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 Subfunction 21 : Test for residency

 AH = 82
 AL = 21
 If MODRES is resident, AX will be set to 5722h, and BX will hold the version
 number in hex (i.e. 0120h means version 1.20). DX:CX will point to a table
 containing the available devices. Note that this information is not available
 in the old versions of MODRES.

 The table has the following format:

 +--------------------------------------------------------------------------+
 | Offset | Len | Contents                                                  |
 |--------+-----+-----------------------------------------------------------|
 |   0    |  20 | The name of the device - blank padded and null            |
 |        |     | terminated.                                               |
 |   20   |  2  | Always seems to be FFFF hex.                              |
 |   22   |  2  | Usually the port associated with the device. This will    |
 |        |     | be set to zero if the device is not available.            |
 |   24   |  2  | Usually the second port associated with stero devices.    |
 |   26   |  2  | The address of the int 8 routine for playing a sample.    |
 |   28   |  1  | The type of device. See table below.                      |
 |   29   |  2  | The address of the int 8 routine for playing a module     |
 |        |     | with all samples < 64K.                                   |
 |   31   |  2  | The address of the int 8 routine for playing a module     |
 |        |     | with some samples > 64K.                                  |
 +--------------------------------------------------------------------------+

 The end of the table will be marked with a FF hex as the first byte in the
 name field. The possible device types known to me are as follows:

 +------------------------------+
 | Number | Name                |
 |--------+---------------------|
 |   0    | Speaker             |
 |   1    | Soundblaster        |
 |   2    | D/A on an LPT port  |
 |   4    | D/A on 1&2 (stereo) |
 |   5    | D/A on 1&2 (mono)   |
 |   7    | Stereo-on-1         |
 |   8    | Disney Sound Source |
 +------------------------------+