Hypothetically, if cost and hardware modifications were not concerns, could SD cards be used in place of RAM to run the original Doom for seven hours? I understand that SD cards use NAND flash and are fundamentally different from actual system memory, but I'm curious whether one card would have enough capacity and whether the idea could work at all.
4 Answers
SD cards can have surprisingly high sequential transfer speeds, but that doesn’t make them suitable as RAM. RAM handles huge numbers of tiny reads and writes with extremely low latency. Flash memory is much slower for that pattern and has a finite number of write cycles, so using it as working memory would cause severe stuttering and eventually wear the card out.
Even just updating the screen repeatedly would be hard on flash memory. At 640×480, 30 frames per second, and three bytes per pixel, the display data alone is about 27 MB of writes per second. If the same flash blocks were reused as a frame buffer, they could wear out very quickly; spreading writes across the whole card would help, but it still wouldn’t provide usable RAM performance.
The practical answer is that no reasonable number of SD cards would make this work. One card has enough space, but adding more cards would not solve the latency and write-endurance problems. With specialized hardware, caching, and an unusual memory controller, you might be able to experiment, but it would be far slower and less reliable than simply using a few megabytes of actual RAM.
Capacity-wise, one very small SD card would be more than enough. The original Doom needed only a few megabytes of RAM and roughly 12 MB of storage, so even a 1 GB card would have plenty of room for the operating system, game, and extra data. The problem is not capacity—it’s that an SD card cannot act like normal RAM.
So the limiting factor is the repeated writing and access latency, not how much storage the card has?