Background: The RTL80192AS incorrectly negotiates the link. The switch thinks, it is half-duplex, but the RTL8019 uses full-duplex. If the RTL8019 transmits a packet while the switch is transmitting, the switch will discard the packet. To avoid this, the RTL8019 must be set to half-duplex. However, this can be done via EEPROM only, which is not available on Ethernut 1. Rev-F boards "emulated" the EEPROM by setting all bits to 1 (via resistor), which automatically enables full-duplex. So we need to set a specific EEPROM bit to 0. The solution was to emulate the EEPROM by the ATmega. In order not to waste port pins for this, Ethernut 1 temporarily uses the upper address bits to emulate an EEPROM.