powerful system. Over the next few pages we will cover some of these in more detail, concluding with a worked example:

• Memory Technology Devices (MTD) is the Linux kernel component which gives easy access to Flash devices. It has lots of features which make handling Flash easy, but does take some effort to understand, so we have provided this MTD reference page.
• The initial program loader (or to give it its full title, the SuperH Initial Program Loader plus GDB stub: sh-ipl+g). This performs the functions of a simple BIOS: in particular setting up the memory interfaces, providing some basic run time services, and some control over booting into the main OS.
• The Linux kernel can be built so that when it starts running, it first decompresses itself. In boot from ROM systems this can be used to store a compressed kernel in ROM, decompressing into RAM.
• Some of the many file systems which are provided with Linux are ideal for use in embedded applications. In particular there is a static (and very compact) ROM file system, a writable Flash file system, or RAM file systems which can be initialised from compressed contents. We give some background on file systems and some hints on choosing file systems.

We bring all of these aspects together with a worked example of a typical boot from ROM system.