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DECLARING SERVICES

: A virtual device uses the Begin_Service_Table and End_Service_Table macors to 
: declare its services. The macors mark the start and end of a list containing the names
: of the services and optionally the names of the segments containing the services. The
: declaration must be made in the same file as the virtual device declaration (that is, in
: the file containing the Declare_Virtual_Device macro) and must be preceded with the 
: definition of a device-specific symbol. The following example shows service table 
: declaration for the sample virtual device, VSAMPLED:
: Create_FSAMPLED_Service_Table equ 1
: Begin_Service_Table VSAMPLED
:         VSAMPLED_Service VSAMPLED_Get_Version, LOCAL
:         VSAMPLED_Service VSAMPLED_Service_1
:         VSAMPLED_Service VSAMPLED_Servie_2,VxD_ICODE
: End_Service_Table VSAMPLED
: In this example, the Create_VSAMPLED_Service_Table symbol, defined just before the
: service table declaration, directs the Begin_Service_Table macro to create a service
: table for VSAMPLED. Once the declaration starts, the VSAMPLED_Service macro
: actually declares the services. This required macro is created by the
: Begin_Service_Table macro, and is only available within the servie table declaration.
: The name of each service must exactly match the service's defined name, that is, the
: name given with the BeginProc macro.
: When declaring a service, the LOCAL option must be given after the service name if the
: service is defined in the same source file containing the service table declaration. In
: other words, the service table automatically declares a service as an external service
: unless the LOCAL option is given. In the preious example, the
: VSAMPLED_Get_Version serivce is define in the same file containing the service
: table.
: Similarly, a segment name must be given afters the service name if the service is not
: defined in the VxD_CODE segment. In the previous example, the VSAMPLE_Service_2
: service is defined in the VxD_ICODE segment.
: The order in which services are declared is important. The first declared service
: for a virtual device must be the Get_Version service.( This service clears the carry flag and
: returns with the virtual device's version number in the AX register.) Also, any new
: services added to the virtual device must be declared at the end of the table(or in
: explicitly reserved slots in  the table). Since the VMM relies on the order of the servies
: in the table to correctly link to the services, inserting a new service in the middle of the
: table requires that all virtual devices using the virtual device's service be rebuilt. for
: convenience, the service table declaration should be placed in an include file so that
: other virtual devices can import the services by including the file rather than recreating
the declaration.


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