Acronis os selector 8.0 User Manual
2.5 Boot
Sequence
2.5.1 Very
Beginning
Whenever a computer is turned on or rebooted, control is given to BIOS
(Base Input/Output System) that is stored in the computer ROM. BIOS
initializes and tests the hardware and then loads the first sector from the
boot disk device (usually it is the first hard disk, and the sector is the MBR)
and passes control to it. All the actions that follow depend on the contents of
this sector.
(Base Input/Output System) that is stored in the computer ROM. BIOS
initializes and tests the hardware and then loads the first sector from the
boot disk device (usually it is the first hard disk, and the sector is the MBR)
and passes control to it. All the actions that follow depend on the contents of
this sector.
Some words should be said about how the hard disks are counted. BIOS
assigns hard disks sequential numbers starting from 080h, the sequence is
defined by the order in which the disks are plugged into IDE controllers
(Primary Master, Primary Slave, Secondary Master, Secondary Slave), next
follow the SCSI hard disks. This sequence would be broken if you change the
boot sequence in the BIOS setup. Thus if you have set that booting should
be done from hard disk E (do not confuse letters that are assigned to the
hard disks by BIOS with partition letters!), then the sequence would start
with the disk that would have otherwise been the third (usually its Secondary
Master). In Acronis OS Selector BIOS hard disk sequence is used, but the
initial number is 1.
assigns hard disks sequential numbers starting from 080h, the sequence is
defined by the order in which the disks are plugged into IDE controllers
(Primary Master, Primary Slave, Secondary Master, Secondary Slave), next
follow the SCSI hard disks. This sequence would be broken if you change the
boot sequence in the BIOS setup. Thus if you have set that booting should
be done from hard disk E (do not confuse letters that are assigned to the
hard disks by BIOS with partition letters!), then the sequence would start
with the disk that would have otherwise been the third (usually its Secondary
Master). In Acronis OS Selector BIOS hard disk sequence is used, but the
initial number is 1.
2.5.2
Booting without the Boot Manager
Usually the MBR of the hard disk contains the code that is written there by a
standard partitioning program (FDISK) and performs the following actions:
standard partitioning program (FDISK) and performs the following actions:
• Searches the partition table for the first partition that is marked as
active;
• Attempts to load into memory the first sector of the partition found. Such
sector of a partition is called the boot sector;
• Passes control to the loaded sector.
The boot sector usually contains the code that attempts to boot an operating
system from the partition. Each operating system has its own boot code.
system from the partition. Each operating system has its own boot code.
2.5.3
What does the Boot Manager do?
Installing a boot manager on your computer slightly changes the boot
sequence. Usually the boot manager writes its own code into the MBR that
loads into memory, not the boot sector of an operating system, but the boot
manager’s code. A boot manager usually offers you a choice of operating
systems to boot, and the booting of the chosen operating system happens
only after your selection.
sequence. Usually the boot manager writes its own code into the MBR that
loads into memory, not the boot sector of an operating system, but the boot
manager’s code. A boot manager usually offers you a choice of operating
systems to boot, and the booting of the chosen operating system happens
only after your selection.
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