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<html><head> <title>Operating Systems FAQ :: Bootloaders and Bootmenus</title> <link rel=stylesheet type="text/css" href="default.css"></head><body><TABLE border="0" width="100%"> <TR> <TD><H2><A name="boot_sector">Boot Sectors</A></H2> </TD> </TR> <TR> <TD>The computer follows a set sequence, one of the first things it doesis perform its POST, Power On Self Test.<p>The POST is designed to detect faults in the system (no display,no keyboard, etc) and look for a bootable device(floppy, scsi, hard disk, cdrom)<p>In modern machnes you can set the order the BIOS determins what toboot from, "A, C, CDROM / C Only / CDROM, C, A" etc.<p>Back in the "old" days, the BIOS would first look for a floppy disk indrive A: and then it would look in drive C:.<p>Once the BIOS finds a boot sector with a bootable ID (0x55, 0xAA),it loads it into memory at a specifically set location, 0x0000:0x7C00,segment 0x0000, address 0x7C00.<p>Once the sector is loaded, execution starts at this address.<p>BIOS, all being made by different manufacturers, set its registersup differently according to who its maker is. *NEVER* assumewhat the contents of registers might be, for example, NEVER assumethat DS/SS/ES etc will be initialised to 0 because not all bios do this.(and this has caught many people unawares!)<p>Floppy disks and hard disks boot differently. There are slight differencesin the way they go about things.<p>Floppies are pretty simple, the BIOS loads the bootsector into 0x0000:0x7C00and proceeds to execute it. Its a one step process.<p>A hard disk is different. A hard disk has an MBR (Master Boot Record),this is because a hard disk can have multiple partitions and any one ofthem can be bootable.<p>The BIOS loads the MBR (sector 0 of the hard disk) into memory at0x0000:0x7C00 and executes it, now typically (ie: there is no standard)that sector moves its code down to 0x0000:0x0600 area and continuesto execute itself from there.<p>Once it has moved itself, it will proceed to scan the partition tablefor a bootable partition. When it finds a bootable partition it loadsthe first sector of that partition into 0x0000:0x7C00 and executes it.(Now you see why it moves its code down!)<p>The entire process is quite simple. But things get much harderwhen you decide you want your bootsector to load a file or anoperating system kernel.<p>(see the examples <a href="files/bootr01.zip">bootr01.zip</a> and <a href="files/smbmbr03.zip">smbmbr03.zip</a>) </TD> </TR></TABLE><P> </P><TABLE border="0" width="100%"> <TR> <TD><H2><A name="mbr">Master Boot Record</A></H2> </TD> </TR> <TR> <TD>Every hard disk has an MBR (Master Boot Record) as the very first sector of the disk. <p> The MBR contains a table of four entries that are used for what is known as Primary Partitions. <p> This table is filled in by the disk paritioning programs such as FDisk and Disk Druid, etc. <p> The layout of this table corresponds to;<pre>Offset Size (bytes) Description0x00 1 Boot Indicator (0x80=bootable, 0x00=not bootable)0x01 1 Starting Head Number0x02 2 Starting Cylinder Number (10 bits) and Sector (6 bits)0x04 1 Descriptor (Type of partition/filesystem)0x05 1 Ending Head Number0x06 2 Ending Cylinder and Sector numbers0x08 4 Starting Sector (relative to begining of disk)0x0C 4 Number of Sectors in partition</pre> <p> (see the examples <a href="files/bootr01.zip">bootr01.zip</a> and <a href="files/smbmbr03.zip">smbmbr03.zip</a>) </TD> </TR></TABLE></body></html>⌨️ 快捷键说明
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