Control Bus

What is Control Bus?

A control bus refers to the bus of a computer that is used by the processor for communicating with several devices connected to it. This communication typically happens via physical connections between them with the use of printed circuits and cables.

Technically, a control bus signifies the connection that carries information as well as reports the status of the communication between several devices and the CPU.

Understanding Control Bus

In computer terminology, a control bus is considered to be an important part of the bus architecture of a computer that carries commands from the processor and the return status signals from the components connected to the computer system.

These buses usually come with different types of control lines used for different purposes, however, the common types of lines that you will find in all microprocessor are:

• A line for read instructions which when active or in logic 0 state indicates that the CPU is reading the device
• A line for status which indicates the operational status of the bus
• A line for interrupt which tells the CPU to postpone sending signals for a period of time
• A line for write instructions which when active or in logic 0 state indicates that the CPU is writing on the device and
• Byte enable line which is actually a group of lines that specify whether the size of the data is 8, 16, 32, or 64 bytes.

In addition to that, there may also be a few other types of additional lines in a control bus depending on the type of the microprocessor such as:

• Transfer ACK line that delivers acknowledgement that the data has been read by the device
• Bus Request or BR, BREQ, or BRQ lines which carry signals to indicate that the devices wants to use the data bus and is sending a request for it
• Bus Grant or BG or BGRT lines which carry signals to indicate that the processor has approved access to the device to use the bus
• Interrupt Request or IRQ line which carries signals from a low priority device that is requesting for an access to the CPU
• Clock signals line which is usually used to synchronize data between a device and the CPU and
• Reset line which, when active indicates that the CPU will carry out a hard reboot.

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Typically, those systems that come with more than one bus master have these extra control bus lines to send signals more smoothly and easily to control and determine which bus master will drive the address bus.

This will typically help in preventing contention in the address bus.

When the processor writes any data to the main memory of the system, the signal is transmitted by it to the write instruction or command line.

In the same way, when the CPU needs to read data, it sends the signal to the read instruction line.

It is only when these signals are transmitted, the CPU will be able to send or receive data from the main memory.

There is an input channel in the control bus which is accessed usually to invoke operations in the context of the hard applications on the beans.

This channel has similar properties as a service stimulating endpoint.

This will allow specifying an output channel if a particular operation has the same return value as the one a user wants to send to a downstream channel.

The signals or messages are run by the control bus on the channel as expressions. However, before running it, there are a few specific steps followed which are:

• Taking the message
• Compiling the body and
• Add text.

This supports any method that can configure several implementations.

Uses of Control Bus

A control bus is mainly used by the Central Processing Unit or CPU for determining data processing as well as in the communication with the other devices connected to it.

Due to its features and functionalities, the control bus is used to ensure that the system is functional and more proficient by ensuring proper coordination between the components.

If there were no control buses in a computer, then the CPU would not be able to figure out whether or not the system is being able to send or receive data properly.

The control bus also determines the direction in which the read or write information should flow.

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Apart from that, this bus is also used for synchronizing the command signals of the CPU so that it reaches properly to the desired components of the computer and the slower external devices connected to it.

Moreover, this bus is also used to carry clock pulses.

Control Bus Functions

The main function of the control bus is to ensure that during data transmission there are no collisions. Ideally, it manages the activity between the computer and the several devices connected to it.

Ideally, it is the set of discrete wires present that helps the control bus to perform separate functions from that of the other two buses, data bus and address bus. These specific functions include:

• Read – This function signals that a memory read operation is about to happen
• Write – This function signals that a memory write operation is about to happen
• Clock – This function synchronizes all signals that pass into and out of the processor
• Reset – This function clears all the data from the buses and registers
• Interrupt – This function informs the processor to save the ongoing process and run interrupt processes until it is completed and then continue its activities and
• Non Maskable Interrupt or NMI – This function tells the processor to save the current process, run interrupt until the process is completed, then resume its activities.

All these functions are separate but related to the control of operations of the computer and therefore are considered to be the functions of a control bus.

Direction of Control Bus

Typically, a control bus is bidirectional in nature. This allows sending data to and from the devices. However, technically, it can be said that the control bus operates in mixed directions where a few of its lines go in while some come out.

These lines of the control bus perform specific functions to control and coordinate different microprocessor operations by performing partly as bidirectional and partly as unidirectional.

Looking into it deeply, it can be said that a control bus is not really bidirectional.

The main reason to say so is that it comes with a combination of both input as well as output pins.

Normally, you can say that a control bus is unidirectional. This is because, most of the time the signals in this bus are generated from one particular point or device and are communicated to another at a different place.

This phenomenon is pretty much contradictory to the bidirectional principle even though the signals may move in both directions but may not be through the same wire.

On the other hand, at some times it can be considered as bidirectional because the microprocessor sometimes needs to know the condition of the bus transactions.

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Also, in the case of the embedded processors, there may be some specific types of I/O pins that can be reconfigured dynamically for input or output.

This typically conforms with the definition of bi-directional to some extent.

These pins are usually used in only one way but the program may initialize it for the embedded application and tell the processor whether the pin is input or out pin.

Though this is more of a configuration phenomenon rather than bidirectional use, there are still some boards and applications that may change the direction of the pin according to the need and the processor may allow it.

Therefore, the direction of the control bus will depend on how exactly it is defined and what purposes it solves.

Questions & Answers:

What is a Control Bus Example?

The most common control bus examples and its signals are the output of processing and learning blocks that are sent to different device interfaces such as displays, motors, speakers and others to generate the preferred output in the real material world.

Some of the most used control signals are

I/O read
I/O write
Memory read
Memory write and
Opcode fetch.

An example of a control bus will also be a line of the bus used to indicate that the processor is reading from or writing to the main memory currently.

What Does Control Bus Avoid?

The control bus avoids data collisions and bus contention when data is transmitted through the data bus.

What is a Not Control Bus Signal?

Usually, control signals are of two types namely, clocks and the signals that set up channels for communication and related to data flow control. Anything other than that is not a control bus signal.

You can say Reset is not a control bus signal because it does not have a direct connection to affect the performance of the CPU with respect to the ongoing data processing.

Conclusion

So, coming to the end of the article you now know that the control bus of the system bus of a computer plays a significant role in the operation of the processor.

It has different functions to perform to ensure that the system is operational and efficient enough to handle data sets given to it for processing.