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  File formats for storing border graphics on ZX Spectrum
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  The following family of raw file formats is designed for efficient storage
  of ZX Spectrum graphics that involve drawing at the border. By design,
  these files have no identifiers and no metadata; they can only be
  recognised by the extension and/or their specific lengths (in bytes).

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  *.BSC files
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  Files with extension *.bsc are intended for storing a standard ZX Spectrum
  image (6912 bytes), which is extended to also cover the border area.
  These files are always 11136 bytes long.

  [6144]	Standard ZX Spectrum bitplane;
  [768]		Standard ZX Spectrum attributes;
  [4224]	The border image data (see detailed description below).

  The border is aligned to the standard ZX Spectrum character grid and is
  stored as a sequence of colours of 8x1 border "pixels". A typical ZX
  Spectrum viewer software would only be able to display stripes that contain
  three or more border "pixels" of the same colour in a row. This requirement
  can be relaxed near the edges of the visible area and also around the
  vertical edges of the screen, therefore, this file format does not
  explicitly prohibit having stripes of one or two border "pixels". It is the
  responsibility of viewer software to somehow deal with the combinations of
  border "pixels" that cannot be displayed.

  A full image contains 48 x 304 border "pixels" and, therefore, spans the
  effective resolution of 384 x 304 pixels. Every byte of the border image
  data stores the colours of two border "pixels":

  	Bits 0..2:	colour of left pixel;
  	Bits 3..5:	colour of right pixel;
  	Bits 6,7:	unused.

  Since it would be wasteful to store border image under the ZX Spectrum
  screen, the border image data is split into three sections:

  64x[24]	64 lines of the border image above the ZX Spectrum screen;
  192x[4+4]	for 192 lines of the screen, 8 border "pixels" are stored to
  		the right of the screen and 8 "pixels" to the left;
  48x[24]	48 lines of the border image below the screen.

  Note that on a specific ZX Spectrum model, a viewer software is allowed to
  additionally truncate as many border lines as necessary for its normal
  operation.

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  *.BMC4 files
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  Files with extension *.bmc4 are intended for storing a multi-colour image
  for ZX Spectrum with 8x4 effective attribute size, which is extended to
  also cover the border area. These files are always 11904 bytes long.

  [6144]	Standard ZX Spectrum bitplane;
  [768]		Standard ZX Spectrum attributes for even lines of the
  		multi-colour image (i.e. for upper halfs of the characters);
  [768]		Standard ZX Spectrum attributes for odd lines of the
  		multi-colour image (i.e. for lower halfs of the characters);
  [4224]	The border image data (see detailed description above)