The kernel allows to pass <irq>,<io>,<mem_start>,<mem_end>,<name> options to interfaces using the netdev= option and the deprecated ether= one (kernel-parameters.txt)

Surprisingly, the mem_start option does not match at all its initial meaning (in the context of shared memory). And the Linux devices drivers book does not clearly states that it is now used otherwise.

The Ethernet HOWTO documents it as PARAM_1, which is not very meaningful on purpose. Some drivers uses this mem_start/PARAM_1 field for debugging purposes, some for DMA channel selection, and most for media type (if_port, which can be IF_PORT_10BASET or IF_PORT_100BASETX for example). This is the case in the epic100 driver, which I stumbled upon on old bug #888