Boot Process
Steps
Most systems have 2 phases:
configure and start the hardware.
find the operating system and run it.
Boot code - machine instructions used by the computer.
When it is starting, power is applied to a CPU:
it reads instructions from a specific location in memory - typically ROM.
the instructions in ROM force the system to probe for and configure hardware.
then searches for a device that may contain additional boot code:
disks reserve space for boot code, but it isn't always used.
its boot code is executed, and attempts to locate and load an operating system.
the process after te bootable disk is found is platform-specific.
Boot code characteristics
Has a specific location.
The instructions are in machine code:
0xB400
MOV AH,00
on a storage device it is difficult to distinguish random data from machine code
Systems with BIOS (Basic Input/Output System)
BIOS boots by reading the first sector on a hard disk and executing it which has space limitations.
This boot sector code in turn locates and runs additional code in the first sector of the partition.
Which locates and loads the actual operating system.
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