AL = Drive number (0=A, 1=B, etc)
        DS:BX = Disk Transfer Address (buffer)
        CX = Number of sectors to read
        DX = First relative sector to read
Return: CF = 1 if error
            AL = error code issued to INT 24h in low half of DI
            AH = 80h if attachment failed to respond
                 40h if seek operation failed
                 20h if controller failed
                 10h if data error (bad CRC)
                 08h if DMA failure
                 04h if requested sector not found
                 03h if write-protected disk
                 02h if bad address mark
                 01h if bad command
Note: ORIGINAL FLAGS ON STACK!
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