prep revision 1.6
1 $NetBSD: prep,v 1.6 1998/01/09 18:46:32 perry Exp $ 2 3First and foremost, before beginning the installation process, MAKE 4SURE YOU HAVE A RELIABLE BACKUP of any data on your hard disk that you 5wish to keep. Mistakes in partitioning your hard disk may lead 6to data loss. 7 8Before you begin, you should be aware of the geometry issues that may 9arise in relation to your hard disk. First of all, you should know 10about sector size. You can count on this to be 512 bytes; other sizes 11are rare (and currently not supported). Of particular interest are 12the number of sectors per track, the number of tracks per cylinder 13(also known as the number of heads), and the number of cylinders. 14Together they describe the disk geometry. 15 16The BIOS has a limit of 1024 cylinders and 63 sectors per track for 17doing BIOS I/O. This is because of the old programming interface 18to the BIOS that restricts these values. Most of the big disks 19currently being used have more than 1024 real cylinders. Some have 20more than 63 sectors per track. Therefore, the BIOS can be instructed 21to use a fake geometry that accesses most of the disk and the fake 22geometry has less than or equal to 1024 cylinders and less than or 23equal to 63 sectors. This is possible because the disks can be 24addressed in a way that is not restricted to these values, and the 25BIOS can internally perform a translation. This can be activated 26in most modern BIOSes by using 'Large' or 'LBA' mode for the disk. 27 28NetBSD does not have the mentioned limitations with regard to the 29geometry. However, since the BIOS has to be used during startup, 30it is important to know about the geometry the BIOS uses. The 31NetBSD kernel should be on a part of the disk where it can be 32loaded using the BIOS, within the limitations of the BIOS 33geometry. The install program will check this for you, and 34will give you a chance to correct this if this is not the case. 35 36If you have not yet installed any other systems on the hard disk 37that you plan to install NetBSD on, or if you plan to use the 38disk entirely for NetBSD, you may wish to check your BIOS 39settings for the 'Large' or 'LBA' modes, and activate 40them for the hard disk in question. While they are not needed 41by NetBSD as such, doing so will remove the limitations mentioned 42above, and will avoid hassle should you wish to share the disk with 43other systems. Do NOT change these settings if you already have 44data on the disk that you want to preserve! 45 46In any case, it is wise to check your the BIOS settings for the 47hard disk geometry before beginning the installation, and write 48them down. While this should usually not be needed, it enables 49you to verify that the install program determines these values 50correctly. 51 52The geometry that the BIOS uses will be referred to as the "BIOS 53geometry", the geometry that NetBSD uses is the "real geometry". 54 55Sysinst will try to discover both the real geometry and BIOS 56geometry. If a Master Boot Record (MBR) has been written to 57your disk by DOS, Windows 95, NT, or another system, sysinst should 58have no trouble find the BIOS geometry. If the disk has not had a MBR 59written to it, it may be much more difficult to get the BIOS geometry. 60 61It is *important* that sysinst know the proper BIOS geometry to be able 62to get NetBSD to boot, regardless of where on your disk you put it. 63It is less of a concern if the disk is going to be used entirely for 64NetBSD. If you intend to have several OSes on your disk, this becomes 65a much larger issue. 66 67Another issue with geometry is with SCSI disks. The geometry reported 68by the disk often does not address all addressable sectors on the disk. 69sysinst will assist you if you want to choose a fake geometry for 70NetBSD to use so it can get access to more sectors. This is not 71the same as the BIOS fake geometry. 72