On the Tegra modules, the CPU shares the memory with the graphic core (see http://en.wikipedia.org/wiki/Shared_graphics_memory ). The Tegra T20 needs this amount to be fixed and specified at boot-time, this memory is called carveout by NVIDIA.
Usually Linux reports substantially less RAM than the module specification states (e.g. 126MB on a Colibri T20 256MB). Carveout, the framebuffers and memory used inside the Kernel (around 25MB) add up to the difference to the total RAM size.
One can adjust it from within U-Boot as follows:
setenv memargs 'mem=218M@0M fbmem=6M@218M nvmem=32M@224M' save
This change results in around 190MB available RAM for user space applications.
Note: Graphics/multimedia performance might be severely crippled should you lower it much from its default values.