MIDI driver module
------------------
v0.06 30/10/23

This module is a key component of the Midi Support system and
should be placed in the MidiSupport Modules directory.
It replaces the Acorn MIDI module or indeed any module that
reports itself as MIDI. From the users perspective, operation
is as the Acorn module with the following exceptions.

  Differences from the Acorn MIDI Specification:
    Fast Clock mode is the default.
    The default Tx Timing clock rate is 90 bpm in fast clock mode.
    There is no Midi Interpreter, so SWI's and *commands that
    control the internal sound system are not implemented.
  
  Commands not implemented.
    MidiSound
    MidiTouch
    MidiChannel
    MidiMode
    
  Additional Commands:
    MidiTxChannel  Same as the SWI MIDI_SetTxChannel
    Midi4Debug     Sets options and displays status
    
  SWI's not implemented:
    MIDI_SoundEnable
    MIDI_SetMode
    MIDI_SynchSoundScheduler
    MIDI_SetBufferSize
    MIDI_Interface
    
  Additional SWI's: (for compatibility with USBMidi, module operation
                    not affected)
    MIDI_USBInfo
    MIDI_Options
    MIDI_Remap
    
  Errors not reported:
    'O' UART overrun
    'F' UART framing error
    'V' Internal sound system voice error
    'T' Tx Fifo overflowed
    'L' Internal sound system note error
