[R] R objects layout

[Faheem Mitha]

Hi,

Can anyone point me to documentation of how R objects such as vectors, 
lists, matrices and data frames are laid out in memory? For example, which 
of these are laid out as a 1-d array, 2-d array and so forth?

The R Internals document seems like the natural place to look, but I don't 
see it there. If such things are only documented in source code, could you 
point me to the relevant place(s)?

(Please CC me; I'm not subscribed.)
                                                             Thanks, Faheem.