tying to debug strange bugs with sdcardio.
wedge state change for i-cache. ISsue #29
begin reimplmenting sd controller wrapper to deal with bizarre
write bugs.  Start with just some registers that can be read and
written, and a counter that records the number of writes. Issue #30.
fix decode address for sdcard controller.  drive sdcard lines
to allow synthesis to succeed. Issue #30
sdcard registers can be read and written from fastio and serial
monitor.
add test bench for IO mapper.
refactor various parts of the io mapping subsystem to try to fix
IO mapping bugs.
clean out unused files and reduce ghdl simulation warnings.
fix simulation problems and some warnings in ghdl.
fix compilation problems.
fix bug with memory reading where colour ram would keep being
read until next fastio access.
don't latch read values. Issue #31
fix testiomapper commands. Issue #31
remove waitstate on kickstart ROM, and show contents of Z register
in debug. Issue #31
more refactoring to prevent latching of read values. Issue #31
remove defunct file from transfer target.
make kickstart ROM read on separate bus from CIAs and SDCARD
controller as workaround for Issue #31.
change ISE synthesis settings to (hopefully) avoid memory
exhaustion problem with previous commit. Issue #31.
add 512 byte RAM for use as sector buffer to get rid of
sector_buffer being inferred as 4096 1-bit registers, and so
hopefully decongest the design.
Issue #31.
first cut at using block ram for sector buffer.
Writing to sectors will definitely not work yet.  Issue #31.