BLOCKED FILE SAVING AND LOADING BY J.G.HARSTON
               ==============================================
                        VERSION 1.01, 23 AUGUST 1987

The Spectrum tape routes save the saved data as one single block. The tape
routines on some other platforms save tape data in a series of shorter
blocks. This gives the ability to rewind after loading errors and retry.
This code demonstates how to do this with the Spectrum.

Saved data format
-----------------
Data is saved in blocks of 256 bytes or longer, each block has a standard
header indicating that block's length and where it is to be loaded to. The
final block is indicated by having a length of less than 256 bytes. If the
final block of actual data is a full block then a zero-length block is saved
after it to end the data.

    Multiple sets of:
      +-----
      | Header, length=nn
      | Data of nn bytes
      +-----
    If final block length>255 bytes, additional block:
      +-----
      | Header, length=0
      | zero-length data block
      +-----

The final zero-length block has to be a header block of zero bytes length as
the Spectrum tape routines read the block type byte before checking the
length, so it ends up also being the parity byte, so has to be zero to avoid
a Tape loading error.

Blocks.s
--------
Blocks.s is source code to save and load blocked data. The file information
is stored in memory and then an entry point is called.

On entry to SAVE:
  START           = start address
  LENGTH          = data length
  BLKLEN          = maximum block size to use, must be at least 256
  HEADER+0        = file type
  HEADER+1 to +10 = filename
  HEADER+15 to 16 = extra address

On entry to LOAD:
  HEADER+0        = file type
  HEADER+1 to +10 = filename

SAVE.s, LOAD.s and *SAVE, *LOAD
-------------------------------
LOAD.s and SAVE.s are source for *LOAD and *SAVE commands runnable with the
JGH ROM to load and save blocked data.

*SAVE filename START+LENGTH
*SAVE filename START+LENGTH:BLKLEN

Saves a code file, START, LENGTH and BLKLEN are in hexadecimal. If BLKLEN is
present, uses that as the maximum block size, otherwise uses a block size of
512. BLKLEN must be 256 or larger.

*LOAD filename

Loads the file into memory as specified by the headers.

Notes
-----
The demonstration code loads all the data and stops if any loading error
occurs. A further development would be to call lower-level tape routines and
catch any loading errors and prompt the user to rewind and retry.