move update of memory_read_value outside conditionals.
Fixed monitor memory read bug (can read even when CPU running) and
also increase max clock of design to 99MHz :)
But CPU still fetching wrong data it seems...
add debug output that confirms that opcodes are being read fine
from IO in simulation, although they are not when synthesised.
In fact, synthesised memory reads by the CPU seem to always return $FF,
even though the serial monitor can read and write all memory just fine.
show opcode on seg led.