Take the Bus

IBM's original PC had a bus that was 8 bits wide and ran at 4MHz. The PC/AT expanded the bus to 16 bits wide and upped its speed to 6MHz. Today, we call the 16 bit PC bus industry-standard architecture (ISA), and its speed has crept up to 8MHz.

ISA has two significant shortcomings that limit the performance of an ISA NEXTSTEP system. One is the data bus: Only 16 bits wide, it transfers data at about 16MB/sec. If you plug a video adapter into the ISA bus, NEXTSTEP won't give you anything but 2 bit gray. Also, ISA's address bus is only 24 bits wide. Since 24 bits can only address 16MB of memory, an ISA SCSI controller that reads data from a disk destined for memory above the lower 16MB must write it to a buffer in the lower 16MB; the CPU must then copy the data to its destination. This is called double buffering.

EISA (Extended Industry Standard Architecture) runs at 8MHz, and both its data and address buses are 32 bits wide, eliminating the need for double buffering and giving it a maximum bandwidth of 32MB/sec. EISA provides ample performance for SCSI and Ethernet but still leaves much to be desired for graphics performance. Hence, local bus.

Local bus refers to a bus that is connected directly to the processor, similar to the computer's memory bus, an idea originally pioneered by Dell. Other PC vendors followed suit, developing a host of proprietary and incompatible implementations.

To standardize the local-bus arena, the Video Electronics Standards Association developed the VESA Local Bus, often referred to as VLB or simply VESA. Like EISA, VLB is 32 bits wide. Local bus, however, runs at the processor's external speed: A '486 DX2/66 has a local bus running at 33MHz, giving it a throughput of 132MB/sec.

With the exception of Compaq and Dell, most vendors have dropped their proprietary local-bus implementations in favor of VESA. (All the machines using C&T's Wingine chip set also have proprietary local-bus implementations, since Wingine is inherently incompatible with VESA.)

Competing with the VESA Local Bus is Intel's Peripheral Component Interconnect, or PCI. Unlike local bus, PCI is independent of the computer's CPU. Thus, a PCI card can work with, say, Intel's Pentium or Motorola's PowerPC. The advantage that VESA holds over PCI is price. It is expected that VESA will remain dominant in '486 machines and that PCI will become dominant in Pentium machines.

NEXTSTEP works just fine with ISA, EISA, VESA, and PCI systems. NEXTSTEP won't work with MicroChannel Architecture (MCA) computers, such as IBM's PS/2 series.