memdisk

Memdisk is a component of the Syslinux project that enables booting operating systems from disk images by loading the image into memory and presenting it to the system as a virtual disk. By copying the image into RAM, memdisk allows a BIOS to boot from the in-memory image as if it were a physical drive, including the image’s boot sector or bootloader.

Memdisk is commonly used in Syslinux-based boot menus to boot from images stored on a USB drive, CD, or network location. It supports several image formats, including floppy, hard disk, and ISO9660 images, allowing users to boot legacy systems or otherwise unsupported images that cannot be launched directly by the primary bootloader. The typical workflow involves invoking memdisk from the bootloader with a path to the image and a format indicator, after which control is transferred to the image’s own boot process.

Practical considerations include memory requirements and compatibility. Since the entire image must reside in RAM, the available system memory sets an upper bound on the size of bootable images. Large images can lead to long load times or boot failures on machines with limited RAM. Memdisk is not a general-purpose virtualization tool; the emulated disk is read-only during the boot process, and the success of boot depends on the image’s internal boot sequence and the host BIOS/bootloader compatibility.

Memdisk was developed as part of Syslinux and has been widely used in live CDs, rescue disks, and multi-boot configurations to provide access to a broad range of operating systems from disk images. It was authored within the Syslinux project, notably by H. Peter Anvin, and remains a useful option for legacy and image-based boot scenarios.