Local Buses

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.
• Each bus hopes to boost microcomputer performance for I/O-intensive tasks. 
• They are two types of Local Buses. Namely 1) VL-Bus and a 2) PCI local bus.

o    VL-Bus : 

     

1.     VL-Bus specification was introduced by the VESA (Video Electronics Standards Association). 
2.      VL-Bus added peripheral components and connectors to the existing motherboard's 486 local bus and was available first. 
3.      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. 
4.     This design is vanishing.

o    PCI (Peripheral Component Interconnect): 

1.      A PCI chip set adds a 64-bit-wide bus between the microprocessor and peripherals to offer a 64-bit data path. 
2.      This chip supports speeds of 66-MHz. 
3.      PCI can transfer data either 32- or 64-bits at a time. This architecture is developed by Intel, Compaq, DEC, IBM and NCR. 
4.     PCI technology incorporates a managing layer to route and manage data for efficient handling of high-speed data transfers between the microprocessor and peripherals.
5.     Its design goals are to produce a low-cost, high-performance interface and support future generations of peripherals. 
6.     PCI provides excellent compatibility, higher throughput and automatic configuration of peripheral cards. 
7.     PCI also has features such as expandability and plug-and-play flexibility.

o    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.