[R] Is there any way to release memory in running time?

[wanr@ucalgary.ca]

Hi all,

I am doing some intensive computation right now. My system is Pentium4 3.20G 
+ 1.0G RAM + WindowsXP + R1.9.1. It seems my computer is very powerful. 
However, when I do some simple matrix algebra operations based on a matrix 
(DD) with dimension 5000000 by 2, I found that the consumption of RAM is 
huge. For example, the command a <- 1 - DD[,2] eats my 100M RAM. Does anyone 
know how to release memory in the middle of program (running time)? BTW, the 
command rm() does not free memory unless you quit R as I as know. Thanks in 
advance.

Rui