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BIOS interrupt calls perform hardware control or I/O functions requested by a program, return system information to the program, or do both. A key element of the purpose of BIOS calls is abstraction - the BIOS calls perform generally defined functions, and the specific details of how those functions are executed on the particular hardware of the system are encapsulated in the BIOS and hidden ...
INT 13h is shorthand for BIOS interrupt call 13 hex, the 20th interrupt vector in an x86-based (IBM PC-descended) computer system.The BIOS typically sets up a real mode interrupt handler at this vector that provides sector-based hard disk and floppy disk read and write services using cylinder-head-sector (CHS) addressing.
On the original IBM PC and XT, if no bootable disk was found, the BIOS would try to start ROM BASIC with the interrupt call to interrupt 18h. Since few programs used BASIC in ROM, clone PC makers left it out; then a computer that failed to boot from a disk would display "No ROM BASIC" and halt (in response to interrupt 18h).
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]
A modern PC's UEFI or BIOS firmware supports booting from various devices, typically a local solid-state drive or hard disk drive via the GPT or Master Boot Record (MBR) on such a drive or disk, an optical disc drive (using El Torito), a USB mass storage device (USB flash drive, memory card reader, USB hard disk drive, USB optical disc drive ...
Using some higher-level interface (e.g. Video BIOS) Using another lower-level device driver (e.g. file system drivers using disk drivers) Simulating work with hardware, while doing something entirely different [9] For software: Allowing the operating system direct access to hardware resources; Implementing only primitives
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A USB flash drive needs to be connected to the system, and be detected by it; One or more partitions may need to be created on the USB flash drive; The "bootable" flag must be set on the primary partition on the USB flash drive; An MBR must be written to the primary partition of the USB flash drive