Described herein is a method of controlling memory usage in a computer system having limited physical memory, wherein one or more application programs execute in conjunction with an operating system. At a first memory usage threshold, the operating system requests at least one of the application programs to limit its use of memory. At a second memory usage threshold that is more critical than the first memory usage threshold, the operating system requests at least one of the application programs to close itself. At a third memory usage threshold that is more critical than the first and second memory usage thresholds, the operating system terminates at least one of the application programs without allowing its further execution.