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ProDOS requires for each opened file a user-supplied page-aligned 1024 byte i/o buffer. This makes the generic POSIX file i/o library contained in the cc65 C-library quite heavyweight. In contrast the lightweight pfs implementation uses the uIP packet buffer as ProDOS i/o buffer. Therefore: - Only one file may be open at any time. - That file may not be open while the uIP packet buffer is used by uIP. The open()/read()/close() sequence should be completed before Contiki event scheduling or inside handling a single Contiki event. - The uIP packet buffer must be large enough to hold the ProDOS I/O buffer. Depending on the position of the uIP buffer in memory this means between 1024 and 1024 + 256 bytes. Therefore in an Ethernet environment setting the MTU_SIZE to at least 1266 is safe (So the default of 1500 is just fine).