ASID -> SidStation MIDI SysEx data protocol
*******************************************
interpretation by 
Jouni Paulus, jobe(a)iki.fi, 19.12.2003
rev 2, 20.12.2003

start playing .sid data:
  F0 2D 4C F7
  
stop playback:
  F0 2D 4D F7
  
display characters on LCD:
  F0 2D 4F <charseq> F7
  <charseq> = <char>* (zero or more chars)
  <char>    = ASCII_CODE bitwise_and 7F
  
send command to SID:
  F0 2D 4E <command> 7F
  <command> = <mask1><mask2><mask3><mask4><msb1><msb2><msb3><msb4><data>
  <maskX> = in this byte the bit number "id" is set for write to the
            corresponding reg according to the table below, 
            e.g. write to regs 0 and 1 and not to 2,3,5,6,7 causes the
            <mask1> to be "000011" = 0x03 etc
  <msbX>  = similarly to <maskX>, containing the MSB (8th bit) of the
            data to be written to the corresponding register
  NB: If two consecutive writes occure to the registers 0x04, 0x0B or 0x12 
      within on sent frame, the second write will be indicated as writes
      to the registers 0x19, 0x1A and 0x1B. This is probably due to some 
      timing matters. 
 
  <data> = actual data for the registers
           each byte containing the 7 lowest bits of the data byte
           data bytes are sent only for the registers that have been indicated
           in the maskX bytes, i.e. in binary representation the number of 1-bits
           in all 4 mask bytes is equal to the numbers of bytes in <data>

  NB2: There is still some things to be cleared with the writes to filter
       registers, 0x15 and 0x16.
           
   id  reg hex
  -------------
   00  00  00 LSB
   01  01  01
   02  02  02
   03  03  03
   04  05  05
   05  06  06
   06  07  07 MSB
  -------------    
   07  08  08 LSB
   08  09  09
   09  10  0A
   10  12  0C
   11  13  0D
   12  14  0E
   13  15  0F MSB
  -------------
   14  16  10 LSB
   15  17  11
   16  19  13
   17  20  14
   18  21  15
   19  22  16
   20  23  17 MSB
  -------------
   21  24  18 LSB
   22  04  04
   23  11  0B
   24  18  12
   25  25  19 <= secondary for reg 04
   26  26  1A <= secondary for reg 11
   27  27  1B <= secondary for reg 18 MSB