ISA (Industry Standard Architecture)
8 bit and 16 bit expansion slots used by PC, XT, and AT designs. Often called IBM Standard Architecture. The problems with the ISA bus grew as systems became faster and more powerful. The problems included: slow bus speed, a limited number of interrupts, lack of busmaster support, complex configuration and poor electrical grounding. Most of the problems were not significant early in the history of DOS computing, but came to the surface as the range of tasks and peripherals grew.
It also allows for bus mastering although only the first 16 MB of main memory is available for direct access. In reference to the XT bus architecture it is sometimes referred to as "AT bus architecture".
EISA (Extended Industry Standard Architecture)
A bus standard for IBM compatibles that extends the ISA bus architecture to 32 bits and allows more than one CPU to share the bus. The bus mastering support is also enhanced to provide access to 4 GB of memory. Unlike MCA, EISA can accept older XT bus architecture and ISA boards. Developed by several independent manufacturers (Compaq, AST, etc.) to standardise 32 bit operation and combat IBM's MCA, or more specifically, the cost of it's licensing agreement. Still expensive, it was largely used for servers.
MCA (Micro Channel Architecture)
Proprietary 32-bit expansion bus designed for multiprocessing. Introduced by IBM in 1987, it was used by some (but not all) PS/2 models. It eliminated potential conflicts that arose when installing new peripheral devices. MCA is *not* compatible with either EISA or XT bus architecture so older cards cannot be used with it. Technically brilliant but ultimately killed by IBM's ridiculous licensing terms for the technology.
VLB (VESA Local Bus)
Short lived, high speed, 32 bit extension to the ISA bus promoted by the VESA (Video Electronics Standards Association).
Generically, a local bus is one connecting a processor to memory, usually on the same circuit board as opposed to a backplane and therefore faster.
PCI (Peripheral Component Interconnect)
High speed bus developed by Intel to support the demands of Pentium and 486 based computers.
A standard for connecting peripherals to a personal computer, designed by Intel and released around Autumn 1993. PCI is supported by most major manufacturers including Apple Computer. It is technically far superior to VESA's local bus. It runs at 20 - 33 MHz and carries 32 bits at a time over a 124-pin connector or 64 bits over a 188-pin connector. An address is sent in one cycle followed by one word of data (or several in burst mode). Processor independent, it can work with other processor architectures as well.
Technically, PCI is not a bus but a bridge or mezzanine. It includes buffers to decouple the CPU from relatively slow peripherals and allow them to operate asynchronously.
AGP (Accelerated Graphics Port)
A bus specification by Intel which gives low-cost 3D graphics cards faster access to main memory on personal computers than the usual PCI bus.
AGP dynamically allocates the PC's normal RAM to store the screen image and to support texture mapping, z-buffering and alpha blending. AGP operates at 66 MHz, doubled to 133 MHz, compared with PCI's 33 Mhz. AGP allows for efficient use of frame buffer memory, thereby helping 2D graphics performance as well. It provides a coherent memory management design which allows scattered data in system memory to be read in rapid bursts. AGP reduces the overall cost of creating high-end graphics subsystems by using existing system memory.
|
