README

This sample program creates a listing of the files in the root directory of the
inserted SD card (WHICH DOES NOT CONTAIN ANY IMPORTANT DATA). The first primary
partition of the SD card must be a FAT16 or FAT32 partition. Extended partitions
or other file systems are not supported.

After creating the directory listing a file "circle.txt" is created in the root
directory and some text is written to it. After closing the file it is re-opened
and its contents is read and written to the screen.

Circle uses different interfaces for SD card access, depending on the Raspberry
Pi model and the system configuration. On Raspberry Pi 1-3 and Zero the SDHOST
interface is used by default, but not if NO_SDHOST or REALTIME is defined. In
this case the EMMC interface is used. The Raspberry Pi 4 has an additional EMMC2
interface for SD card access, which is used in any case.

The EMMC driver was ported from [https://github.com/jncronin/rpi-boot]. See the
notices in emmc.cpp! The SDHOST driver was ported from Raspbian Linux. See the
notices in sdhost.cpp!
