The numbers you program into the 765 are not in bytes. Those are counter values (usually a up counter) so that putting in 255 means 1, the however the count values are prescaled such that in some cases putting in 254 does not yeild 2 of something but 4 or even 8 [(0-n) * base]. Example are HLT (head load time) and HUT (head unload time) and even they are affected by what the 8/4 mhz clock supplied. Byte counts like the gaps are similar. So a gap length if FBh might really be 4*16 gap bytes. One note: those counters have different prescale values for FM then MFM. Hope this helps and I'm really testing 25 year old memory from my NEC apps engineeing days. No it does not in the fullest sense. The WD and NEC designs are programtically as different as night and day. The level of internal automation is higher for the NEC but doing that takes control from the programmer. NEC used to have a part (upD371) that was like the 1771 in that it required a fair amount of processer attention at the per byte level to affect transfers especially formatting. Where the 765 family all the programmer supplies for formatting is the initial command and then the sector header info (CHRN) on a per sector IO. The result is smaller format buffer at the expense of limited format options. IE:the general layout of a track is allways the same save for gaps and sector lengths. Correct and the information provided may help in deciding what those counters corospond to. There are two cases of gap 4, gap 4A(index mark) and gap 4B(EOT to index). I've seen gap 4b refered to as gap 5. In the nominal IBM gap 4A is index gap and 4B is the end of track filler gap. FYI: gap 4 it not optional on 765 and excluded in 7265. Functionally its use is been abused and sometimes not included (1771/1791 it's optional). Yes for the fill to EOT. The INDEX signal is important to the 765 for start of track, end of track and read fail. Allison