Implement write-protect for IMSAI memory boards

[udo-munk]

Priority: low
Difficulty: easy

The standalone BASIC interpreter write protects it's memory, so it can't be poked to death.
Just set the 1KB pages in the page table to MEM_WPROT from the output to the port.
See BASIC source and manual for refernce.

[dmcnaugh]

I've noticed the MPU-A and MPU-B boot-roms hit the write protect IO port (0FEh) a number of times also. I think I read that the California Computer Systems FDC http://www.s100computers.com/Hardware%20Folder/CCS/FDC/FDC.htm hacks the write protect signals to implement banked boot ROM switching. Interesting but probably irrelevant.