Introduction to Computer Buses

 

1. Introduction to Computer Buses

A bus is a set of electrical pathways that facilitate communication between different components inside a computer. Buses are responsible for transferring data, memory addresses, and control signals among the CPU, memory, and peripheral devices. The speed and efficiency of buses directly impact the computer’s overall performance.

There are two main categories of buses:

• Internal Buses: Used for communication inside the computer (e.g., Control Bus, Data Bus, Address Bus).

• External Buses: Used for communication with external devices (e.g., USB, PCI, SCSI, EISA, MCA).

2. Primary Internal Buses

The three primary internal buses in a computer system are the Control Bus, Data Bus, and Address Bus. These buses work together to ensure smooth communication between the CPU, memory, and I/O devices.

2.1. Control Bus

The Control Bus is responsible for carrying control signals that manage the operations of the entire system. It ensures that all components function in sync by sending instructions from the CPU.

Functions of the Control Bus:

1. Coordinating Data Transfers: It tells the memory or I/O device when to read, write, or respond.

2. Managing CPU Instructions: Helps direct execution of instructions by enabling and disabling components at the right time.

3. Handling Interrupts: Allows peripherals to alert the CPU when they require attention.

4. Synchronizing Components: Ensures that data transmission occurs at the correct time with clock signals.

Control Signals in the Control Bus:

• Read Signal (RD): Instructs memory or a device to send data to the CPU.

• Write Signal (WR): Instructs memory or a device to receive data from the CPU.

• Clock Signal: Synchronizes communication timing.

• Interrupt Requests (IRQ): Signals the CPU that a device needs attention.

Characteristics of the Control Bus:

• It is bidirectional (can send and receive control signals).

• The number of control lines varies depending on the system architecture.

• Works in conjunction with the Data Bus and Address Bus.

2.2. Data Bus

The Data Bus carries the actual data being transferred between the CPU, memory, and I/O devices. The width of the Data Bus determines how much data can be transmitted at once.

Data Bus Width and Its Impact:

Data Bus Width	Amount of Data Transferred
8-bit Data Bus	1 Byte (8 bits) per cycle
16-bit Data Bus	2 Bytes (16 bits) per cycle
32-bit Data Bus	4 Bytes (32 bits) per cycle
64-bit Data Bus	8 Bytes (64 bits) per cycle

A wider Data Bus means the computer can transfer more data per cycle, improving performance.

Characteristics of the Data Bus:

• It is bidirectional (allows data to move between CPU, memory, and I/O).

• The width of the Data Bus affects system performance.

• Modern systems use 64-bit Data Buses for higher speeds.

2.3. Address Bus

The Address Bus is responsible for carrying memory addresses from the CPU to memory or I/O devices. It tells the system where data should be stored or retrieved from.

Impact of Address Bus Width on Memory Access:

Address Bus Width	Maximum Addressable Memory
8-bit Address Bus	256 Bytes (2⁸)
16-bit Address Bus	64 KB (2¹⁶)
32-bit Address Bus	4 GB (2³²)
64-bit Address Bus	16 Exabytes (2⁶⁴)

A wider Address Bus allows the CPU to access a larger memory space.

Characteristics of the Address Bus:

• It is unidirectional (only sends addresses, not data).

• Determines the amount of memory the CPU can access.

• Modern computers have a 64-bit Address Bus, allowing them to support huge memory capacities.

3. External Buses (Peripheral Buses)

Apart from internal buses, computers use external buses to connect peripheral devices. These include SCSI, EISA, and MCA, which are older technologies that paved the way for modern standards like PCI and USB.

3.1. SCSI (Small Computer System Interface)

SCSI (pronounced "skuzzy") is a high-performance bus standard designed for connecting multiple devices such as hard drives, scanners, and CD-ROMs.

Features of SCSI:

• Supports up to 15 devices on a single bus.

• Uses daisy-chaining to connect multiple peripherals.

• Faster than older interfaces like IDE.

• Commonly used in enterprise storage and servers.

Versions of SCSI and Their Speed:

SCSI Version	Data Transfer Rate
SCSI-1	5 MB/s
SCSI-2	10 MB/s
Fast SCSI	20 MB/s
Ultra SCSI	40 MB/s
Ultra-320 SCSI	320 MB/s

Advantages of SCSI:

• Allows multiple devices on one connection.

• High-speed data transfer compared to IDE.

• Reliable and widely used in servers and workstations.

Disadvantages of SCSI:

• More expensive than IDE and SATA.

• Requires specialized controllers.

3.2. EISA (Extended Industry Standard Architecture)

EISA is a 32-bit expansion bus developed in 1988 to improve upon ISA (Industry Standard Architecture).

Features of EISA:

• Backward compatible with ISA.

• Used in high-end workstations and servers.

• Clock speed: 8.33 MHz.

• Data transfer rate: Up to 33 MB/s.

Advantages of EISA:

• Allowed for faster and wider data transfers.

• Compatible with older ISA cards.

Disadvantages of EISA:

• Expensive and complex compared to ISA.

• Replaced by PCI (Peripheral Component Interconnect).

3.3. MCA (Micro Channel Architecture)

MCA was introduced by IBM in 1987 as a proprietary high-performance bus system.

Features of MCA:

• 32-bit architecture (faster than ISA).

• Bus mastering (devices can communicate without CPU intervention).

• Used in high-performance IBM computers.

Advantages of MCA:

• Higher speeds compared to ISA.

• Improved multitasking due to bus mastering.

Disadvantages of MCA:

• Not backward compatible with ISA.

• Expensive and proprietary, limiting its adoption.

• Eventually replaced by PCI, which was more open and widely accepted.