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The original IBM Personal Computer ROMs hooked INT 18H (originally to invoke Cassette BASIC) and INT 19H, as these two interrupts were used for the boot process. INT 19h is called to initiate the boot process, and INT 18h was called to start Cassette BASIC from ROM when the boot process found that none of the possible boot devices was bootable.
A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS Setup function by pressing a special key when the computer is first turned on (such as Delete, F1, F2, F10 or F12), and then changing the boot order. [6]
If the sector is read successfully, some BIOSes will also check for the boot sector signature 0x55 0xAA in the last two bytes of the sector (which is 512 bytes long), before accepting a boot sector and considering the device bootable. [b] When a bootable device is found, the BIOS transfers control to the loaded sector.
A bootable MBR device is defined as one that can be read from, and where the last two bytes of the first sector contain the little-endian word AA55h, [nb 6] found as byte sequence 55h, AAh on disk (also known as the MBR boot signature), or where it is otherwise established that the code inside the sector is executable on x86 PCs.
When a system on a chip (SoC) enters suspend to RAM mode, in many cases, the processor is completely off while the RAM is put in self refresh mode. At resume, the boot ROM is executed again and many boot ROMs are able to detect that the SoC was in suspend to RAM and can resume by jumping directly to the kernel which then takes care of powering on again the peripherals which were off and ...
A bootable device can be storage devices like floppy disk, CD-ROM, USB flash drive, a partition on a hard disk (where a hard disk stores multiple OS, e.g Windows and Fedora), a storage device on local network, etc. [7] A hard disk to boot Linux stores the Master Boot Record (MBR), which contains the first-stage/primary bootloader in order to be ...
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In these cases a computer can often be "redirected" to boot from a USB device through use of an initial bootable CD or floppy disk. [ 5 ] [ 6 ] [ 7 ] Some Intel-based Macintosh computers have limitations when booting from USB devices – while the Extensible Firmware Interface (EFI) firmware can recognize and boot from USB drives, it can do ...