Check Extensions Present

Entry:
    AH = 41h
    BX = 55AAh
    DL = Drive number
Exit:
    carry clear
        AH = Major version of extensions
        AL = Internal Use Only
        BX = AA55h
        CX = API subset support bitmap
    carry set
        AH = error code = 01h, Invalid Command

This function is used to check for the presence of the INT 13h extensions for a particular drive. If the carry flag is returned set, the extensions are not supported for this drive.

If the carry flag is returned cleared, BX must be checked for the value AA55h to confirm that the extensions are present. If this value is correct, the value of CX must then be checked to determine what subset of this API is supported on this drive. At least one subset must be supported. Bit 0 of CX set indicates that the extended disk access subset is supported. Bit 1 of CX set indicates that the removable drive control subset is supported. All other bits are reserved, and must be returned 0.

For this version of the spec, major version must be returned 1. The major version is incremented when this API changes. (AL) may be used by the BIOS implementor as a minor version/revision field. However, it must not be tested by invoking software.