Initial RAM disk image
An initial RAM disk holds files, programs, or modules that are not included in the kernel image but are required for booting.
For example, booting from DASD requires the DASD device driver. If you want to boot from DASD but the DASD device driver has not been compiled into your kernel, you must provide the DASD device driver module on an initial RAM disk. If your image contains all files, programs, and modules that are needed for booting, you do not need an initial RAM disk.
Distributions often provide specific RAM disk images to go with their kernel images.