Secure Digital or SD Card is a generic flash card for storing data. The standard extends the older MultiMediaCard (MMC) by adding faster transfer modes and a protected area designed for use by digital restrictions management functions of entertainment devices. Nearly all MMC and SD cards are block devices using NAND memory.
Though the MMC and SD native protocols are proprietary, all SD cards and virtually all modern MMC cards include support for a well-documented variant of the SD protocol used over the SPI bus. Devices such as the Supercard may use SPI mode, or they may implement the (faster) native SD mode.