I use aboot to boot my Alpha. It has a config file called /etc/aboot.conf. You can see my config below. Things worth noting are configs 0 and 8 (which are the ones I use, the other ones are only for rescue use.
Config 0 just boots my standard kernel, with an initrd containing my SCSI drivers.
Config 8 boot the same kernel, but with a serial console (that's how I've captured all the bootup messages).
To boot with a serial console, I just disconnect the keyboard/mouse, connect a serial cable to the top serial port, and boot the machine. In SRM I then give this command:
Digital AlphaPC 164 366 MHz Console V5.5-1, Jul 14 1999 12:37:45 >>>boot -flags 8
# NOTICE: You have a /boot partition. This means that # all kernel paths are relative to /boot/ 0:1/vmlinuz-2.4.16 root=/dev/hda5 initrd=initrd-2.4.16.img 1:1/vmlinuz-2.4.3-12 root=/dev/hda5 console=ttyS0 2:1/vmlinuz-2.4.3-12 root=/dev/hda5 console=ttyS0 console=tty0 3:1/vmlinuz-2.4.3-12 root=/dev/hda5 single 4:1/vmlinuz-2.4.3-12 root=/dev/hda5 floppy=nodma,nofifo 5:1/vmlinuz-2.4.3-12 root=/dev/hda5 6:1/vmlinuz-2.2.19 root=/dev/hda5 ether=10,0x300,eth0 7:1/vmlinuz-2.4.14 root=/dev/hda5 ether=10,0x300,eth0 cycle=366000000 8:1/vmlinuz-2.4.16 root=/dev/hda5 initrd=initrd-2.4.16.img console=ttyS0
To be able to login on the serial console, you need to start a getty on it as well. To start it at bootup, modify /etc/inittab. Here's the relevant bit from mine:
6:2345:respawn:/sbin/mingetty tty6 S1:12345:respawn:/sbin/mgetty -r -s 9600 ttyS0 vt100