... resulted in figuring out the whys and hows.

The EPROM was intended to hold a monitor and work with any '51 including the romless - that's why it's mapped at the bottom. One of the aims of it was to provide a loader, to load programs to RAM, which therefore had to be utilized as fully as possible - that's why the RAM is mapped to both code and xdata space and the whole upper 32kB. The bottom 32kB had to fit also the memory-mapped peripherals (LCD, keyboard and reserve) which are there for educational purposes - hence only 16kB space remained for the EPROM. There is no 27C128 manufactured for ages, so you can use either 27C256 or 27C512, but mapped only to 16kB. The jumper allows to manually select one of two banks in the '512.

But, if you don't need to use the RAM, you can simply drop a 27C512 into the RAM's socket. It will be mapped to the upper 32kB of both xdata and code space, and you can select one of two 32kB banks of it manually via P3.6 (which is /WR, but if you don't have RAM you don't need WR either...).

"Fixing" the EPROM socket would involve also remapping it somehow to the xdata space, otherwise the space of EPROM overlapped by internal code memory of the '51 would be lost...


JW