What is the "base address" in ifconfig output on some systems?

On the Busybox system I'm using, the output of busybox ifconfig -a includes a field for "Base address":

~ # busybox ifconfig -a
eth0      Link encap:Ethernet  HWaddr 00:D0:93:4D:AE:4F  
          inet addr:192.168.9.159  Bcast:192.168.9.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1 errors:0 dropped:0 overruns:0 frame:0
          TX packets:140 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:60 (60.0 B)  TX bytes:7136 (6.9 KiB)
          Base address:0xd000 

eth1      Link encap:Ethernet  HWaddr 00:D0:93:4D:AE:50  
          inet addr:192.168.9.160  Bcast:192.168.9.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:20 errors:0 dropped:0 overruns:0 frame:0
          TX packets:21 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:1826 (1.7 KiB)  TX bytes:1678 (1.6 KiB)
          Base address:0x1000 

eth2      Link encap:Ethernet  HWaddr 00:D0:93:4D:AE:51  
          BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)
          Base address:0xd000

I have been unable to find any information on what this represents, despite extensive searching. Can someone tell me what it is, since it's clearly considered important and relevant enough to include in ifconfig output?