Methods, systems, and apparatus, including computer programs encoded on computer storage media, relating to software execution. One of the methods includes executing, on a computer including a single memory for storing data and instructions, a virtual environment including a data memory and an instruction memory, the instruction memory configured to be unreadable by instructions stored in the instruction memory; receiving, at the virtual environment, a software module comprising multiple instructions; and performing validation of the software module including: identifying, in the software module one or more calls to the single memory; and verifying that the one or more calls to the single memory are in the data memory.