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Chapter: Embedded and Real Time Systems - Process and Operating Systems

Interprocess Communication Mechanisms

1. Shared Memory Communication 2. Message Passing 3. Signals



Processes often need to communicate with each other. Interprocess communication mechanisms are provided by the operating system as part of the process abstraction.


In general, a process can send a communication in one of two ways: blocking or nonblocking. After sending a blocking communication, the process goes into the waiting state until it receives a response.


Nonblocking communication allows the process to continue execution after sending the communication. Both types of communication are useful. There are two major styles of interprocess communication: shared memory and message passing.


Shared Memory Communication:


Figure 3.9 illustrates how shared memory communication works in a bus-based system. Two components, such as a CPU and an I/O device, communicate through a shared memory location. The software on the CPU has been designed to know the address of the shared location.


The shared location has also been loaded into the proper register of the I/O device. If, as in the figure, the CPU wants to send data to the device, it writes to the shared location. The I/O device then reads the data from that location. The read and write operations are standard and can be encapsulated in a procedural interface.


As an application of shared memory, let us consider the situation of Figure 6.14 in which the CPU and the I/O device want to communicate through a shared memory block. There must be a flag that tells the CPU when the data from the I/O device is ready.

The flag, an additional shared data location, has a value of 0 when the data are not ready and 1 when the data are ready. If the flag is used only by the CPU, then the flag can be implemented using a standard memory write operation. If the same flag is used for bidirectional signaling between the CPU and the I/O device, care must be taken. Consider the following scenario:


·        CPU reads the flag location and sees that it is 0.


·        I/O device reads the flag location and sees that it is 0.


·        CPU sets the flag location to 1 and writes data to the shared location.


·        I/O device erroneously sets the flag to 1 and overwrites the data left by the CPU.


Message Passing:


Message passing communication complements the shared memory model. As shown in Figure 3.10, each communicating entity has its own message send/receive unit. The message is not stored on the communications link, but rather at the senders/ receivers at the end points.


In contrast, shared memory communication can be seen as a memory block used as a communication device, in which all the data are stored in the communication link/memory.


Applications in which units operate relatively autonomously are natural candidates for message passing communication. For example, a home control system has one microcontroller per household device—lamp, thermostat, faucet, appliance, and so on.


The devices must communicate relatively infrequently; furthermore, their physical separation is large enough that we would not naturally think of them as sharing a central pool of memory.


Passing communication packets among the devices is a natural way to describe coordination between these devices. Message passing is the natural implementation of communication in many 8-bit microcontrollers that do not normally operate with external memory.




Another form of interprocess communication commonly used in Unix is the signal. A signal is simple because it does not pass data beyond the existence of the signal itself. A signal is analogous to an interrupt, but it is entirely a software creation. A signal is generated by a process and transmitted to another process by the operating system.


A UML signal is actually a generalization of the Unix signal. While a Unix signal carries no parameters other than a condition code, a UML signal is an object. As such, it can carry parameters as object attributes. Figure 3.11 shows the use of a signal in UML. The sigbehavior ( ) behavior of the class is responsible for throwing the signal, as indicated by <<send>>.The signal object is indicated by the <<signal>> stereotype.

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