important at boot time if the user wants to control the boot process.
TODO:
* implement timing for write and format commands
* more accurate timing for seek, recalibrate and maybe others
* raw floppy access slows down the emulation, use separate threads (???)
- irq line goes low with when reading the first result byte
- the drive status bits are only modified after reading the first result byte
- handling of floppy command 'sense interrupt status' simplified
- improved debug messages for i/o port reads and writes
