Bootloader

The bootloader of the operating system is a system software that provides loading of the operating system immediately after turning on the computer ( POST procedure) and initial boot .

Bootloader operating system:

• provides the necessary tools for dialogue with a computer user (for example, the bootloader allows you to select the operating system to boot);
• brings the computer hardware to the state necessary to start the kernel of the operating system (for example, on non- x86 architectures, before starting the kernel, the loader must correctly configure virtual memory );
• loads the kernel of the operating system ( Ntoskrnl.exe in the case of MS Windows) into RAM . Booting the kernel of the operating system does not necessarily occur from the hard drive . The bootloader can receive the kernel over the network. The kernel can be stored in ROM or loaded via serial interfaces (this can be useful at an early stage of debugging the created computer system);
• generates parameters passed to the kernel of the operating system (for example, parameters are passed to the Linux kernel that indicate how to connect the root file system);
• transfers control to the kernel of the operating system.

On computers of the IBM PC architecture , the bootloader is launched by the BIOS software written in the ROM of the computer after the successful completion of the POST procedure. Let us describe the procedure by which the IBM PC is loaded from the HDD: the BIOS reads 512 bytes of the first sector of the HDD ( MBR ) into RAM at the address 0x00007C00 ( 0x07C0: 0x0000 in real mode ), then control is transferred to the read code. This code reads and analyzes the partition table of the hard drive, and then, depending on the type of bootloader, either transfers control to the boot code of the active partition of the hard drive (see Initial boot of the computer ), or it independently loads the kernel from the drive (for example, network or removable ) random access memory and transfers control to it. Initially, the bootloader operates in real addressing mode with the A20 address line disabled, which creates certain difficulties when writing bootloaders.