Quite apart from what others have pointed out regarding the "hidden" feature that isn't, I had homebrew development systems for both Acorn Atom, C64, ZX Spectru

Quite apart from what others have pointed out regarding the "hidden" feature that isn't, I had homebrew development systems for both Acorn Atom, C64, ZX Spectrum and a few other machines that effectively banked the RAM. There was the main 48KB of RAM on the Speccy, and a further 128KB in banks that stored the assembly program, the editor, and other tools in a protected space. The 16KB ROM was swapped out for a custom blown 16KB EEPROM that performed the copy from banked RAM to the Spectrum's main RAM, and then the custom EEPROM swapped back with the regular Spectrum ROM, and the machine performed a reset/jmp to execute the actual code I needed. This was most definitely not an MMU in the true sense of the word, but it did give access to a "protected" memory space, a dedicated kernel mode effectively, that the regular Spectrum could not access.