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Detail:
  lr was being used as a temporary register to store the buffer handle in
whilst checking for ownership.  If DeviceFS didn't own the buffer the call
was being made on, it then used this value in a MOV pc, lr thus going who
knows where...

Admin:
  Checked and still works.  Appears to cure previous problem where issuing
Upcall 8 could cause a branch through zero error.

Version 0.55. Tagged as 'DeviceFS-0_55'