MIX is a hypothetical computer used in Donald Knuth's monograph, The Art of Computer Programming (TAOCP). MIX's model number is 1009, which was derived by combining the model numbers and names of several contemporaneous, commercial machines deemed significant by the author. ("MIX" also represents the value 1009 in Roman numerals.)
The 1960s-era MIX has since been superseded by a new (also hypothetical) computer architecture, MMIX, to be incorporated in forthcoming editions of TAOCP. Software implementations for both the MIX and MMIX architectures have been developed by Knuth and made freely available (named "MIXware" and "MMIXware", respectively).
Several derivatives of Knuth's MIX/MMIX emulators also exist. GNU MDK is one such software package; it is free and runs on a wide variety of platforms.
Their purpose for education is quite similar to John L. Hennessy's and David A. Patterson's DLX architecture, from Computer Organization and Design - The Hardware Software Interface.
Over to under
Enter your wonder baby
Submerge into me baby
And take me to higher
Submerge within the light
Illuminate my night
And let your atmosphere surround me
Submerge within the light all night
Till we become the sun
See if I'm breathing
Because I'm not sure
Tonight if I'm alive
Words are demeaning
They can't describe
Submerge within the light