altmem —
Alternative memory disk driver
The altmem driver enables use of physical memory that is
  normally inaccessible by the machine-dependent
  pmap(9) as a swap device.
When an alternative memory disk device is present, this device is
    generally preferred to hard disk-based swap space. See the
    swapctl(8) and
    fstab(5) man pages for
    instructions on how to assign priorities to swap devices.
  - /dev/altmem??
- block mode alternative memory disk devices.
- /dev/raltmem??
- raw mode alternative memory disk devices.
Thealtmem device driver appeared in
  NetBSD 6.0.