Thanks Paul for your suggestions. I try to provide some answers. The original IBM AT BIOS that I have on my TYPE 1 motherboard has two prom labelled: 6181028 and 6181029. The present drive in my IBM PC AT is a 3.5" 720K and the boot is not made ??by it or rather I think the bios expects to have an A drive as 1.2 MB which I have not. Therefore I downloaded from here: http://www.minuszerodegrees.net/bios/bios.htm an AMI BIOS SUITABLE FOR 5170 (Dated 30MAR89) which made available the only position 47 to add hard drives. Biosutil works well: I read the bios AMI, i save it in the file, allows you to change the position 47 with the geometry inserted for the new hard disk, recalculates the checksum of the file and save it to a even file and an odd file 32KB each . The saved file is BIN type and not hex. I am not in a position to know whether the calculated CheckSum is correct with respect to the expected one. With the AMI bios at boot i heard a beep and then it boot from the hard disk, but with the bios changed to make the geometry of the hard disk to a permanent position 47 nothing happens. no reply. I thought I had reversed the two EPROM, inverting them it does not change the result. Not being able to load the dos disk with the biosutil from my 720kb drive and i not having to drive 1.2 mb I can not boot dos and biosuil with original firmware. But it could not be true that biosutil allows you to edit an existing position. Dear Fritz, I can not fully understand how to use an EPROM with a network card and XTide. Why an unmodified AT bios at its boot if it can not boot from any drive, it should load the contents from an EPROM of the network card and run it? And what should I save in the EPROM of the network adapter so that it allows to see my hard drive? Enrico