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THE ETHERNET DEVELOPMENT BOARD

Part 2: The Software and Firmware Exposed
by Fred Eady

Start ý The Basics ý Registers ý Bus Interface Registers ý Status and Control Registers ý InitChip ý Initiate Transmit Registers ý Address Filter Registers ý Receive and Transmit Frame Locations ý CS8900A-CQ Transmit and Receive Operations ý The Next Read ý Broadcast ý ARP ý No Cheating ý Tiger Woodsý Putter ý Whatýs the Point? ý And It Programs, Too! ý Sources and PDF

BROADCAST

In the DLC area of the Sniffer screen shot (see Photo 2), a frame size of exactly 60 tells you that thereýs probably been some padding of the base data to meet the minimum frame requirements. The highlighted areas (hex dump and DLC area) of Photo 3 show the DA filled with "f"s indicating a broadcast frame was received. This is obvious to the Sniffer because it blatantly says "BROADCAST" twice on the same line.

Photo 3ýNotice the highlighted areas of the text area correspond with highlighted areas in the hex dump area. (enlarge)

The source address highlighted in Photo 4 belongs to a PC running an SMC NIC under Windows 98.

Photo 4ýI ran a Sniffer autodiscovery to get the MAC addresses of all of the cards on the Florida-room 192.168.1.0 network, and the SMC cards had a common "00E029" at the beginning of each NICýs MAC address. (enlarge)

Photo 5 highlights an element of the Ethernet header that the Ethernet development board firmware will key on to process the rest of the frame. These two bytes (0806) tell the Ethernet development board code that the frame transferred from the CS8900A-CQ to the PIC is an ARP frame. The rest of the ARP code in Listing 13 checks every field you see in the Sniffer shot (see Photo 6). If it all matches (which it does), then an ARP request has been tendered and the Ethernet development board must assemble and transmit an ARP reply. I threw in Photo 7 to delineate the padded bytes.

Photo 6ýThis shot is like being lost at sea and thirsty. Everything around you spells "water" but one essential ingredient is missing. In this case itýs the target hardware address. (enlarge)

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