Secure Digital card

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The XO laptop has 1 MMC/SD/SDHC slot. 8-gigabyte memory cards work. As larger cards are created, they will also be tested (-gnu).

XO uses a custom microchip for SD card access, Camera and Flash Enabler, or CaFE. CaFE includes SDHC support. SDHC is compatible at a hardware level with SD. Pierre Ossman wrote the Linux driver support for SDHC. SD cards up to something like 256 GB should be supported. cafe is driven by the sdhci driver. cafe follows the standard host controller interface for SD controllers.

Here is how SD block addressing works: The address is 32 bits. In the CaFe chip, which is compatible with the standard "SDHCI" spec, the address goes in the 32-bit chip register at offset 8. For old SD cards (not the new SDHC - High Capacity careds), the address is a byte address, so the maximum size is 4 Gbytes. For the newer SDHC cards, the address is a block number, assuming 512 byte blocks, so the maximum size is 2 TBytes. None of this is CaFe-specific, BTW. It is standard SD.