Memory mapping utility routines.

Summary

Memory mapping utility routines.
Functions
map_data	Map data to mpr 3 and 4.
remap_data	Update value of mpr 3 and 4 if needed.
unmap_data	Restore values of mpr 3 and 4.

Map data to mpr 3 and 4.

_bl	data bank.
_si	data address.

_bx	previous values of mpr 3 and 4.
_si	remapped data address.

Update value of mpr 3 and 4 if needed.

_si	data address.

_si -

Restore values of mpr 3 and 4.

_bx	mpr 3 and 4 values.