The Dreamcast uses the SD card connected directly to
the serial
 port, it would take some circuitry to connect a CF into that port. 
 [...???...]
  There is a way to connect an SDCard into the serial port and load
 games from that.  Look for dreamshell and your life will be enlighted
 :) 
 
It's not clear from what I've been able to find that the adapters
between the serial port and SD cards are passive (as opposed to having
active circuitry in them); I haven't found any description of what's in
them beyond "it Just Works with this software".  Do you have reason to
think the adapter is a passive device?
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