[pedrow]

Is the aspect ratio part (needing to stretch from 8:5 to 4:3) because the IBM PC didn't have square pixels?

[vikingerik]

Correct, although the nitpick is that it's not exactly the IBM PC itself, but the CGA video card output. Its resolution of 320x200 is 8:5, and so the pixels aren't square on a 4:3 CRT.