SRAM Chip. Entering the data either manually through DIP Swithces or from the bus
In the early days of computing, this was quite common: computers had front panel switches to 'toggle in' the low-level instructions to get the system up and running, along with lights to display the memory contents. My exposure to this was on the Data General Eclipse minicomputer. Early hobbyist computers had this also (e.g., Altair 8800). So it's a total retro thing!





Since you have just one address and one data, you can treat the DIP switch access as if it were a single-cycle DMA. And your manual/auto switch? Treat it as 'run/halt'. And, add 'reset' as well to get the machine to vector back to zero.

DIP switches are fiddly, hard to actuate and will wear out quickly. I suggest toggles.

Finally, since you have an FPGA at your disposal you could wire everything up to it, and implement the CPU of your choice inside. Depending on your mood you could play with 8080, 6502, 6800, 4004...