2.5 Memory Ranges
Many devices are assigned address space in main memory. It's sometimes called "shared memory" or "memory-mapped IO" or "IO memory". This memory is physically located inside the physical device but the computer accesses it just like it would access memory on memory chips. When discussing bus-resources it's often just called "memory", "mem", or "iomem". In addition to using such "memory", such a device might also use conventional IO address space. To see what mem is in use on your computer, look at /proc/iomem. This "file" includes the memory used by your ordinary RAM memory chips so it shows memory allocation in general and not just iomem allocation. If you see a strange number instead of a name, it's likely the number of a PCI device which you can verify by typing "lspci".
When you insert a card that uses iomem, you are in effect also inserting a memory module for main memory. A high address is selected for it by PnP so that it doesn't conflict with the main memory modules (chips). This memory can either be ROM (Read Only Memory) or shared memory. Shared memory is shared between the device and the CPU (running the device driver) just as IO address space is shared between the device and the CPU. This shared memory serves as a means of data "transfer" between the device and main memory. It's Input-Output (IO) but it's not done in IO space. Both the card and the device driver need to know the memory range.
ROM (Read Only Memory) on cards is a different kind of iomem. It is likely a program (perhaps a device driver) which will be used with the device. It could be initialization code so that a device driver is still required. Hopefully, it will work with Linux and not just MS Windows. It may need to be shadowed which means that it is copied to your main memory chips in order to run faster. Once it's shadowed it's no longer "read only".
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