The Texas Instruments PCI-4520 CardBus Controller is a hardware component that allows a laptop or desktop computer to use CardBus devices. CardBus is an expansion card standard that was used primarily in laptops to add functionality such as networking, sound, or storage devices via a PC Card slot (also known as a PCMCIA slot). The PCI-4520 is a specific model of CardBus controller from Texas Instruments, which enables the computer to interface with such cards through the PCI (Peripheral Component Interconnect) bus.