http://www.ualberta.ca/CNS/RESEARCH/LinuxClusters/64-bit.html
The -mcmodel flag selects the memory model or memory access modes for a program. Three memory models are supported by the compilers: small(text+data<2GB), medium (text<2GB, data>2GB), and large (text>2GB, data>2GB). Memory segments less than 2GB can be accessed with direct references, while larger segments require indirect or relative references.
64-bit Programming Issues