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Defining Services

A virtual device defines its service by using the BeginProc and EndProc macros and 
the Service or Async_Service options. The macros mark the start and end of the
procedure code for the service. The options identify the procedure as a service. The
following example shows the definition for the service VSAMPLED_Get_Version:

BeginProc VSAMPLED_Get_Version, Service
        mov     ax,030ah
        clc
        ret
EndProc VSAMPLED_Get_Version

The Async_Service option is for services that can be called asynchronously, that is,
during processing of an interrupt. Asynchronous services must be re-entrant and must
not call VMM or virtual device services that are not also asynchronous.

The VMM and standard virtual devices use two calling conventions for their services:
register-based and 32-bit C-language based. These calling conventions specify the
spelling for the service name, the method for passing parameters and returning values,
and the registers to preserve.

For register-based services, the service name should not begin with an underscore(_).
All the parameters are passed in registers, and results are returned in register.
Typically, the service preserves all registers it does not explicitly use to return values in.
For C-language-based services, the service name must begin with an underscore(_).
All parameters are passed as 32-bit vlaues on the stack. Results (if any) are returned in
the EAX register(for 32-bit values), ot the EAX and EDX registers(for 64-bit values).
The service preserves the EBX,ES,FS, and GS registers as well as the ESI and EDI
registers. Only the flags and the EAX,ECX, and EDX registers are modified.

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