Can I install both SunOS and Solaris on the same machine,

and choose between them at boot time?

Yes, that is possible. All partitions other than the system
partitions (typically /, /usr, /var and /opt) can be shared by the
two OSes. All partitions, including the system partitions, can be
mounted and accessed by either OS.

The easiest way to set this up is to do separate suninstalls on two
different disks. Then just choose the appropriate disk at boot
time with the PROM's "boot" command.

Setting up both OSes on one disk is a little harder, but not much.
You need to partition the disk to allow for both OSes. Almost any
partition layout is possible, but one common setup might be:

a: / for Solaris 2
b: swap (shared)
c: The usual (whole disk)
d: / for Solaris 1
e: /usr for Solaris 1
g: /usr for Solaris 2

Again, it's most reliable to use suninstall to do the installations.
If for some reason you choose not to use suninstall, make sure you run
installboot for both bootable partitions.

With this setup, you choose between the two OSes in the PROM's "boot"
command as follows:

To boot Solaris 2: boot
To boot Solaris 1: boot disk:d

NOTE: In boot PROM versions <= 2.5, the "disk:d" syntax is not supported,
and the PROM cannot boot from root partitions that begin or end beyond 1GB.

Since there is no hardware which supports both SunOS 4.x
and Solaris 10, this is no longer relevant for S10.

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