The System Bus
The control lines, the address bus and the data bus constitute the system bus and are guided in parallel to the bus slots. Several bus architectures used in microcomputers are: AT, ISA, EISA, IBM Micro channel, VESA Local Bus, PCI etc.
Different system bus architectures and the branches represent processor types that have developed over the years.
The term XT stands for eXtended Technology. It is the successor of the IBM PC with an 8086 processor and an internal 16-bit data bus. The term AT stands for advanced technology.
It is the successor of the PC XT with 80286 microprocessor and 16-bit slots. The AT bus developed by IBM is commonly known as ISA which stands for industrial Standard Architecture.
The ISA bus has a speed of 8 MHz, and a maximum throughput of 8 MB per second. EISA is the abbreviation of Extended ISA. EISA defines a 32-bit extension for the ISA or AT bus to integrate the 80386/80486 32-bit processors.
EISA is downward compatible with XT and AT buses. XT and ISA adapters can be used with EISA bus. At a bus speed of 8 MHz. EISA can achieve a maximum throughput of 33 MB per second.
Micro channel is a bus system of IBM PS/2 personal computer. This bus is designed for an 8-bit, to 32-bit data bus and it support multitasking operating systems on a hardware level. Unlike EISA, the micro channel is incompatible with PC XT and ISA bus.
VL bus is also know as VL local bus. It is abbreviation of VESA local bus, architecture introduced by Video Electronics standard Association (VESA). The VL bus is a 32-bit bus, running at either 33 or 40 MHz. the maximum throughput is 133 MB per second at 33 MHz or 148 MB per second at 40 MHz. the most common VL bus adapters are video adapters, hard-disk controllers and network interface cards.
The PCI Bus
The term PCI stands for peripheral component interconnect. The PCI specification introduced by Intel defines a local bus that allows up to ten PCI complaint expansion cards to be plugged into the computer.
One of these ten cards must be the PCI controller card, and others are video card, network interface card, SCSI interface or any other basic I/O function.
The PCI controller exchanges information with the computer’s processor as 32-bit or 64-bit words and allows intelligent PCI adapters to perform certain tasks concurrently with the main processor.
PCI can operate at a bus speed of 32 MHz, and can manage a maximum throughput of 132 MB per-second with 32-bit data or 264 MB per-second with 64-bit data path. Unlike the VL bus, PCI is processor independent. Figure 4.6 shows the typical PCI/ISA bus system.
With the exception of IBM micro channel bus, all PCI and VESA local bus systems support ISA slots. EISA system also support ISA slots as part of their basic design.
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