Archive file header:
  0000: "**I am a BBC BFM file**",<cr>
  0018: Date in format: "Wed,10th May 1989",<cr>
  002A: Time in format: "08:45",<cr>
  0030: Comment string<cr>

Entries:
  0000: "BEGIN:",<cr>
  0007: Filename<cr>
  n   : File length
  n+4 : File load address
  n+8 : File exec address
  n+12: File contents

End of archive file:
  eeee: "END:",<cr>