[XT] [AT]
Expects: AH    10H
DL    drive: 80H-81H=hard disk 0-1
                                                                  
Returns: AH    BIOS disk error code if CF is set to CY
                                                                  
Info: This determines if the drive specified in DL is ready and can
process a command.

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INT 13H 10H: Test Drive Ready