Update Cisco Catalyst Software

I had to update the software of a new Cisco Catalyst 4948 yesterday.
As usual I did:

copy tftp://<hostname>/<filename> bootflash:
conf t
boot system flash bootflash:<filename>
exit
reload

But the switch ignored the new software image.
During boot it said:

Booting first image from bootflash

Solution: The config-register was set to 0x2101 right out of the box. I had to change this to 0x2102. With this value the switch honors the bootvar and boots into the configured system image.

conf t
config-register 0x2102
exit
reload

The config-register has different meanings on different cisco plattforms.
For the Catalyst 4948 the config-register contains a bit field explained at:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/31sga/configuration/guide/supcfg.html#wp1051990