The active context is the operating system facility that underlies the implementation of D11. D11 requests from local D11 programs are translated into protected procedure calls that enter the D11 window system kernel. Because protected procedure calls can be implemented so that they are inexpensive relative to a standard Unix context switch and no protocol needs to be packed, sent through a transport, and unpacked, D11 protected procedure calls should be a net win over local use of the X11 protocol using conventional transports. This section details the working of active contexts and protected procedure calls for a Unix style operating system.