What is MLO file?
I have had the Beagle-xM for a while now. If you have worked on it, you must have noticed the MLO file, which is required to be present in the bootpartition of the Micro SD card. The file is necessary for booting the system. So ever wondered, what is this MLO file? No? What are you doing with the Beagle-xM anyways? Yes? Well, then read on.
As far as i know, all TI SOC’s like DM368 and DM3730 (on which i have worked) have a ROM bootloader. On sytem start up or reset, the ROM bootloader(RBL) starts running. The job of this RBL, is to do some very bare minimum initialisation and then find and boot from a device such as MMC card or flash. The RBL has got the capability to boot from a variety of devices like NOR flash, NAND flash, MMC, USB or UART. To know everything in detail,read the 26th chapter of the DM3730 Technical Reference Manual having the TI document number SPRUGN4N.
Now, how does the chip know which device to boot from you ask?. The chip has got a set of pins, sys_boot[4:0] pins, whose state decides what memoryor peripheral to boot from. Also, there is a predefined booting order in the case of both memory booting or peripheral booting. For example, let’s say the state of sys_boot[4:0] pins is 0b01110. Then, the booting order will be XIPwait, DOC, USB, UART3 and finally MMC1. XIPwait is Execute in Place memory with wait monitoring and DOC is DiskOnChip memory. You can find this information on Page 3549 of the DM3730 TRM.
In the case of Beagle-xM, the state of the sys_boot pins cause it to boot from the MMC card at the start. As a result, on power on, the ROM will initialise the SD/MMC device, detect a card and look for a boot image. The ROM is capable of reading from the card with a filesystem. It will search for a file named MLO which should be located on the boot partition, on a partition of type FAT16/32. So, this explains the name MLO.
Now, let’s talk about the flow of control. The sys_boot pins specify where the first external boot loader MLO can be found. After the MLO is located and executed, it passes the control to the second external boot loader viz. uboot. Have you seen the uboot.bin file in the boot partition of the micro SD card? No? Jeez, have you been sleeping? U-boot is the application which passes control to the Linux system. The main goal of u-boot is to retrieve the Linux kernel and provide the kernel with information about the location of the Linux filesystem. U-boot can be configured to retrieve the kernel from a variety of options like NFS or flash. U-boot then boots the Linux kernel. The Linux kernel mounts the Linux root filesystem.
In case, you are interested further in knowing the boot sequence, below is a link, which describes the boot sequence.
It uses AT91RM9200 as the reference, but, still should be very informative enough. Also, you can use this link to browse the source.
Hope you found this “What is MLO file?” post informative.
Addendum: Mikkel Nielsen shared quite a few important links and had this to quote. You can also find this in the comments.
Think it could also be Memory LOader, as it is a file containing (apart from metadata) binary code that is loaded directly into internel SoC memory.
TI also refers to it as X-loader
This suggests it is short for Memory LOcator.
Some history about why it is like that here.