The Driver Capabilities Structure (DCS) is maintained by the physical device driver and is passed by reference to the kernel and client FSDs in the GET DRIVER CAPABILITIES command. The kernel and client FSDs must not modify the structure, as it is shared by FSDs and the physical device driver. A DCS has the following format:

┌──────────────────────────────┬──────────────────────────────┐
│Field                         │Length                        │
├──────────────────────────────┼──────────────────────────────┤
│Reserved.  Must be zero.      │WORD                          │
├──────────────────────────────┼──────────────────────────────┤
│DD_VerMajor                   │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│DD_VerMinor                   │BYTE                          │
├──────────────────────────────┼──────────────────────────────┤
│DD_Capabilities               │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DD_Strategy2                  │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DD_SetFSDInfo                 │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DD_ChgPriority                │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DD_SetRestPos                 │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│DD_GetBoundary                │DWORD                         │
└──────────────────────────────┴──────────────────────────────┘

DD_VerMajor

The major version number of the interface the physical device driver supports, equal to 01h in the first release. Old major versions do not function correctly with a file system using a newer version.
DD_VerMinor
The minor version number of the interface the physical device driver supports, equal to 01h in the first release. Old minor versions support a strict subset of the functionality found in newer versions.
DD_Capabilities
A bit field describing the capabilities of the physical device driver:

Bits 0-2

Reserved. Must be zero.
Bit 3
If set, supports disk mirroring.
Bit 4
If set, supports disk duplexing.
Bit 5
If set, driver does not block in Strategy2. LAN Server and LAN Manager products using the HPFS 386 file system require that bit 5 be set to guarantee that the physical device driver does not block in Strategy2.
Bits 6-31
Reserved. Must be zero.
DD_Strategy2
The 16:16 entry point for the strategy routine that supports multi-request asynchronous I/O.
DD_SetFSDInfo
The 16:16 entry point for DD_SetFSDInfo. The value returned is 0:0, if the service is not provided by the physical device driver.
DD_ChgPriority
The 16:16 entry point for DD_ChgPriority. The value returned is 0:0, if the service is not provided by the physical device driver.
DD_SetRestPos
The 16:16 entry point for DD_SetRestPos. The value returned is 0:0, if the service is not provided by the physical device driver.
DD_GetBoundary
The 16:16 entry point for DD_GetBoundary. The value returned is 0:0, if the service is not provided by the physical device driver.

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