Selectable drive startup (sometimes referred to as selectable boot) allows you to control the startup sequence of the drives in your server.  Each time you turn on the server, it checks the drives as it looks for the operating system.  The order in which the system checks the drives is the drive-startup sequence. 

In most cases, there is no need to change the default drive-startup sequence. However, if users are working with multiple hard disk drives, multiple operating systems, different size diskette drives, or they are planning to do remote initial program load (RIPL) from a drive in a network server, they might want to change the sequence.

The default sequence checks the primary diskette drive for a startable diskette. If one is present, the operating system or program is loaded from the diskette.  If not, the system checks the primary hard disk for an operating system.  If one is present, the operating system is loaded from that hard disk drive (Ed. Although CDROM is a valid choice on later systems, it is very demanding. Basically, not supported).  If not, the system displays either the diskette and F1 prompts or the I9990305 error code.

Example:  Default Startup Sequence
 
The example below shows the default startup sequence for the system.
 
Startup Number          Device
1.......[2.88MB 3.5-inch Diskette Drive 0]
2.......[SCSI hard disk slot 1 (ID, LUN).:6,0]
3.......[Not selected]
4.......[Not selected]
 
Note:  The selections and device types vary.

If you start the system from a diskette, the drive containing the diskette becomes drive A, regardless of the defined sequence, and the first hard disk selected in the startup sequence becomes drive C.  You can choose a startup sequence of up to four drives. 
   You can customize the startup sequence by changing the order in which the system checks the drives.  You decide which four drives are the first to be checked, and the order in which the system checks them. 

Note:  When you change the startup sequence, the drive letters also might change. The operating system assigns the drive letters when the system starts. Letters A and B always are assigned to diskette drives. Also, if a CD-ROM drive has a startable (bootable) CD mounted at startup time, the system views the CD-ROM drive as a diskette drive and assigns it drive letter A or B. Subsequent drive letters can be assigned to any type of drive, based on the operating system or the device drivers used. 

Warning: If you changed your startup sequence, you must be extremely careful when doing write operations (for example, copying, saving, or formatting). You can accidentally overwrite data or programs if you select the wrong drive. 

To change the startup sequence: 
 1. Turn on the system and when the F1 prompt appears on the screen, press F1.  The system programs Main Menu appears. 
    Note:  For disk-array models, insert the Reference Diskette into the primary diskette drive, then turn on the system. Or, if the system is turned on already, insert the Reference Diskette and press Ctrl+Alt+Del. 
 2. Select Set features from the Main Menu. 
 3. Select Set startup sequence and follow the instructions on the screen. 

Nice Graphic 

    To make a change, use the Up Arrow key or the Down Arrow key to highlight one of the bracketed items.  Use F5 and F6 to scroll through the list of available choices.   When you are finished with your changes, press F10 to save them.  If the same drive is selected twice in the sequence, an asterisk appears next to the second occurrence to indicate a conflict.  You must resolve all conflicts before you can save the changed sequence.
    Note:  As with all procedures that change the configuration, you should back up the System Partition (for non-array models).  For disk-array models, which do not have a System Partition, the updated configuration information is saved to your Reference Diskette. 

To Change the Startup Sequence
 
To change the startup sequence, do the following:
 
1.  Start the system programs from the hard disk.
2.  Select Set features from the Main Menu.
3.  Select Set startup sequence and follow the instructions on the screen.
 
Example:  Changing the Startup Sequence
 
Assume the following drives are installed in the system:
 
   Diskette Drives
    -   3.5-inch diskette drive (primary)
    -   3.5-inch diskette drive (second)
    -   5.25-inch diskette drive (third)
   Hard Disk Drives
    -   400MB hard disk drive (primary hard disk drive) divided into two 200MB partitions.
 
If you want to start the system from a 3.5-inch diskette, a 5.25-inch diskette, or the hard disk drive, you can define the startup sequence as follows:
 
  Startup Number          Device
 
  1.......[2.88MB 3.5-inch Diskette Drive 0]
  2.......[1.2MB 5.25-inch Diskette Drive 2]
  3.......[SCSI hard disk slot 1 (ID, LUN).:6,0]
  4.......[Not selected]
 
In this example, the system looks for a startable diskette in the primary 3.5-inch diskette drive and then looks for a startable diskette in the 5.25-inch diskette drive.  If there is not a startable diskette in either drive, the system loads the  operating system from the System Partition on the hard disk drive.