The DMA memory map, explained

Physical memory is not one flat block of RAM. The chipset reserves parts of the address space for devices, for firmware, for things that are not memory at all. A memory map is the list of the ranges that really are RAM.

A reader that has one knows where it may look. A reader without one has to discover the layout as it goes.

• Speed - probing unknown space costs reads, and reads are the one thing a DMA setup spends.
• Stability - touching a reserved range is not always harmless. On some boards and chipsets it is enough to hang the machine.

That is the whole reason the subject comes up so often: it is the difference between a setup that feels instant and one that stutters, and occasionally between a session and a reboot.

The loader generates the map itself the first time it runs, and reuses it afterwards. It is kept as a plain text file (elusive_mmap.txt) next to the loader, or in ProgramData when the loader path contains characters the underlying library cannot take.

You do not have to run any other tool, and you do not have to copy a file from a forum post. If a map is already there, it is reused as it is.

A memory map describes one machine, in one configuration. Let a new one be generated when:

• you change or add RAM in the game PC;
• you change the motherboard or the CPU;
• a BIOS update changes how memory is reserved;
• reads start failing on a machine where they used to work.

Deleting the file is enough: the next run writes a new one.

A map that no longer matches the machine shows up as reads that return nothing, or as a game that is found but whose entities never make sense. Delete the file, start again, and watch the log line that reports how many regions were written.

If the loader says it could not open the FPGA device at all, the map is not your problem - the card is. The guide on a card that is not detected takes that case in order.