Rafal Kupiec
3e1a7c1932
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Reviewed-on: #1 Reviewed-by: Piotr Likoski <likoski@noreply.codingworkshop.git> Co-authored-by: Rafal Kupiec <belliash@codingworkshop.eu.org> Co-committed-by: Rafal Kupiec <belliash@codingworkshop.eu.org>
63 lines
3.6 KiB
Markdown
63 lines
3.6 KiB
Markdown
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title = 'XT Boot Loader'
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date = 2024-06-16T23:43:56+02:00
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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
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Extensible Firmware Interface (UEFI) of the computer and is responsible for loading the rest of ExectOS. The procedure is
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quite different for UEFI and BIOS systems. However currently, legacy BIOS boot procedure is not supported by ExectOS.
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### UEFI
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The Unified Extensible Firmware Interface (UEFI) has support for reading both the partition table as well as file systems.
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UEFI does not launch any boot code from the Master Boot Record (MBR) whether it exists or not, instead booting relies on
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boot entries in the NVRAM. The UEFI specification mandates support for the FAT12, FAT16, and FAT32 file systems, as well
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as ISO9660 for optical discs. Any conformant vendor can optionally add support for additional file systems.
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UEFI launches EFI applications, e.g. boot loaders, boot managers, UEFI shell, etc. These applications are usually stored
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as files in the EFI system partition (ESP). The applications can be launched by adding a boot entry to the NVRAM or from
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the UEFI shell.
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### Startup Process
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When a PC is powered on its UEFI firmware initializes the hardware components of the system. This includes CPU, memory,
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and device initialization. Firmware has its own boot manager, which is responsible for loading UEFI applications stored
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in the EFI System Partition (ESP), a dedicated partition on a data storage device, formatted with any from the supported
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filesystem and attempts to load the selected EFI application (usually an OS bootloader) into memory and transfers control
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to it.
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XTLDR accesses the file system on the ESP partition and reads the xtldr.ini file to determine what Operating Systems are
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present. Also additional configuration, like debugging options is loaded. Afterwards, XTLDR loads additional modules, in
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order to extend its functionality as well as to support additional file systems, that are normally not supported by UEFI.
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This allows to boot Operating System kernel from the partition other to ESP. Then boot loader menu is presented to the
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user.
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After a user choose the position from the list, or timeout exceeded, XTLDR continues with loading additional modules,
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specific to the chosen Operating System. If a non XTOS is selected, XTLDR loads the associated kernel file listed in the
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configuration file and gives it control. Otherwise, if XTOS is selected, boot loader loads registry HIVE data, kernel
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and boot drivers. Finally, starts the kernel, passing to it all the necessary information.
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### XTLDR Configuration
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XTLDR's first action is to read the xtldr.ini file. It describes whole configuration for the boot loader as well as allows
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the user to choose which Operating System to boot from at the menu. it also allows the user to pass preconfigured options
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to the kernel. Regardless an OS, the location of the Operating System is written as an ARC (Advanced RISC Computing) path.
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Below is an example of the xtldr.ini file:
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```
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[XTLDR]
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Debug=COM1,115200
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Default=ExectOS
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KeepLastBoot=TRUE
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Modules=beep
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Timeout=10
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Tune=400 880 2 988 2 783 2 392 2 587 3
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[ExectOS]
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SystemName="ExectOS Operating System"
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SystemType=XTOS
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BootModules=xtos_o
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SystemPath=multi(0)disk(0)rdisk(0)partition(1)/ExectOS
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KernelFile=xtoskrnl.exe
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Parameters=DEBUG=COM1,115200
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```
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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)
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file in project's GIT repository.
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