So Chuck ... do you know exactly how this works (as opposed to how it is documented)? 128 bytes/sector is a special case in the 765 (at least it is documented as such). According to the NEC documents, for the sector read and write commands, if you give it a value of 0 for 'n' (6th byte of the command = Number of bytes/sector), the actual number of bytes read/written is determined by the 9th byte written 'DTL' (DaTa Length). The docs don't go into much detail, and I haven't fooled with it much - I just set DTL to 128 when I am reading or writing 128 byte sectors (n=0). A few questions which are not clearly answered by the docs: under "READ DATA" "When N=0, then DTL defines the data length which the FDC must treat as a sector. If DTL is smaller than the actual data length in a sector, the data beyond DTL in the sector is not sent to the data bus. The FDC reads (internally) the complete Sector performing the CRC check, and depending upon the manner of the command termination, may perform a multi-sector read operatiohn. When 'N' is non-zero, then DTL has no meaning and should be set to FF hexidecimal" under "WRITE DATA" there is mention of N=0 or DTL ... but DTL is shown as a part of the write commands, and generally described as "DTL - Data Length - when N is defined as 00, DTL stands for the data length which users are going to read out or write into the sector". The read description seems to imply that the FDC "knows" the actual length of a sector, and only reads the number of bytes specified, while calculating the CRC over the whole sector. - How does it "know" the sector size ... the 'n' field in the ID header is the only place in the sector data that defines the size. - Does it assume the sector is 128 bytes? - or Does it actually read (and check CRC) on a smaller sector? -What is the actual behaviour during write? - Does it fill the remainder of a 128 byte sector with some value? - or Does it actually write a smaller sector? under "FORMAT TRACK" (in the chart notes) "(3) In MFM mode the FDC cannot perform a read/write/format operation with 128 bytes/sector (N = 00)" This implies that 128/MFM simply doesn't work - not even for format (which you indicated earlier actually does work). Is this a fairly reliable assumption? Does DTL have any effect on 128/MFM reads or writes? You mentioned that 80 bytes get transferred instead of 128. Does changing DTL make a difference? Or is it always 80 bytes? Dave -- dave12 (at) Dave Dunfield dunfield Firmware development services & tools: www.dunfield.com (dot) com Classic computers: http://www.classiccmp.org/dunfield/