Bus Communication DDIs

To provide communication protocols over an I/O bus, the ChorusOS operating system implements a layered architecture composed of the following:

• A physical (low) layer which includes the bus control DDI, the local bus communication DDI and the remote bus communication DDI.

• A logical (middle) layer which includes the bus multiplexor DDI.

• A protocol (high) layer which includes the ethernet driver (using the multiplexor DDI) and the TCP/IP stack.

The physical layer abstracts the bus architecture, enabling the portability of all upper layers. The main task of the physical communication driver is to make shared memory resources accessible from any board within the communication domain.

Typically, among all physical drivers running on a given CPU board, there is one driver that provides access to the board local memory (exported to the bus). All other physical drivers provide access to the remote memory (imported from the bus). Thus, the total number of physical drivers on a CPU board (visible for the multiplexer) is always equal to the number of CPU boards communicating over the bus (or buses). Another task of the physical layer is to provide interrupt services allowing a cross interrupt to be sent from one CPU board to another.

The logical communication driver (multiplexer) uses services provided by the physical drivers (shared memory and cross interrupts) to implement a low-level communication protocol over the bus. While various implementations of such a communication layer are possible, the ChorusOS operating system implements a basic communication protocol providing simplex (unidirectional) communication channels over the bus.

For more information on these DDI's refer to the buscom(9DDI) and busmux(9DDI) man pages.