Understanding the -A and the -H flags

Hi! I'm trying to understand the interaction between the -A and -H RTS flags. The documentation at http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/runtime-control.html says that if you use -H (with or without an argument) it implicitly implies some value of -A. However, it's not clear to me what value -A will get and how that value is related to the value of -H. For example, if I set the suggested heap size to 1G, using -H1G, surely the size of the nursery (-A) won't be "whatever is left over," but something more reasonable e.g. the size of the L2 cache? Perhaps it would make sense to document the actual algorithm used to set -A given -H (with and without argument.) Cheers, Johan