There is a delay between the index pulse and the start of bits coming in from the drive (e.g. dma started). It is in the range of 135-200 microseconds. This delay breaks down as follows: 55 microseconds for software interrupt overhead (this is the time from interrupt to the write of the DSKLEN register); 66 microsecs for one horizontal line delay (remember that disk I/O is synchronized with Agnus' display fetches). The last variable (0-65 microseconds) is an additional scan line since DSKLEN is poked anywhere in the horizontal line. This leaves 15 microseconds unaccounted for. In short, you will almost never get bits within the first 135 microseconds of the index pulse, and may not get it until 200 microseconds. At 4 microsecs/bit, this works out to be between 4 and 7 bytes of user data delay. Forewarned is Forearmed. ------------------------ Amiga, Inc. may make enhancements to the disk format in the future. Amiga, Inc. intends to provide compatibility within the trackdisk device. Anyone who uses these raw routines is bypassing this upward-compatibility and does so at her own risk.
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