Compaq AlphaServer DS20E Reference Manual page 88

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Examples
In the following OpenVMS example, root_number is set to 2 and boot_flags is set
to 1. With this setting, the system will boot from root directory SYS2.SYSEXE
to the SYSBOOT prompt when you enter the boot command.
P00>>> set boot_osflags 2,1
In the following OpenVMS example, root_number is set to 0 and boot_flags is set
to 80. With this setting, you are prompted for the name of the secondary
bootstrap file when you enter the boot command.
P00>>> set boot_osflags 0,80
4-10 DS20E Reference Guide