How to sort millions of rows of data in a file with less/meagre memory

Question

(From here)

I attended an interview last week and this question was asked:

How do you sort a billion rows of data in a file with only 640KB of memory in

Answer 1

Heapsort would be my reccomendation. It's relatively quick when n is large, and you only have to look at three elements with definite indecies at once.

That being said, my intuition tells me that sorting a billion rows on an 8080 even in C would be unfeasibly slow.