Bus mastering allows components on the same system bus to communicate directly, improving performance by conserving resources and reducing instruction time. It is primarily available through the PCI bus, with video cards gaining direct access to RAM. Bus mastering is more beneficial for IDE/ATA buses than PCI, as devices like hard drives and optical drives have little memory of their own and rely heavily on system RAM.
A computer “bus” is a path by which data is transmitted between the various devices in the computer. Bus mastering is a process by which components connected on the same system bus can communicate directly with each other without the need for an intermediary. In general, bus mastering tends to improve performance by conserving system resources and reducing the time it takes for instructions to pass through the system.
On modern computers, bus mastering is available primarily through the Peripheral Component Interconnect (PCI) bus. On the computer motherboard, the PCI slots are located on the bottom half of the physical board. The PCI bus includes the PCI-Express (PCI-E) slot or slots, into which the system video card or cards are inserted. Other devices mounted on the PCI bus include network and sound cards. Other uses for bus mastering focus on Integrated Drive Electronics (IDE) and Advanced Technology Attachment (ATA) channels.
The primary use for bus mastering allows the video card to gain direct access to the computer’s random access memory (RAM). Under normal circumstances, if the video card wants to use system memory, it must first work by communicating with the Central Processing Unit (CPU). Without a bus master, the CPU acts as an intermediary to pass instructions between the two components. With bus mastering, the CPU is taken out of the equation, improving response times and conserving CPU cycles, thus improving performance.
While most modern systems support bus mastering, the real-world performance advantage may not apply. This is because the video card of the system, in many cases, does not need to convey information to and from the system’s RAM. Modern video cards contain onboard RAM for specific uses of video, obviating the need to communicate with the computer’s general system RAM. Some video cards contain several gigabytes of RAM on the card itself, meaning any additional RAM is superfluous.
Bus mastering may have questionable value on the PCI bus, but using it on IDE/ATA buses is much more beneficial. The primary devices connected to the IDE/ATA buses include the computer’s optical drives and hard drives; these devices contain little or no memory of their own. Therefore, data transfer between these devices and the general system RAM is substantial, as any program or file loaded from the hard drive or optical disc must first be placed into the system RAM before it can be used by the computer.
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