switch RAM and ROM mode with memory_region_set_readonly

[Hiroko Shimizu]

Hello,
I want to switch  the memory read-only  mode with
memory_region_set_readonly() function for writing a value to memory under a
specific condition. I checked that I could switch the mode using
memory_region_set_readonly(), but I couldn't write a value to the memory
region when the mode was false.
What should I do?