Basics of boot-loader


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Describes how a bootloader works and how it starts-up the system.

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Basics of boot-loader

  1. 1. Bootloaders
  2. 2. What ? How ? Why ?
  3. 3. What is a bootloader ? ● ● A boot loader is a computer program that loads the main operating system or run time environment for the computer after completion of the self-tests. Will be covering the loading and working of a bootloader with respect to the physical memory implementation of the x86 based systems.
  4. 4. Booting ● The term boot is short for bootstrap or bootstrap load and derives from the phrase to 'pull oneself up by one's bootstraps'. The usage calls attention to the requirement that, if most software is loaded onto a computer by other software already running on the computer, some mechanism must exist to load the initial software onto the computer.
  5. 5. ● Two major sections – Lower memory (< 1MB) – High memory ● First 640 KB backed by RAM ● Reserved memory for BIOS ● Real Mode – – Can only access lower memory – ● 16 bit instructions BIOS interrupts are available Protected mode – 32 bit instructions – Can access high memory – BIOS interrupts are not available
  6. 6. Boot Sequence ● Turn on the Power button. ● CPU pins are reset and registers are set to specific value. ● CPU jumps to address of BIOS (0xF0000), the reset vector. ● ● ● ● BIOS run POST (Power-On Self Test) and other necessary checks. BIOS jumps to MBR (Master Boot Record) of the boot disk. Primary Bootloader runs from MBR and jumps to Secondary Bootloader. Secondary Bootloaders loads Operating System.
  7. 7. When in BIOS ● A firmware interface ● Initialize and test the system hardware components (POST) ● Provides interface to access hardware (with interrupts) ● ● ● ● Stored on a non-volatile ROM/flash memory chip on the motherboard Table of interrupt vectors resides in the beginning that contain the segmented addresses of the interrupt handlers. The number of interrupt is essentially an index into this table. Checks each device to see if it's bootable Loads the first sector (boot sector) from the disk to memory address 0x007C00
  8. 8. Boot Sector ● ● A Master Boot Record (MBR) is the first sector of a data storage device that has been partitioned. The MBR sector may contain code to locate the active partition and invoke its Volume Boot Record. A Volume Boot Record (VBR) is the first sector of a data storage device that has not been partitioned, or the first sector of an individual partition on a data storage device that has been partitioned. It may contain code to load and invoke an operating system installed on that device or within that partition. ● First 446 bytes are the primary boot loader ● Next sixty-four bytes are the partition table ● Ends with two bytes that should be 0x55AA. These numbers act as validation that this sector is the boot sector or Master Boot Record.
  9. 9. First stage ● Resides in the boot sector of disk ● Loaded by the BIOS. ● Should be covered in 446 bytes due to size constraints in the boot sector. ● Load segment registers (%DS, %ES, %FS, %GS, %SS). ● Set the stack pointer. ● Reset disk controller. ● Reads the 2nd stage bootloader and loads it into memory. ● Jump to it.
  10. 10. Second Stage ● ● ● ● ● ● Query the BIOS for the size of lower and upper memory( INT 12h BIOS interrupt). Loads the kernel from the boot disk into memory, prepare the system for executing the kernel, and jump to the kernel entry point. Switch into protected mode and load the kernel into high (> 1 MB) memory. BIOS services used to read sectors from the boot disk are available only in real mode and real mode cannot access high memory. Repeatedly switch back and forth between real mode and protected mode while reading the kernel from the floppy disk. Small kernel can be preloaded entirely into low memory before transitioning into protected mode.
  11. 11. Starting up … finally!! ● ● ● ● ● ● ● Kernel needs startup arguments while loading, but this cannot be done at runtime. Solution: Configuration files (grub.cfg,initramfs) The bootloader loads the kernel as well as the temporary filesystem before the actual file system is loaded. Initramfs(initial ram file system) : successor of initrd. Solves the chicken-egg problem. An archive of the initial file system that gets loaded into memory. The Linux kernel mounts it as root file system and starts the init process from it. initramfs needs to contain all of the device drivers and tools needed to mount the real root filesystem. After all these, /sbin/init, /etc/init, /bin/init, and /bin/sh are called in the order; the first user process is init and they read etc/events.d or etc/inittab. Hence the system starts.
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