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## 6. How do i know the BIOS number for my SCSI disks

The contribution from Marc Tanguy (mtanguy@ens.uvsq.fr), 2001-09-27

## 6.1 The theory

Actually, it exists two ways to know it :

If you have an adaptec scsi card (2940u2, 29160, 39160), you simply use the 'diagnose' mode (using BIOS v3.10.0 recommended). It must be activated in the scsi card BIOS menu. Then you just have to wait and see something like :

... | ID | LUN | Vendor | Product | Rev | Size | Sync | Bus | HD#
... | 0 | 0 | QUANTUM | ATLAS10K2 | DDD6 | 17GB | 160 | 16 | 80h
... | 1 | 0 | QUANTUM | ATLAS10K2 | DDD6 | 17GB | 160 | 16 | 81h
... | 2 | 0 | IBM | DDRS | DC1B | 4GB | 80 | 16 | 82h
... | 3 | 0 | IBM | DNES | SAH0 | 9GB | 80 | 16 | 83h

If you don't own an adaptec card, you have to know what is the 'booting' disk (usually ID 0, but not necessary, it can be defined in the scsi card BIOS) where LILO is going to be found and start : this is the first disk so it has number 0x80. Then it's very simple, the BIOS follows the IDs.

By example :

    ID 0 -> boot  -> 0x80
    ID 1 -> empty
    ID 2 -> disk  -> 0x81
    ID 3 -> disk  -> 0x82

or

    ID 0 -> disk  -> 0x81
    ID 1 -> empty
    ID 2 -> disk  -> 0x82
    ID 3 -> boot  -> 0x80
    ID 4 -> disk  -> 0x83

This part doesn't care at all of what is installed on the scsi drives. But you should note that if you use an ID higher than the SCSI adapter it c an be a problem. So you should always try to set the SCSI adapter ID after the SCSI devices IDs.

## 6.2 How to swap linux and NT booting ?

OK, but NT must be the first disk to boot, so i want it in 0x80, but i already have LILO and a full ext2 only drive on 0x80 and my NT drive is in 0x83. How can i 'swap' linux and NT ? This a very easy : you just have to tell BIOS that NT drive is now 0x80 and the Linux drive is 0x83.

    other=/dev/sdd1
          label=nt
          map-drive = 0x83
          to = 0x80
          map-drive = 0x80
          to = 0x83

This change will produce a warning :

      Warning: BIOS drive 0x8? may not be accessible

but if you know what you are doing it will run without problem.

I used it on this configuration which has a Red Hat Linux 7.1 and a Windows 2000 Pro :

    Name        Flags      Part Type  FS Type          [Label]        Size (MB)

    Disk Drive: /dev/sda - 0x80
    sda1        Boot        Primary   Linux ext2       [/boot]            24.68
    sda2                    Primary   Linux Swap                         139.83
    sda3                    Primary   Linux ext2       [/usr]           3150.29
    sda4                    Primary   Linux ext2       [/home]         15044.04

    Disk Drive: /dev/sdb - 0x81
    sdb1                    Primary   Linux Swap                         139.83
    sdb2                    Primary   Linux ext2       [/]              3150.29
    sdb3                    Primary   Linux ext2       [/opt]           1052.84
    sdb4                    Primary   Linux ext2       [/public]       14015.88

    Disk Drive: /dev/sdc - 0x82
    sdc1                    Primary   Linux ext2       [/var]           1052.84
    sdc2                    Primary   Linux ext2       [/tmp]            106.93
    sdc3                    Primary   Linux ext2       [/cache]         1052.84
    sdc4                    Primary   Linux ext2       [/chroot]        2352.44

    Disk Drive: /dev/sdd - 0x83
    sdd1        Boot        Primary   NTFS             [WINDOWS_2000]   9162.97

My full /etc/lilo.conf :

    boot=/dev/sda
    map=/boot/map
    install=/boot/boot.b
    prompt
    default=Linux
    read-only
    compact
    image=/boot/vmlinuz
      label=Linux
      root=/dev/sdb2
    other=/dev/sdd1
      label=Windows
      map-drive = 0x83
      to = 0x80
      map-drive = 0x80
      to = 0x83

## 6.3 Miscellaneous

I just plugged a new scsi drive, and now LILO refuse to boot, what's going on ?

When you plug a disk, you must be careful with the IDs. If you add a drive between two already plugged disks the BIOS numbers are changed :

             Before            ---->             After
    scsi id -       - BIOS id        scsi id -          - BIOS id
    ID 0    - disk  - 0x80           ID 0    - disk     - 0x80
    ID 1    - empty                  ID 1    - new disk - 0x81
    ID 2    - disk  - 0x81           ID 2    - disk     - 0x82 !!

If you change the BIOS ids, you have to re-evaluate them.

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