User`s guide

Upon return, register A is set to zero if the function is successful. However, if the
BDOS Error Mode is one of the return modes (see Function 45), and a physical error
is encountered, register A is set to OFFH, 255 decimal, and register H is set to one of
the following values:
01 - Disk I/O error
04 - Invalid drive error
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BDOS Function Calls CP/M 3 Programmers Guide