I'm slightly uncertain how memory mapped spaces work.
Essentially, they map a local buffer over some memory space on the
i/o device in question, such that reads or writes from the local buffer
access the device's memory space.

So lets say I have 20 bytes in the device's buffer; if I read 10bytes
from the buffer, will the following 10 bytes be available from the start
of the buffer at the next read?  ie: is this a ring?

I suspect this may depend on the device, but do most devices act this way?