Polls the sd controller until the read flag is set, at which point it
reads 32 bits of data from the controller.
long response codes (such as CID) are not supported in hw or sw.
These series of commands are enough to read the first 512b block off of
the sd card. The RCA is hard coded to the sd card that I have on hand,
since response codes are not supported
This code creates a shared mapping in high ram, makes sure that that
works, and also makes sure that the memory which is mapped over is left
unchanged.
This only includes the method to set the pairs of displays individually.
The functions to write a single 16 or 32 bit value have not been
implemented yet, nor has the mask function.
