MSX Tape I/O
-------------

BIT AND BYTE ENCODING

Information on tape is stored as square waves of 1200 or 2400 baud. The
encoding used is FSK (frequency shifting key). The next table shows the
relation between the selected recording speed, the bits and the pulses
put on tape:

bps   bit   encoding          freq (Hz)
                    _______
1200   0    _______|       |  1200
                ___     ___
1200   1    ___|   |___|   |  2400

                ___
2400   0    ___|   |          2400
              _   _
2400   1    _| |_| |          4800

Each byte is put on tape as a sequence of 11 bits:

 0 b0 b1 b2 b3 b4 b5 b6 b7 1 1  (one startbit, 2 stopbits, LSb first)

Futhermore 2 kinds of headers are used: short and long, a long header being
4 times as long as a short one. They consist of a series of square wave pulses
(bit value 1):

bps   header   freq (Hz)   nr of pulses 
1200  short    2400        4000             (5/3  sec)
1200  long     2400        16000            (20/3 sec)
2400  short    4800        8000
2400  long     4800        32000


FILE FORMATS

Files on tape are stored in two blocks. The first block is the filedescriptor,
the second contains the filedata.  Three types of files exist:
BASIC file, memory dump and data file.

BASIC File (CSAVE/CLOAD)
  filedescriptor: long header
                  10x #D3       (filetype)
                  6 bytes filename

  file data:      short header
                  tokenized BASIC
                  7x #00

memory dump (BSAVE/BLOAD)
  filedescriptor: long header
                  10x #D0
                  6 bytes filename

  file data:      short header
                  2 bytes start address
                  2 bytes end address
                  2 bytes exec address
                  memory dump

data file (SAVE/LOAD/OPEN)
  filedescriptor: long header
                  10x #EA
                  6 bytes filename

  file data:      short header
                  block of 256 bytes

                  short header
                  block of 256 bytes

                  ....
                  last block contains 256x #1A (^Z/EoF)