What is a bus in a CPU

What is a bus system?

The totality of all connections for sending and receiving data between the individual components and devices (functional units) of a computer is referred to as a bus system. It is practically a complex system of miniature electronic conduction paths. This includes:

rigid conductor tracks on the circuit boards (main board, local bus) and
flexible lines between the hardware components (peripheral bus).

A so-called internal bus connects the CPU-internal elements, while the connections between the CPU and the other components of a computer are made by an external bus.

Local bus system
The hardware components belonging directly to the microprocessor are housed on the main board (motherboard). These functional units are electrically connected by a local bus system (Fig. 1).

1) As of Pentium, the coprocessor is part of the microprocessor at Intel.

Peripheral bus system:
The peripheral bus is also connected via an expansion interface. Even the interconnection in the network up to and including the Internet can be viewed as part of this peripheral bus system.

local and peripheral bus

In principle, all functional units connected to a bus system can send and receive information. In order to prevent different devices / components from sending data at the same time and to ensure that only the devices / components for which the currently transmitted data are intended are ready to receive, bus systems must be managed very precisely. This is usually done by the processor, often also by its own bus controller. This allocates fixed times for use to the functional units.

Which bus systems are used?

Two or more different, complementary bus systems can definitely be accommodated on a motherboard. This architecture is chosen for reasons of cost, for example. Various bus systems are used, including:

ISA and EISA bus:
The ISA bus (Industry Standard Architecture = bus according to industry standard architecture) and the EISA bus (Enhanced ISA = further development of the industry standard bus architecture) were the first bus industry standards.
The ISA bus introduced in 1981 was the bus standard for the first PCs. It was optimally matched to the 286 Intel processor. In its original version it was 8 bits wide and worked with a clock frequency of 4.77 MHz. Later it was expanded to 16 bit (and 8 MHz) for the AT PCs and then also referred to as the AT bus. It is still sometimes found in computers today when it comes to simple expansion cards, such as a sound card or a game port card.
The EISA bus for IBM-compatible systems, developed in 1988 by various hardware manufacturers under the leadership of Compaq, was overall much more powerful than the ISA bus. EISA slots are practically duplicated ISA slots and can therefore also accommodate ISA plug-in cards. The EISA bus also works with a bus width of 32 bits (for processors from 80386 upwards) and a clock frequency of 8.33 MHz. The EISA bus quickly lost its importance after 1993, as more powerful and at the same time cheaper new bus developments became established (VESA bus, PCI bus).

PCI bus:
The PCI bus (Peripheral Component Interconnect = connection bus for expansion components) has established itself. Today it is an integral part of all IBM-compatible systems, as well as that of Apple Macintosh PCs and Digital Alpha workstations. The bus system specially developed and introduced by Intel in 1992 for the Intel Pentium is an industrial standard that is standardized down to the last detail and can (and will) be copied by computer manufacturers. It is used to connect (maximum 10) expansion cards to the processor, e.g. B. sound, graphics or network cards.
The PCI bus has a 32-bit data and address bus, is clocked at 33 MHz and has a maximum transmission rate of 132 Mbytes / s.
The PC-BIOS (Basic Input Output System) automatically recognizes the expansion cards and configures them independently.
The data exchange in this bus system does not necessarily have to run via the microprocessor. To reduce the workload, PCI components can also exchange data with one another on the PCI bus.

AGP bus
Realistic graphics have always been a particular problem for computers. A new solution is offered by the AGP bus (Accelerated Graphics Port), a special graphics bus that is designed as an independent slot on the graphics card and is directly connected to the main memory and the processor via the chipset. This allows any size of RAM to be shared. There is no need for the CPU to access the graphics card; the data exchange takes place in the considerably faster main memory. This, the reservation of lines for the exclusive use of graphics, higher bus clocks (66MHz, 100 MHz and more with a data width of 32 bits), the use of the pipeline method and the dual use of signals through the rising and falling pulse edges are currently (at least theoretical) transfer rates of 1 GByte / s possible.

PCI-Express (PCIe):

PCIe is the successor to PCI and AGP and offers a higher data transfer rate compared to its predecessors. Graphics cards that require particularly high data transfer rates as well as other internal components such as sound cards, modems, etc. can be connected via PCIe. Usually there are several PCIe slots on a mainboard, which differ in the transmission speed. The graphics card is then plugged into the fastest PCIe slot.

Despite this very different physical structure, PCIe is fully compatible with PCI in terms of software, so that neither operating systems, drivers nor application programs have to be adapted.