Computer Buses: Types and Functions

• A bus consists of physical connections (cables, circuits) shared by multiple hardware components for communication. These connections link memory and I/O devices to the CPU.

Functions of Buses

• Buses facilitate data sharing between computer peripherals in serial or parallel formats, exchanging 1, 2, 4, or 8 bytes at a time, depending on the number of bits. The buses can be 8-bit, 16-bit, 32-bit, or 64-bit.
• Address lines in a bus match the processor, allowing data to be sent to specific memory locations.
• Buses supply power to connected peripherals.
• Buses provide a system clock signal to synchronize peripherals with the system.

Types of Computer Buses

• Address Bus consists of wires to identify memory locations; its size determines addressable locations and is unidirectional.
• Data Bus consists of wires transferring data between computer parts. It transfers instructions to/from the processor and is bidirectional, with its size determining transmittable data volume.
• Control Bus consists of connections that carry control information between the CPU and other devices for signal controllers. It transports orders and synchronization signals. Plus transmits hardware response signals with a bidirectional flow.
• Example: a system with a 4-bit, 16-bit and 20-bit can address 16 Bytes, 64 KB and 1 MB of memory, respectively.
• Example: A 16-bit/32-bit bit bus can transmit 16 bits / 32 bits of data at a time.
• Example (Control Bus): memory read instructs memory to provide data to the CPU, memory write tells memory to store data from the CPU, and interrupt request signals the CPU for device processing.

Bus Structure

• Bus structure is the arrangement of buses to facilitate data transfer and control within a computer system.
• Single Bus Structure shares a single bus to connect hardware like the CPU, memory, and I/O devices, commonly found in simple systems such as PCs and small embedded systems.
• Simplicity provides easy deployment and administration.
• Cost-Effectiveness has lower expenses because fewer buses are less wiring.
• Maintenance is easier because fewer components are involved.
• Multi-Bus Structure uses multiple buses, with one fetching instructions and others fetching data. It overcomes single-bus bottlenecks and enhances performance and scalability.
• Limitations: the rate that data is transferred is significantly slower, because all components feed of this bus creating a bandwidth bottleneck and thus a slower performance
• Scalability Issues occur when more components can intensify the bandwidth and cause expansion problems of the system.
• Advantages of Double Bus Structure
• Improved Performance occurs as efficiency is increased because the system is able to manage more data at once due to the buses (one for memory / I/O)
• Better Bandwidth Utilization from each bus working on decreasing traffic and increase the speed of data exchange.
• Scalability is easier due to multiple buses accommodating more components without degrading performance.
• Disadvantages of Double Bus Structure
• Increased Complexity means the design is cumbersome requiring several parts to be fitted properly.
• Higher Cost is a major drawback from more buses and wiring brings an increase to the overall system.
• Maintenance id Challenging because identifying problems are complex due to the numerous parts and contact points.

Bus Arbitration

• Bus arbitration determines which device controls the bus, especially when multiple devices need access, using methods like:
• Priority-based assigns priority levels for access.
• Round-robin grants access in turns.
• Random grants access at random (less common due to potential inefficiencies).

Direct Memory Access (DMA)

• DMA allows I/O devices to directly transfer data to memory, reducing CPU involvement, increasing system efficiency by freeing the CPU to focus on other tasks.
• Direct Memory Access (DMA) uses hardware for accessing the memory, that hardware is called a DMA Controller. It transfers data between I/O devices and main memory with minimal processor interactions.
• DMA controllers interface the data bus and I/O Devices, transferring data without processor intervention and containing an address unit.
• Types of Direct Memory Access (DMA)
• Single-Ended DMA controllers operate by reading and writing from a single memory address. They are the simplest DMA.
• Dual-Ended DMA controllers can read and write from two memory addresses and is more advanced than single-ended DMA.
• Arbitrated-Ended DMA works by reading and writing to several memory addresses, and is more advanced than Dual-Ended DMA.
• Interleaved DMA read from one memory address and write from another memory address.
• Working of DMA Controller
• Address register contains the address to specify the desired location in memory.
• Word count register contains the number of words to be transferred,
• Control register specifies the transfer mode.
• All registers in the DMA appear to the CPU as I/O interface registers which allows the CPU to both read and write into them, through programming control, via the data bus.

Modes of DMA transfer:

• Burst Mode: Buses are handed over to the CPU by the DMA only after complete data transfer.
• Cycle Stealing Mode: Buses are handed over after each byte transfer, generating a continuous request for bus control
• Transparent Mode: DMA doesn't require a bus, operating when the CPU executes the transaction.

DMA Advantadges

• Faster memory operations and data transfer are achieved.
• The CPU is uninvolved during data transfer.
• Operations require minimal clock cycles.
• DMA distributes workloads appropriately, reducing CPU load.
• DMA Controller increases the complexity of the software.

DMA Disadvantages

• DMA is costly because of extra operations.
• DMA suffers from cache-coherence issues.
• DMA Controller increases the overall cost of the system.