The planar modes are how the chip operates naturally, and chain 4 is the hack (sort of! - I mean, chain 4 is of course how the system is designed to be used).

The planar modes are how the chip operates naturally, and chain 4 is the hack (sort of! - I mean, chain 4 is of course how the system is designed to be used). There's a bit about this on the Wikipedia page: https://en.wikipedia.org/wiki/Mode_13h https://en.wikipedia.org/wiki/Mode_13h