Agat 840k MFM, 300rpm, 250000 bps 5"25 80 Tracks, 21 Sectors per track, Sector 256 bytes, 2 sides, 860160 bytes formatted. Un-Formatted track capacity 6250 bytes. ============================================================================= DOS Track format 1. GAP1 13x 0xAA bytes 2. 21 sectors of the following format: 2.1 Desync: 0xA4, 2 ms zero level interval, 0xFF 2.2 Address field: 0x95, 0x6A (2 byte, address field prologue), volume number (1 byte, default 0xFE), Track number (1 byte, 0x00 - 0x9F), Sector number (1 byte, 0x00 - 0x14), 0x5A (1 byte, address field epilogue) 2.3 GAP2 5x 0xAA bytes 2.4 Desync: 0xA4, 2 ms zero level interval, 0xFF 2.5 Data field: 0x6A, 0x95 (2 byte, data field prologue), 256 Data Bytes, CRC (1 byte), 0x5A (1 byte, data field epilogue) 2.7 GAP3 22x 0xAA bytes 3. There is NO GAP4, after last sector track is exactly 6250 bytes Agat is using Apple 2 Disk checksum algorithm. Despite of being called "simple arithmetic sum" it is more complicated: function CalcCRC ($data) { $c=0; for ($n=0;$n<=255;$n++) { if ($c>255) {$c++; $c=$c & 255;} $c=$c+$data[$n]; } return $c & 255; } Sector numbering is 0..20, Sector order and interleave is not important. Usual sector order is 0..20 Address field has no disk side information. Tracks are numbered 0..159 and present on disk in following order: Track 0, Side 0: Track 0 Track 0, Side 1: Track 1 Track 1, Side 0: Track 2 Track 1, Side 1: Track 3 And so on. Acknowledge: Oleksandr Kapitanenko (http://www.torlus.com/floppy/forum/viewtopic.php?f=19&t=1385)