service as a subroutine
Any service except those invoked by hardware interrupts which is called explicitly by an application can be handled this way.
service as a process
The process providing the service is reffered to as a server while the calling process termed a client.
Through this client-server approach requires the OS to maintain a number of permanent server processes that listen and respond to various requests, it has important benifits such as suitable for a distributed system, convenient for different types of services, greater degree of fault tolerance and enforces a strict segregation of functionalities.
In summary, services which maintain a global view of resources and must be constantly avaiable are uauslly implemented as autonomous concurrent processes (e.g. UNIX scheduler), while others are constructed as subroutines which called as part of existing processes.
