1. A computing device that uses segmentation to safely execute native code, comprising: a processing element that executes the native code; and a memory configured to store code and data for the processing element; a segmentation mechanism within the processing element which limits the native code executing on the processing element to accessing a specified segment of memory; an instruction-processing unit within the processing element, wherein the instruction-processing unit is configured to execute a user-level instruction that causes the segmentation mechanism to limit memory accesses by the native code to the specified segment of the memory; and a descriptor table in the memory that comprises memory segment descriptors that define one or more segments in the memory, wherein upon executing the user-level instruction, the instruction-processing unit accesses the descriptor table.