can someone please explain the comment
   
   ## NFS boot options; default on sparc is the bootparam protocol
   options         NFS_BOOT_BOOTPARAM
   #options        NFS_BOOT_BOOTP
   #options        NFS_BOOT_DHCP
   
   which can be found in may of the sparc config files.  I'm
   particularly thinking of the first comment.
   
   In what sense is booparam "the default"?  In the sense that it
   does not need to be defined?  I recently had an experience where
   I upgraded from 1.3.2 to 1.4_BETA, and I now had to add the
   NFS_BOOT_BOOTPARAM define in order for a diskless boot to succeed
   (otherwise it would fail to mount the root file system).

is it the 'default' in that it is uncommented on the sparc,
as shown above from your snippet.
   
   Thus, I think something must have changed between 1.3.2 and
   1.4_BETA which makes the above mentioned comment misleading.


yes.  all the nfs boot code was made conditional on the type
of support needed (bootparams, bootp, or dhcp), and kernel options
were created for these.  much like the exec options were created
some time ago (exec_aout, exec_elf32, etc.) to allow ports to not
have, eg, a.out support.  the old code (i think) included bootparams
unconditionally, which wasn't much use for non-sun folks.


these options are documented in options(4) also;  you could have
looked there :-)