PC Hardware

VESA Local Bus (VLB)

The problems posed by MCA and EISA designs meant that developers needed an improved bus architecture to speed up graphics adapter performance and keep up with the evolving technologies as new 32-bit operating systems such as Microsoft Windows 3.1 gained popularity. The latter's graphical user interface (GUI) required a much faster display adapter, because every pixel (not just lines of character data) had to be represented and refreshed. About the same time, laser printers and graphics programs like PageMaker and CorelDRAW fostered the "desktop publishing revolution." The hardware industry developed the VESA local bus (VLB) to meet the need for a faster expansion interface. (VESA, the Video Electronics Standards Association, was the driving force behind the new bus technology.) Found only in 386 and 486 machines, the VLB had a short lifespan. The cards based on this design are connected directly to the system-bus side of the PC's external data bus (see Figure 10.5).

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Figure 10.5 VESA local bus design

The speed of the system data bus is based on the clock rate of the motherboard's crystal. During the heyday of the VLB, this was usually 33 MHz, and VLB cards usually ran at half that rate, far outpacing the ISA bus. Some cards ran as fast as 50 MHz, using the full speed of the souped-up system bus. That often caused system crashes, because 50 MHz was outside the VLB specification.

The chip design for the VLB controller was relativity simple, because many of the core instructions were hosted by the ISA circuits already on the motherboard, but the actual data passes were on the same local bus as the one used by the CPU.

The design specification provides two other performance-boosting features: burst mode and bus mastering. In burst mode, VLB devices gain complete control of the external data bus for up to four bus cycles, passing up to 16 bytes (128 bits) of data in a single burst. Bus mastering allows the VLB controller to arbitrate data transfers between the external data bus and up to three VLB devices without assistance from the CPU. This limit of three devices also limited the maximum number of VLB slots to three and called for the use of a coprocessor. Display-system design is covered in more detail in Tutorial 11, "The Display System: Monitors and Adapters."

The actual connectors on the motherboard resemble an ISA slot with an additional short slot aligned with it. On systems that support this interface, one to three slots are located on the side of the motherboard closest to the keyboard connection.