Cepheus (4.10.21) is a SPARCStation. At boot time SPARCstation's are under the control of the Sun OpenBoot. Obtain the OpenBoot prompt with the keyboard L1 (which might be labelled Stop)and A simultaneously. The prompt is a simple ``ok''. Note that the prompt can even be obtained from within SILO (the Linux Loader for Sparc) or using the halt command in SILO. Then boot the machine from CD-ROM with:
ok boot cdrom
At the boot prompt choose s (SCSI) and the system then boots from the CD-ROM. To boot single user simply use boot -s.