Acorn DFS
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OSWORD &7D - Read disk cycles, corrupting &E00-&FFF and parts of &1000-&10FF
OSWORD &7E - Read disk size, corrupting &E00-&FFF and parts of &1000-&10FF
OSWORD &7F - Read/write disk sectors, corrupting parts of &1000-&10FF

NOTE:
DFS 0.90 traps **ALL** OSWORDs >&7F, treating any high numbered OSWORD call
as OSWORD &7D, overwriting whatever is in memory at &0E00-&0FFF. This bug
is fixed in DFS 0.98.