A system and method for directing download and execution of an application. A runtime engine receives an identifier of a block of shared memory from a directing process. The shared memory stores a map indicating which portions of an application file have been stored in a local copy of the application file. The runtime engine also receives an instruction from the directing process to execute the local copy when the local copy stores less than the entire application file. As the application executes, the runtime engine identifies requests from the application to access portions of the local copy and uses the map to determine whether the requested portions have been stored in the local copy. If the requested portions are not stored in the local copy, the runtime engine prevents the executing application from accessing the local copy until the map indicates the requested portions have been stored therein.