Import XTLDR documentation

content/documentation/xtldr.md

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+title = 'XT Boot Loader'
+date = 2024-06-16T23:43:56+02:00
+draft = true
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+XTLDR (abbreviation of XT Loader) is the boot loader dedicated for XTOS. It is the first program launched by the Unified
+Extensible Firmware Interface (UEFI) of the computer and is responsible for loading the rest of ExectOS. The procedure is
+quite different for UEFI and BIOS systems. However currently, legacy BIOS boot procedure is not supported by ExectOS.
+
+### UEFI
+The Unified Extensible Firmware Interface (UEFI) has support for reading both the partition table as well as file systems.
+UEFI does not launch any boot code from the Master Boot Record (MBR) whether it exists or not, instead booting relies on
+boot entries in the NVRAM. The UEFI specification mandates support for the FAT12, FAT16, and FAT32 file systems, as well
+as ISO9660 for optical discs. Any conformant vendor can optionally add support for additional file systems.
+
+UEFI launches EFI applications, e.g. boot loaders, boot managers, UEFI shell, etc. These applications are usually stored
+as files in the EFI system partition (ESP). The applications can be launched by adding a boot entry to the NVRAM or from
+the UEFI shell.
+
+### Startup Process
+When a PC is powered on its UEFI firmware initializes the hardware components of the system. This includes CPU, memory,
+and device initialization. Firmware has its own boot manager, which is responsible for loading UEFI applications stored
+in the EFI System Partition (ESP), a dedicated partition on a data storage device, formatted with any  from the supported
+filesystem and attempts to load the selected EFI application (usually an OS bootloader) into memory and transfers control
+to it.
+
+XTLDR accesses the file system on the ESP partition and reads the xtldr.ini file to determine what Operating Systems are
+present. Also additional configuration, like debugging options is loaded. Afterwards, XTLDR loads additional modules, in
+order to extend its functionality as well as to support additional file systems, that are normally not supported by UEFI.
+This allows to boot Operating System kernel from the partition other to ESP. Then boot loader menu is presented to the
+user.
+
+![XTLDR Boot Menu](/images/exectos/xtldr_boot_menu.png)
+
+After a user choose the position from the list, or timeout exceeded, XTLDR continues with loading additional modules,
+specific to the chosen Operating System. If a non XTOS is selected, XTLDR loads the associated kernel file listed in the
+configuration file and gives it control. Otherwise, if XTOS is selected, boot loader loads registry HIVE data, kernel
+and boot drivers. Finally, starts the kernel, passing to it all the necessary information.
+
+### XTLDR Configuration
+XTLDR's first action is to read the xtldr.ini file. It describes whole configuration for the boot loader as well as allows
+the user to choose which Operating System to boot from at the menu. it also allows the user to pass preconfigured options
+to the kernel. Regardless an OS, the location of the Operating System is written as an ARC (Advanced RISC Computing) path.
+Below is an example of the xtldr.ini file:
+```
+[XTLDR]
+Debug=COM1,115200
+Default=ExectOS
+KeepLastBoot=TRUE
+Modules=beep
+Timeout=10
+Tune=400 880 2 988 2 783 2 392 2 587 3
+
+[ExectOS]
+SystemName="ExectOS Operating System"
+SystemType=XTOS
+BootModules=xtos_o
+SystemPath=multi(0)disk(0)rdisk(0)partition(1)/ExectOS
+KernelFile=xtoskrnl.exe
+Parameters=DEBUG=COM1,115200
+```
+A full list of configuration options with their descriptions can be found in the [example configuration](https://git.codingworkshop.eu.org/xt-sys/exectos/src/branch/master/bootdata/xtldr/xtldr.ini)
+file in project's GIT repository.