The C64 had the KERNAL OS that loaded from ROM, only had like 49K free in BASIC. The C64 DOS was on the 1541 drive which had its own CPU and memory and the DOS was on a ROM.<p>Both the Atari ST and Commodore Amiga had Mac emulators with a serial port or printer port dongle that loaded a Mac 128K ROM onto it to boot up Mac System floppy disks. Atari ST had Magic Sack, Amiga had AMax. Both had the same 68000 CPU as the Mac but had better graphics and sound chips so things ran faster.<p>As I recall there were viruses for the Atari ST, Commodore Amiga, and Macintosh even if most of their stuff was on ROM. Just not as many viruses as DOS had on PC Clones.