Input/output ports are used to connect the
computer to external devices. Input and output standards described in the
previous sections are referred to as external bus standards.
In addition to these external buses,
computers also have internal buses that carry address, data and control signals
between the CPU, cache memory, SRAM, DRAM, disk drives, expansion slots and
other internal devices. Internal buses are of three types, namely the local
bus, the PCI bus and the ISA bus.
Local Bus
This bus connects the microprocessor to the
cache memory, main memory, coprocessor and PCI bus controller. It includes the
data bus, the address bus and the control bus.
It is also referred to as the primary bus.
This bus has high throughput rates, which is not possible with buses using
expansion slots.
PCI Bus
The peripheral control interconnect (PCI)
bus is used for interfacing the microprocessor with external devices such as
hard disks, sound cards, etc., via expansion slots. It has a VESA local bus as
the standard expansion bus.
Variants of the PCI bus include PCI 2.2,
PCI 2.3, PCI 3.0, PCI-X, PCI-X 2.0, Mini PCI, Cardbus, Compact PCI and
PC/104-Plus. The PCI bus will be superseded by the PCI Express bus. PCI
originally had 32 bits and operated at 33 MHz. Various variants have different
bits and data transfer rates.
ISA Bus
The industry-standard architecture (ISA)
bus is a computer standard bus for IBM-compatible computers. It is available in
eight-bit and 16-bit versions.
The VESA local bus was designed to solve
the bandwidth problem of the ISA bus. It worked alongside the ISA bus where it
acted as a high-speed conduit for memory-mapped I/O and DMA, while the ISA bus
handled interrupts and port-mapped I/O.
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