use an eprom (27C2001/27C020 or bigger) or flashrom (29F020 or bigger)
connect pins 22, 24 and 31 of the eprom to GND (pin 16 of the
you might have to connect 31 to 5V (pin 32) if you are using a flashrom.
FOR XBOX v1.0/v1.1
The two unused points are only two additional Adress-Lines for the Chip (with which you can access everything beyong 256 kByte) ... so they are NOT needed.
FOR XBOX v1.2/v1.3 (Thanks to D-BlooD who posted this diagram on forums.xbox-scene.com)
FOR XBOX v1.4/v1.5 (Thanks to pcmerc who posted this diagram
WE is the location you jump on the bottom of the board along with the other point on top that is standard on all motherboards. These are the write enable for reflashing the TSOP.
Pad 33 is also known as link to 32 in previous pinouts.