The following are details about how the bits on the Commodore-Amiga disk are actually written. Gross Data Organization: 3 1/2 inch (90mm) disk double-sided 80 cylinders/160 tracks Per-track Organization: Nulls written as a gap, then 11 or 22 sectors of data. No gaps written between sectors. Per-sector Organization: All data is MFM encoded. This is the pre-encoded contents of each sector: two bytes of 00 data (MFM = $AAAA each) two bytes of A1* ("standard sync byte" -- MFM encoded A1 without a clock pulse) (MFM = $4489 each) one byte of format byte (Amiga 1.0 format = $FF) one byte of track number one byte of sector number one byte of sectors until end of write (NOTE 1) [above 4 bytes treated as one longword for purposes of MFM encoding] 16 bytes of OS recovery info (NOTE 2) [treated as a block of 16 bytes for encoding] four bytes of header checksum [treated as a longword for encoding] four bytes of data-area checksum [treated as a longword for encoding] 512 bytes of data [treated as a block of 512 bytes for encoding] NOTE: ----- The track number and sector number are constant for each particular sector. However, the sector offset byte changes each time we rewrite the track. The Amiga does a full track read starting at a random position on the track and going for slightly more than a full track read to assure that all data gets into the buffer. The data buffer is examined to determine where the first sector of data begins as compared to the start of the buffer. The track data is block moved to the beginning of the buffer so as to align some sector with the first location in the buffer. Because we start reading at a random spot, the read data may be divided into three chunks: a series of sectors, the track gap, and another series of sectors. The sector offset value tells the disk software how many more sectors remain before the gap. From this the software can figure out the buffer memory location of the last byte of legal data in the buffer. It can then search past the gap for the next sync byte and, having found it, can block move the rest of the disk data so that all 11 sectors of data are contiguous. Example: The first-ever write of the track from a buffer looks like this: <GAP> |sector0|sector1|sector2|......|sector10| sector offset values: 11 10 9 ..... 1 (If I find this one at the start of my read buffer, then I know there are this many more sectors with no intervening gaps before I hit a gap). Here is a sample read of this track: <junk>|sector9|sector10|<gap>|sector0|...|sector8|<junk> value of 'sectors till end of write': 2 1 .... 11 ... 3 result of track re-aligning: <GAP>|sector9|sector10|sector0|...|sector8| new sectors till end of write: 11 10 9 ... 1 so that when the track is rewritten, the sector offsets are adjusted to match the way the data was written. Sector Label Area ----------------- This is operating system dependent data and relates to how AmigaDOS assigns sectors to files. Reserved for future use.
[Back to Amiga Developer Docs]