The internal memory on an FPGA very fast, dual ported, and quite small. An FPGA that retails for around $30 has about 70kB of block RAM. Retro computer emulations use the internal memory for features that need the performance or can benefit from dual porting, such as full speed access to the video memory with no access flicker.
The main memory of the retro CPU is implemented with external RAM. This external memory is usually high density dynamic memory of 32MB or more in a single chip. It’s a direct descendant of the 4116 and 4164 dynamic RAM chips of the 70’s and 80’s and is very similar to the DDR RAM in a modern PC. Most FPGA development boards come with 32MB or more of external RAM.
Just like the 41xx RAM chips, these memories require periodic refreshing that is synchronized to the accesses for data. This is implemented using FPGA logic, and complicates the design. But the ability to have megabytes of cheap RAM usually makes it worthwhile. Another downside is that the board layout becomes critical because these parts run at high speeds in 20MHz plus range. Consistent trace lengths, special decoupling etc etc.
After a lot of thought, I’ve decided to use a different approach.
I’m going to use 3.3V static RAMs to avoid the refresh and high speed hassles of the DRAMs. I don’t need megabytes of RAM – 256k would be fine. Also, I can’t use the 5V versions because FPGA pins are all 3.3V and level converters triples the parts count.
The 3.3V parts are SMD only, but not ridiculously so.
A surface mount RAM chip
I also need some ROM storage. Using a 3.3V parallel flash ROM just like the RAM is possible, but that adds another big SMD part.
Instead, I’m going to use one or more SPI flash ROMs and have the FPGA copy the appropriate data into RAM when it boots (just like the Model 3 does when using “ROM” BASIC). The SPI ROMs are tiny and cheap and have only 8 pins.
Next step is to do a simple PCB with these parts. There are a few places that will do them at $10 for 10 boards, as long as they are 100mm x 100mm.