ROM Loader
==========
  info by Richard Storer <rstorer@cyberspace.org> ,
          Ben Castricum  <webmaster@cia.c64.org> ,
          Tomaz Kac      <Tomaz.Kac@uni-mb.si>

Encoding:
---------

  Three different Pulses are used : (these are NOT 100% accurate)

 Short  (s) : 616  T-States
 Medium (m) : 896  T-States
 Long   (l) : 1176 T-States

 Pilot wave:           s
 Sync waves:           l , m
 Data waves:  Bit 0:   s , m
              Bit 1:   m , s
 Trailing Tone wave:   s

 Each Data Byte has: - 8 Data Bits  (LEAST Significant bit First !)
                     - Checksum Bit (XOR of all Bits (starting value is 1))
                     - End waves

 End waves can be: Continue   :  l , m   (if there is more data to follow)
                   End Of Data:  l , s   (if there is no more data)

 Trailing Tone is present after Repeated blocks (see below) only !

Still to find out: 100% Correct timings (Now gathered from TAP files) !
~~~~~~~~~~~~~~~~~~

Structure:
----------

  When C64 Saves data to tape it normally creates 4 tape blocks. First two are
HEADER (always the same length: 202 bytes) and last two are DATA. Why two blocks
for each ? For security reasons... i.e. Header is saved two times (with
different flag and some other stuff) and Data is saved twice too...

So the structure on tape is:

   Header
   Header repeated
   Data
   Data repeated

  Pilot tone of Header is around 25000 waves long (it can be longer or shorter),
the pilot tone of Data is around 6000 waves long (again, it varies). BUT both
repeated blocks have always 79 waves of pilot tone.

  It is a good idea to set the shortest possible pilot tone length to 79 waves,
since a lot of programs save some additional pilot tones which can confuse the
decoder... :)

  Both Header and Data blocks have the following sequence in first 9 bytes:

$89 $88 $87 $86 $85 $84 $83 $82 $81

  Both Header and Data REPEATED blocks have the same sequence with bit 7 reset:

$09 $08 $07 $06 $05 $04 $03 $02 $01

  Normally the Header Repeated and Data Repeated have a Trailing Tone after
the Data, which is normally 78 or 79 s waves ! Some loaders depend on these
being present !

  After those 9 bytes, comes the header block (192 bytes):

    00 File Type: this can be any number from $01 to $05

       $01 - Basic file
       $02 - Data block (for a seq file)
       $03 - Fixed address file
       $04 - Seq file
       $05 - End of tape maker

       $01 (Basic) files are relocatable: the start address is moved to the
       location pointed to by the locations 43 ($2B) and 44 ($2C), and the end
       address is moved by the same amount. This can be overridden by typing
       LOAD"NAME",1,1 (to load the file named NAME) or LOAD"",1,1 (to load the
       first found file): in those cases, the start and end addresses provided
       by the header will always be used. $03 files are fixed-address files: the
       start and end addresses provided by the header will always be used, no
       matter whether the flag ,1,1 is set or not.

    01 Start Address (Low, High byte)
    03 End   Address (Low, High byte)
    05 FileName (16 bytes)
    21 data (171 bytes)

The header block is stored in RAM in the so-called "cassette buffer", from
location 828 ($033C) to location 1019 ($03FB). Normally the data after the
filename are all spaces (ASCII $20), but, in many cases, some data is here: for
example, many turbo loaders put part of their code in here. After the header
block, a checksum byte (XOR of all the 192 header block bytes) is present.

  For the Data and Data Repeated block the Data starts immediately after the
starting 9 bytes sequence. At the end of Data block, a checksum (XOR of all data
bytes) is present.

Still to find out: - Correct pilot tone lengths for Header and Data
~~~~~~~~~~~~~~~~~~