may issue a command to switch to the four-bit SD bus interface, if the SD card supports it. For various card types, support for the four-bit SD bus is either optional or mandatory. After determining that the SD card supports it, the host device can also command the SD card to switch to a higher transfer speed. Until determining the card's capabilities, the host device should not use a clock speed faster than 400 kHz. SD cards other than SDIO (see below) have a "Default Speed" clock rate of 25 MHz. The host device is not required to use the maximum clock speed that the card supports. It may operate at less than the maximum clock speed to conserve power. Between commands, the host device can stop the clock entirely. Achieving higher card speeds The SD specification defines four-bit-wide transfers. (The MMC specification supports this and also defines an eight-bit-wide mode; MMC cards with extended bits were not accepted by the market.) Transferring several bits on each clock pulse improves the card speed. Advanced SD families have also improved speed by offering faster clock frequencies and double data rate (explained here) in a high-speed differential interface (UHS-II). File system Like other types of flash memory card, an SD card of any SD family is a block-addressable storage device, in which the host device can read or write fixed-size blocks by specifying their block number. MBR and FAT Most SD cards ship preformatted with one or more MBR partitions, where the first or only partition contains a file system. This lets them operate like the hard disk of a personal computer. Per the SD card specification, an SD card is formatted with MBR and the following file system: For SDSC cards: Capacity of less than 32,680 logical sectors (smaller than 16 MB ): FAT12 with partition type 01h and BPB 3.0 or EBPB 4.1 Capacity of 32,680 to 65,535 logical sectors (between 16 MB and 32 MB): FAT16 with partition type 04h and BPB 3.0 or EBPB 4.1 Capacity of at least 65,536 logical sectors (larger than 32 MB): FAT16B with partition type 06h and EBPB 4.1 For SDHC cards: Capacity of less than 16,450,560 logical sectors (smaller than 7.8 GB): FAT32 with partition type 0Bh and EBPB 7.1 Capacity of at least 16,450,560 logical sectors (larger than 7.8 GB): FAT32 with partition type 0Ch and EBPB 7.1 For SDXC cards: