VESA Local Bus: Difference between revisions

Revision as of 07:49, 31 December 2019

Revision as of 20:21, 22 January 2020

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[[File:KL ATI Mach 64 VLB.jpg|thumb|left|upright=1.5|An ATI MACH64 [[SVGA]] VLB graphics card]]

In the early 1990s, the [[I/O]] bandwidth of the prevailing ISA bus, 8.33 MB/s for standard 16 bit 8.33 MHz slots, had become a critical bottleneck to PC video and graphics performance. The need for faster graphics was driven by increased adoption of [[graphical user interface]]s in PC operating systems. While IBM did produce a viable successor to ISA with the [[Micro Channel Architecture]] offering a bandwidth of 66 MB/s, it failed in the market due to IBM's requirement to license and payment of licensing fees by hardware manufacturers to use it. While an extension of the royalty-free ISA bus in the form of [[Extended Industry Standard Architecture|EISA]] open standard was developed to counter MCA, it'sits bandwidth of 33.32 MB/s was unable to offer enough improvement over ISA to meet the significant increase in bandwidth desired for graphics.

Thus for a short time, a market opening occurred where video card manufacturers and motherboard chipset makers created their own proprietary implementations of [[local bus]]es to provide graphics cards direct access to the processor and system memory. This avoided the limitations of the ISA bus while being less costly than a "licensed IBM MCA machine". It is important to note that at the time a migration to an MCA architecture machine from an ISA one was not insubstantial. MCA machines generally did not offer ISA slots, thus a migration to MCA architecture meant that any prior investment in ISA cards was made unusable. Additionally, makers of MCA-compatible cards were subject to IBM's licensing fees, which combined with MCA's greater technical requirements and expense to implement (which in itself is not bad: MCA required peripheral cards to not just be "passive" members and made cards active participants in increasing system performance) it did have the effect of making an MCA version of a peripheral card significantly more expensive than its ISA counterpart.