This article offers open source software (oss) for FAT over MMC SDC. We have many options of open source TCP/IP stack implementations. However we don’t have enough choices for open source FAT file system over popular MMC/SDC, because of commercial reasons. Most of the popular flash cards are covered and protected by patents and copy rights. For an experienced engineer it is easy to hack the protocol, but this will bring legal problems if he releases what he found during the reverse engineering.