Com
From Computer History Wiki
Com is listed as the oldest CP/M emulator. The project homepage is http://userweb.windwireless.net/~jimc/com.html.
Com started out as an i8080 emulator for CP/M running on the 68000 cpu, running CP/M. At the time all the OS calls were handed down to the native OS. From there it developed into a port to various UNIX that ran on the 68000 CPU. As time went on the CPU emulation was re-written in various other assembly languages, and the last version has a 'C' version making it now very portable.
Com doesn't run CP/M natively, it's more similar to how WINE works.
