What does a bus in a microprocessor mean?

A bus inside a microprocessor is a collection of wirelines that contain related information. A group of wires required for communication among the microprocessor and peripherals is known as a system bus. A bus is an electrical or digital passage across which bits are transferred between a variety of computer elements. It mainly links all the internal parts to the CPU (Central Processing Unit) and main memory. The width or size of any bus is crucial since it affects the amount of data that is transferred at a certain time period. Buses always transmit data parallelly, in a 32-bit bus, the information is supplied over 32 wirelines simultaneously and each bus comprises a clock speed measurable in MHz. The various types of buses inside a microprocessor are address bus, data bus, and control bus.

Address bus

The address bus carries a unique binary pattern called the address used to determine a memory location or an input/output port. The CPU reads or writes information from the memory by addressing a distinctive location and outputs the location of the data on the address bus, then the memory applies this address to access the proper data.

Every I/O device (Input/Output) appliance has a special address; thus, the CPU uses its address to place it on the address of each device and they shall detect the address by their individual ways and operate accordingly. It comprises a total of 16 wires. The size of the address bus deduces the memory size that is useful for interaction as the microprocessor provides an address on the respective bus to the memory.

For a 16-bit address bar length of an 8085 microprocessor, it usually ranges from 0000 H to FFFF H (H = Hexadecimal). The address bus is a unidirectional quantity and the numerical values can only be sent from microprocessor to memory (one direction). It contains 16, 24, or 32 parallel signal lines. If the CPU consists of N address lines, then it may directly address 2^N address lines. For instance, a 32-bit address-equipped device can address 4 GB of physical memory. Although, the actual quantity of memory that may be accessed is generally lesser than this theoretical limit because of the chipset and motherboard limitations.

Data bus

The data bus is utilized for the exchange of data between the memory and the processor or between the input/output device and processor. It is bidirectional (to and fro flow allowance), the CPU may read data in from memory or may provide the data outside the memory. This is where the actual information gets passed on between the CPU, memory, and I/O components.

Working of the data bus includes firstly, the CPU receiving data from memory, it outputs the address onto the address bus, then the memory outputs the data onto the data bus after which the CPU reads the data from the data bus. When there is a need to write the data onto the memory, the CPU first outputs the address on the address bus and then outputs the data onto the output bus after which the memory then reads and collects the data at an appropriate place. It comprises 8, 16, 32, and 64 parallel sign lines since every wire is capable of transporting 1-bit of data at once, e.g., 8 wire bus can transmit 8 bits (full byte) data. For an 8-bit address bar length of an 8085 microprocessor, it usually ranges from 00 H to FF H (H = Hexadecimal).  

The amount of wireline buses influences the speed of information towards the hardware elements and broader the data bus, the quantity of data to be transmitted will be more. The size of the data bus evaluates a suitable arithmetic method. The data bus also holds instructions from the memory to the microprocessor of the proposed device.

Memory size = 2^A x D

Where,

A = address lines, and

D = data lines.

Control bus

The control bus is the assembly of conducting wires, useful for producing timing and control signals to supervise the respective peripherals. A microprocessor uses the control bus to process data and it contains control signals, which are inclusive of signals for choosing the memory or I/O device from the provided address, data transfer direction, and integration of data transfer for slower devices. They are various lines that have certain functions for coordinating and controlling microprocessor operations. It carries control signals both unidirectional and bidirectional partially. Certain control signals include memory read, memory write, I/O read, I/O write and Opcode fetch. In case a control bus line is a read or write wireline, the memory will read if the wire is low (negligible electricity flowing) and the memory will write if the wire is high (flow of electricity).

Microprocessor operations

The microprocessor performs four operations by utilizing the address bus, data bus, and control bus:

1. Memory Read: It reads data (or instruction) from the memory.
2. Memory Write: It writes data (or instruction) inside the memory.
3. I/O Read: It accepts data from an input device.
4. I/O Write: It sends data to an output device.

The three control signals available on a chip are RD (Read signal), WR (Write signal), and ALE (Address Latch Enable signal). Direct Memory Access (DMA) enables the transmission of data operations between the main memory and I/O subsystems along with restrained CPU intervention. HOLD is an active-high input signal to the microprocessor from another major device appealing to the application of the address and data buses. HLDA is a high active output signal indicating that the MPU (Micro Processing Unit) is yielding control over the buses. The 8085 microprocessor has two pins available for the DMA (Direct Memory Access) mode of I/O communication; the HOLD (Hold) and the HLDA (Hold Acknowledge).

Context and Applications

The topic is related to the following courses:

• Bachelors in Technology (Electrical Engineering)
• Masters in Science (Electronics and Communication)
• Masters in Science (Robotics and Machine Learning)

Practice Problems

Q1. How is the transfer of information done by the buses?

1. Perpendicularly
2. Parallelly
3. Diagonally
4. Vertically

Correct option- b

Explanation: The transfer of information by the buses is done parallelly.

Q2. Which of the following bus types acts in a bidirectional manner?

1. Address bus
2. Control bus
3. Data bus
4. System bus

Correct option- c

Explanation: The data bus acts in a bidirectional manner.

Q3. What is the full form of ALE?

1. Address Latch Enable signal
2. Action Latch Energy signal
3. Address Latent Energy signal
4. Action Latent Enable signal

Correct option- a

Explanation: The full form of ALE is the Address Latch Enable signal.

Q4. What is the effect on the memory if the wireline holds no electricity flow?

1. Memory will write
2. Memory will do nothing
3. Memory will collapse
4. Memory will read

Correct option- d

Explanation: The effect on the memory if the wireline holds no electricity flow is that it will read.

Q5. Which of the following is/are the control signal(s) on the chip?

1. RD
2. WR
3. ALE
4. All of the above

Correct option- d

Explanation: The control signals on the chip are RD, WR, and ALE.

Related Concepts

• LATCH (single word memory circuit)
• Expansion bus
• EISA (Enhanced Industry Standard Architecture)