bus. The Express bus re-uses the pin layout of UHS-II cards and reserves the space for additional two pins that may be introduced in the future. Hosts which implement version 7.0 of the spec allow SD Cards to do direct memory access, which increases the attack surface of the host dramatically in the face of malicious SD cards. Version 8.0 was announced on 19 May 2020, with support for two PCIe lanes with additional row of contacts and PCIe 4.0 transfer rates, for a maximum bandwidth of 3938 MB/s. microSD Express In February 2019, the SD Association announced microSD Express. The microSD Express cards offer PCI Express and NVMe interfaces, as the June 2018 SD Express release did, alongside the legacy microSD interface for continued backwards compatibility. The SDA also released visual marks to denote microSD Express memory cards to make matching the card and device easier for optimal device performance. Bus speed comparison Compatibility NOTE: If the card reader uses the DDR208 controller on the UHS 1 pins, the card reader will perform at 180MB/s on applicable UHS 1 cards Class The SD Association defines standard speed classes for SDHC/SDXC cards indicating minimum performance (minimum serial data writing speed). Both read and write speeds must exceed the specified value. The specification defines these classes in terms of performance curves that translate into the following minimum read-write performance levels on an empty card and suitability for different applications: The SD Association defines three types of Speed Class ratings: the original Speed Class, UHS Speed Class, and Video Speed Class. (Original) Speed Class Speed Class ratings 2, 4, and 6 assert that the card supports the respective number of megabytes per second as a minimum sustained write speed for a card in a fragmented state. Class 10 asserts that the card supports 10 MB/s as a minimum non-fragmented sequential write speed and uses a High Speed bus mode. The host device can read a card's speed class and warn the user if the card reports a speed class that falls below an application's minimum need. By comparison, the older "×" rating measured maximum speed under ideal conditions, and was vague as to whether this was read speed or write speed. The graphical symbol for the speed class has a number encircled with 'C' (C2, C4, C6, and C10). UHS Speed Class UHS-I and UHS-II cards can use UHS Speed Class rating