text: 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!
  