http://wiki.openwrt.org/doc/techref/flash.layout?s[]=trx

There are hard discs,which are considered block devices,and there is flash memory. There types of flash,like NOR flash,SLC NAND flash and MLC NAND flash.
If the flash chip (no matter what type) is connected directly with the SoC and has to be addressed directly by Linux,we call it "raw flash". If the flash chip cannot be addressed directly by the OS (because there is an additional controller chip between flash chip and the SoC),we call it "FTL (Flash Translation Layer) flash". Embedded systems almost exclusively use "raw flash",while USB memory sticks,almost exclusively use FTL flash!

The flash chip on embedded systems is not accompanied by a controller chip,and thus are not "FTL"-devices but "raw flash"-devices. Storage space is treated and addressed as an MTD (Memory Technology Device) and special Filesystems are used. The available storage is not partitioned in the traditional way,where you store the data about the partitions in the MBR and PBRs,but it is done in the Linux Kernel (and sometimes independently in the bootloader as well!). You simply define,that "partition kernel starts at offset x and ends at offset y". The advantage is,that later we can address these partitions by name,rather then specifying the precise start point of the data.

Since the partitions are nested we look at this whole thing in layers:

1. Layer0: So we have the Flashchip,8MiB in size,which is soldered to the PCB and connected to the SoC over SPI (Serial Peripheral Interface Bus).
2. Layer1: We "partition" the space into mtd0 for the bootloader,mtd5 for the firmware and,in this case,mtd4 for the ART (Atheros Radio Test) - it contains mac addresses and calibration data for the wifi (EEPROM). If it is missing or corrupt,ath9k (wireless driver) won't come up anymore.
3. Layer2: we subdivide mtd5 (firmware) into mtd1 (kernel) and mtd2 (rootfs); In the generation process of the firmware (see obtain.firmware.generate) the Kernel binary file is first packed with LZMA,then the obtained file is packed with gzip and then this file will be written onto the raw flash (mtd1) without being part of any filesystem!
4. Layer3: we subdivide rootfs even further into mtd3 for rootfs_data and the rest for an unnamed partition which will accommodate the SquashFS-partition.

→ filesystems

• / this is your entire root filesystem,it comprises /rom and /overlay. Please ignore /rom and /overlay and use exclusively / for your daily routines!
• /rom contains all the basic files,like busybox,dropbear or iptables including default configuration files. Does not contain the kernel. Files in this directory are on the SqashFS partition,and thus cannot be deleted. But,because we use mini_fo filesystem,so called overlay-whiteout-symlinks can be created on the JFFS2 partition.
• /overlay is the writable part of the file system that gets merged with /rom to create a uniform /-tree. It contains anything that was written to the router after installation,e.g. changed configuration files,additional packages installed with opkg,etc. It is formated with JFFS2.

Rather than deleting the files,insert a whiteout,a special high-priority entry that marks the file as deleted. File system code that sees a whiteout entry for file F behaves as if F does not exist.

#!/bin/bash
# shows all overlay-whiteout symlinks
?
find /overlay -type l | while read FILE
  do
    [ -z "$FILE" ] && break
    if ls -la "$FILE" 2>&- | grep -q '(overlay-whiteout)'; then
    echo "$FILE"
    fi
  done

→ user.beginner.fhs

NOTE1: If the Kernel was part of the SquashFS,we could not control where exactly on the flash it is written to (on which blocks its data is contained). Thus we could not tell the bootloader to simply load and execute certain blocks on the flash storage,but would have to address it with path and filename. Now this would not be bad,but in order to that the bootloader would have to understand the SquashFS filesystem,which it does not. The embedded bootloaders we utilize with OpenWrt generally have no concept of filesystems,thus they cannot address files by path and filename. They pretty much assume that the start of the trx data section is executable code.
NOTE2: The denomination "firmware" usually is used for the entire data on the flash comprising the boot loader and any other data necessary to operate the device,such as ART,NVRAM,FIS,etc,but we also use it to name only the parts that are being rewritten. Don't let this confuse you

TODO

cat /proc/mtd
dev:    size   erasesize  name
mtd0: 00020000 00010000 "u-boot"
mtd1: 00140000 00010000 "kernel"
mtd2: 00690000 00010000 "rootfs"
mtd3: 00530000 00010000 "rootfs_data"
mtd4: 00010000 00010000 "art"
mtd5: 007d0000 00010000 "firmware"

The erasesize is the block size of the flash,in this case 64KiB. The size is little or big endian hex value in Bytes. In case of little endian,you switch to hex-mode and enter 02 0000 into the calculator for example and convert to decimal (by switching back to decimal mode again). Then guess how they are nested into each other. Or execute dmesg after a fresh boot and look for something like:

Creating 5 MTD partitions on "spi0.0":
0x000000000000-0x000000020000 : "u-boot"
0x000000020000-0x000000160000 : "kernel"
0x000000160000-0x0000007f0000 : "rootfs"
mtd: partition "rootfs" set to be root filesystem
mtd: partition "rootfs_data" created automatically,ofs=2C0000,len=530000
0x0000002c0000-0x0000007f0000 : "rootfs_data"
0x0000007f0000-0x000000800000 : "art"
0x000000020000-0x0000007f0000 : "firmware"

The flash chip can be represented as a large block of continuous space:

There is no ROM to boot from; at power up the CPU begins executing the code at the very start of the flash. Luckily this isn't the firmware or we'd be in real danger every time we reflashed. Boot is actually handled by a section of code we tend to refer to as the bootloader (the BIOS of your PC is a bootloader).

The partition or partitions containing so called Special Configuration Data differ very much from each other. Example: The ART-partition you will meet in conjunction with Atheros-Wireless and U-Boot,contains only data regarding the wireless driver,while the NVRAM-partition of broadcom devices is used for much more then only that.

If you dig into the "firmware" section you'll find a trx. A trx is just an encapsulation,which looks something like this:

"HDR0" is a magic value to indicate a trx header,rest is 4 byte unsigned values followed by the actual contents. In short,it's a block of data with a length and a checksum. So,our flash usage actually looks something like this:

Except that the firmware is generally pretty small and doesn't use the entire space between CFE and NVRAM:

(NOTE: The <model>.bin files are nothing more than the generic *.trx file with an additional header appended to the start to identify the model. The model information gets verified by the vendor's upgrade utilities and only the remaining data – the trx – gets written to the flash. When upgrading from within OpenWrt remember to use the *.trx file.)

So what exactly is the firmware?

The boot loader really has no concept of filesystems,it pretty much assumes that the start of the trx data section is executable code. So,at the very start of our firmware is the kernel. But just putting a kernel directly onto flash is quite boring and consumes a lot of space,so we compress the kernel with a heavy compression known as LZMA. Now the start of firmware is code for an LZMA decompress:

Now,the boot loader boots into an LZMA program which decompresses the kernel into memory and executes it. It adds one second to the bootup time,but it saves a large chunk of flash space. (And if that wasn't amusing enough,it turns out the boot loader does know gzip compression,so we gzip compressed the LZMA decompression program)

Immediately following the kernel is the filesystem. We use SquashFS for this because it's a highly compressed readonly filesystem – remember that altering the contents of the trx in any way would invalidate the crc,so we put our writable data in a JFFS2 partition,which is outside the trx. This means that our firmware looks like this:

And the entire flash usage looks like this -

That's about as tight as we can possibly pack things into flash.

An image file is byte by byte copy of data contained in a file system. If you installed a Debian or a Windows in the usual way onto one or two harddisc partitions and would afterwards copy the whole content byte by byte from the hard disc into one file:

dd if=/dev/sda of=/media/sdb3/backup.dd

the obtained backup file /media/sdb3/backup.dd,could be used in the exact same manner like an OpenWrt-Image-File.

The difference is,that OpenWrt-Image-File are not created that way

• flash.layout
• header
• obtain.firmware.generate If you want to read about the Image Generator,go back to obtain.firmware and choose the second way.
• About Broadcom Firware Format