![]() Since the DOS boot sector is executed every time you power on or boot your PC, it is very vulnerable to virus attack. Bootable CD-ROMs, if infected by whomever wrote out the disc, could also be the source of an infection. Other removable devices have DOS boot sectors that are defined by the format utility for that device. ![]() This special sector is numbered 0,0 and is called the Boot Sector.Ī hard disk also has a DOS boot sector, but it’s located in a different sector on the hard disk (see note below). storing a short program that either puts a message on the screen saying the disk cannot be used to start the computer if it’s a data disk, or a short program that starts to load the operating system if it’s a system disk which can start the computer (boot disk).storing information about the disk, and.They are written by a WRITE TRACK controller command that interprets F5 and F6 to write those special bytes.Even a non-bootable floppy can contain a virus in the boot sector.When a floppy diskette is formatted (it doesn’t matter if it’s a system or data disk) the very first portion of the disk is set aside for two main purposes: ![]() The A1 and C2 marker bytes are special and don't follow the standard MFM encoding: A1 is missing a clock transition between bits 4 and 5, and C2 is missing a clock transition between bits 3 and 4. ![]() Some gap lengths can differ between variants. Here is a transcription of the byte patterns (in hex) from the datasheet: 80x 4E Gap 4aĬRC is calculated by hardware. You can find details in the datasheet of the controller, and in IBM mainframe manuals. The IBM PC floppy controller chips are descendants of the venerable NEC µPD765, which implements the IBM 34 MFM format as used in the mainframes. ![]() Interleaving is handled by software, I don't know the details. So the bit pattern structure is the same, even though number of sectors per track etc. All IBM floppy variants (5.25″ and 3.5″, DD and HD) use the same floppy controller, just the timing is different. ![]()
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