The VESA Local Bus (usually abbreviated to VL-Bus or VLB) is a short-lived
expansion bus
In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus slo ...
introduced during the i486 generation of
x86 IBM-compatible
personal computer
A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tech ...
s. Created by
VESA
VESA (), formally known as Video Electronics Standards Association, is an American technical standards organization for computer display standards. The organization was incorporated in California in July 1989To retrieve the information, sear ...
(Video Electronics Standards Association), the VESA Local Bus worked alongside the then-dominant
ISA
Isa or ISA may refer to:
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* Mount Isa, Queensland, Australia
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bus to provide a standardized high-speed conduit intended primarily to accelerate video (graphics) operations. VLB provides a standardized fast path that add-in (video) card makers could tap for greatly accelerated
memory-mapped I/O
Memory-mapped I/O (MMIO) and port-mapped I/O (PMIO) are two complementary methods of performing input/output (I/O) between the central processing unit (CPU) and peripheral devices in a computer. An alternative approach is using dedicated I/O pr ...
and
DMA
DMA may refer to:
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* ''DMA'' (magazine), a defunct dance music magazine
* Dallas Museum of Art, an art museum in Texas, US
* Danish Music Awards, an award show held in Denmark
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, while still using the familiar ISA bus to handle basic device duties such as interrupts and
port-mapped I/O. Some high-end 386dx motherboards also had a VL-Bus slot.
Historical overview

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
The GUI ( "UI" by itself is still usually pronounced . or ), graphical user interface, is a form of user interface that allows User (computing), users to Human–computer interaction, interact with electronic devices through graphical icon (comp ...
s in PC operating systems. While IBM did produce a viable successor to ISA with the
Micro Channel Architecture
Micro Channel architecture, or the Micro Channel bus, is a proprietary 16- or 32-bit parallel computer bus introduced by IBM in 1987 which was used on PS/2 and other computers until the mid-1990s. Its name is commonly abbreviated as "MCA", alt ...
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
EISA open standard was developed to counter MCA, its bandwidth of 33.32 MB/s was unable to offer enough improvement over ISA to meet the significant increase in bandwidth desired for graphics. It would be superseded by
Peripheral Component Interconnect
Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format ...
(PCI), starting at speeds of 133 MB/s (32-bit at 33 MHz in the standard configuration)
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
In computer architecture, a local bus is a computer bus that connects directly, or almost directly, from the central processing unit (CPU) to one or more slots on the expansion bus. The significance of direct connection to the CPU is avoiding t ...
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 the cost to migrate to an MCA architecture machine from an ISA machine was substantial. 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. MCA required peripheral cards to not just be "passive" members, cards were made 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.
So while these ad-hoc manufacturer-specific solutions were effective, they were not standardized, and there were no provisions for providing interoperability. This drew the attention of the
VESA
VESA (), formally known as Video Electronics Standards Association, is an American technical standards organization for computer display standards. The organization was incorporated in California in July 1989To retrieve the information, sear ...
consortium and resulted in a proposal for a voluntary and royalty-free local bus standard in 1992. An additional benefit from this standardization (beyond the primary goal of greater graphics card performance) was that other devices could also be designed to utilize the performance offered from VLB; notably, mass-storage controllers were offered for VLB, providing increased hard-disk performance. VLB bandwidth depended on the CPU's bus speed: It started at 100 MB/s for CPUs with a 25 MHz bus, increased to 133 MB/s at 33 MHz and 160 MB/s at 40 MHz, and reached 200 MB/s at 50 MHz.
Implementation
A "VLB slot" itself is an ''additional''
edge connector
An edge connector is the portion of a printed circuit board (PCB) consisting of traces leading to the edge of the board that are intended to plug into a matching socket. The edge connector is a money-saving device because it only requires a si ...
placed in-line with the traditional ISA or EISA connector, with this extended portion often colored a distinctive brown. The result is a normal ISA or EISA slot being ''additionally'' capable of accepting VLB-compatible cards. Traditional ISA cards remain compatible, as they do not have pins past the normal ISA or EISA portion of the slot. The reverse was also true VLB cards are by necessity quite long in order to reach the VLB connector and were reminiscent of older full-length expansion cards from the earlier
IBM XT
The IBM Personal Computer XT (model 5160, often shortened to PC/XT) is the second computer in the IBM Personal Computer line, released on March 8, 1983. Except for the addition of a built-in hard drive and extra expansion slots, it is very simila ...
era. The VLB portion of a slot looks similar to an IBM MCA slot, as indeed it is the same physical 116-pin connector used by MCA cards, rotated by 180 degrees. The IBM MCA standard had not been as popular as IBM expected, and there was an ample surplus of the connector, making it inexpensive and readily available.
Limitations

The VESA Local Bus was designed as a