mem - physical system memory
/dev/mem is a character device file that is used by other programs to examine the physical memory.
Trying to mmap(2) a physical range results either with success, or with an error. When invoking mmap(2) on bad memory range, the kernel will write a message about it to the kernel log.
By default, the kernel limits the areas which can be accessed. The allowed areas are the reserved ranges in physical memory, essentially limiting the access to ROMs and memory-mapped PCI regions on x86.
To create it manually:
mknod /dev/mem c 1 1
chmod 660 /dev/memmmap(2)EINVAL: An access violation was detected.ENOMEM: The requested range would wrap around, creating an access violation.