Buses
A bus is a data pathway between several hardware components inside or outside a computer. It not only connects the parts of the CPU to each other, but also links the CPU with other important hardware. The other important hardware includes memory, a disk control unit, a terminal control unit, a printer control unit, and a communications control unit. The capacity of a bus is expressed as bits. A larger capacity bus is faster in data transfer. For example, a 32-bit bus is faster than an 8-bit bus.
Three Main Bus Architectures
ISA (Industry Standard Architecture): ISA is pronounced i- suh. This is the original PC bus architecture. It includes the 8-bit (PC, XT) and 16-bit (AT) buses in IBM personal computer series and compatibles. Now, it refers specially to the 16-bit AT bus.
MCA (Micro Channel Architecture): A 32-bit bus used in IBM P/S 2 series and other IBM models. This architecture allows multiprocessing that allows several processors to work simultaneously. Micro channel architecture is not compatible with PC bus architecture.
EISA (Extended Industry Standard Architecture): EISA is pronounced eesa. This is a bus standard for PCs that extends the AT bus (the ISA bus) architecture to a 32-bit bus. This architecture also allows more than one CPU to share the bus. The purpose of EISA is to extend and amend the old ISA standard, so that all existing AT expansion boards can work with an EISA slot. Local Buses The performance of a microcomputer is often restrained by the relatively slow video cards and other peripherals, which cannot keep up with today's fast CPUs. A local bus reduces the performance gap between the high-speed microprocessors and slower hard disks, video boards and other peripherals.
There are two local-bus systems available today. Each bus hopes to boost microcomputer performance for I/O-intensive tasks. They are a VL-Bus and a PCI local bus.
- VL-Bus (VESA Local Bus): VL-Bus specification was introduced by the VESA (Video Electronics Standards Association). VL-Bus added peripheral components and connectors to the existing motherboard's 486 local bus and was available first. Performance of the VL-Bus architecture declines sharply when supporting more than two devices, and the specification is currently limited to a 32-bit data path and 33-MHz operation. This design is vanishing.
- PCI (Peripheral Component Interconnect): A PCI chip set adds a 64-bit-wide bus between the microprocessor and peripherals to offer a 64-bit data path. This chip supports speeds of 66-MHz. PCI can transfer data either 32- or 64-bits at a time. This architecture is developed by Intel, Compaq, DEC, IBM and NCR. \PCI technology incorporates a managing layer to route and manage data for efficient handling of high-speed data transfers between the microprocessor and peripherals.
Its design goals are to produce a low-cost, high-performance interface and support future generations of peripherals. PCI provides excellent compatibility, higher throughput and automatic configuration of peripheral cards. PCI also has features such as expandability and plug-and-play flexibility.
- Comparison: Both technologies employ a microprocessor's local bus instead of the system input/output bus to rapidly exchange data between the processor and peripherals.\The VESA design reached the market first and is less expensive than PCI, but PCI is technically superior. A VL-Bus usually supports only two or three local-bus peripherals, while PCI can support up to 10 local buses. PCI uses fewer bus lines than VL-Bus. This enables PCI to eventually cost less to manufacture. PCI is now dominating the market.
