This package provides a win32 program that can be used as a loader for Debian Installer, acting as a more user-friendly boot mechanism than traditional BIOS-based boot.
It is intended to simplify first-time installation of Linux, and for creation of rescue and other special-purpose boot disks. CategoryBootProcess BootLoader last modified Wiki Login. The target aliases allow scripts—and many sysadmins like myself—to use SystemV commands like init 3 to change runlevels.
Of course, the SystemV commands are forwarded to systemd for interpretation and execution. Each target has a set of dependencies described in its configuration file. These dependencies are the services required to run the Linux host at a specific level of functionality.
When all of the dependencies listed in the target configuration files are loaded and running, the system is running at that target level. If so, systemd used those as configuration files to start the services described by the files. The deprecated network service is a good example of one of those that still use SystemV startup files in Fedora.
Figure 1, below, is copied directly from the bootup man page. It shows the general sequence of events during systemd startup and the basic ordering requirements to ensure a successful startup. The sysinit. Although systemd has as one of its design goals to start system services in parallel, there are still certain services and functional targets that must be started before other services and targets can be started. These checkpoints cannot be passed until all of the services and targets required by that checkpoint are fulfilled.
So the sysinit. All of those units, mounting filesystems, setting up swap files, starting udev, setting the random generator seed, initiating low-level services, and setting up cryptographic services if one or more filesystems are encrypted, must be completed, but within the sysinit. After the sysinit.
The basic target provides some additional functionality by starting units that re required for the next target. These include setting up things like paths to various executable directories, communication sockets, and timers. Finally, the user-level targets, multi-user. Notice that the multi-user.
The underlined targets in Figure 1, are the usual startup targets. When one of these targets is reached, then startup has completed. If the multi-user. If graphical. I found that some of the commands did not seem to work properly for me, or that I was not using them correctly. I am not yet certain which was the case, and need to do some more research. This circumvention worked as expected and booted to the alternate kernel. GRUB2 and the systemd init system are the key components in the boot and startup phases of most modern Linux distributions.
Despite the fact that there has been controversy surrounding systemd especially, these two components work together smoothly to first load the kernel and then to start up all of the system services required to produce a functional Linux system. Although I do find both GRUB2 and systemd more complex than their predecessors, they are also just as easy to learn and manage.
The man pages have a great deal of information about systemd, and freedesktop. Refer to the resources, below, for more links. Thank you so much for that article on Linux booting! I wanted to find such a resource for a long time. Something I miss from the past is the old stream of listing boot operations.
I guess maybe this was too scary for some, so we have to have this graphical boot that tells us nothing. That can be fixed. Then run the command:. That command creates a new grub. I find this information extremely useful due to potential for issues and downright crashes during boot. I can tell a lot by watching that information flow by and it sounds like you can, too. I always have to take a deep breath when I mess with the boot process, it's such a PITA when you have an unbootable system.
You are correct. I typed the incorrect path. I have fixed the typo. Thanks for pointing that out. I was going to add that, on a practical level, I have found that the boot messages more often alert you to something that doesn't prevent booting, but may affect some service you need afterward, and gives you a heads up about some failure.
Thank you for the fine summary of the boot and startup process. Consider an update to explain which distros are likely to follow the scheme that you describe. It fills the remaining bytes with zeros and finishes with the two magic bytes 0xaa and 0x A real-world boot sector has code for continuing the boot process and a partition table instead of a bunch of 0's and an exclamation mark. In real mode, calculating the physical address in memory is done as follows:. We have only bit general purpose registers, which has a maximum value of 0xffff , so if we take the largest values the result will be:.
An processor which was the first processor with real mode , in contrast, has a bit address line. How can the CPU access this address in real mode? The answer is in the coreboot documentation:. The Linux kernel has a Boot protocol which specifies the requirements for a bootloader to implement Linux support. This example will describe GRUB 2.
Continuing from before, now that the BIOS has chosen a boot device and transferred control to the boot sector code, execution starts from boot. Its code is very simple, due to the limited amount of space available. It contains a pointer which is used to jump to the location of GRUB 2's core image.
The core image begins with diskboot. The above code loads the rest of the core image, which contains GRUB 2's kernel and drivers for handling filesystems, into memory. As we can read in the kernel boot protocol, the bootloader must read and fill some fields of the kernel setup header, which starts at offset 0x01f1 from the kernel setup code.
You may look at the boot linker script to confirm the value of this offset. S starts from:. The bootloader must fill this and the rest of the headers which are only marked as being type write in the Linux boot protocol, such as in this example with values either received from the command line or calculated during booting. We will not go over full descriptions and explanations for all fields of the kernel setup header for now, but we shall do so when discussing how the kernel uses them.
You can find a description of all fields in the boot protocol. As we can see in the kernel boot protocol, memory will be mapped as follows after loading the kernel:. In my case, X is 0x , as we can see in a memory dump:.
The bootloader has now loaded the Linux kernel into memory, filled the header fields, and then jumped to the corresponding memory address. We now move directly to the kernel setup code. Finally, we are in the kernel! Technically, the kernel hasn't run yet. First, the kernel setup part must configure stuff such as the decompressor and some memory management related things, to name a few. After all these things are done, the kernel setup part will decompress the actual kernel and jump to it.
It may look a bit strange at first sight, as there are several instructions before it. A long time ago, the Linux kernel had its own bootloader. Now, however, if you run, for example,. Actually, the file header. S starts with the magic number MZ see image above , the error message that displays and, following that, the PE header:. It needs this to load an operating system with UEFI support. We won't be looking into its inner workings right now but will cover it in upcoming chapters.
The bootloader GRUB 2 and others knows about this point at an offset of 0x from MZ and jumps directly to it, despite the fact that header. S starts from the. In Nf notation, 2f , for example, refers to the local label In our case, it's label 1: that is present right after the jump, and contains the rest of the setup header. Right after the setup header, we see the. This is the first code that actually runs aside from the previous jump instructions, of course.
After the kernel setup part receives control from the bootloader, the first jmp instruction is located at the 0x offset from the start of the kernel real mode, i.
In my case, the kernel is loaded at the physical address 0x This means that segment registers have the following values after kernel setup starts:. First of all, the kernel ensures that the ds and es segment registers point to the same address. Next, it clears the direction flag using the cld instruction:.
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