Application execution environment for a small device with partial program loading by a resident operating system

Abstract

Small application programs called “applets” are defined by pseudo-code (p-code) instructions. Each applet is divided into software components that are executed by a processor, operating as a state machine. The applets are originally written in a high level programming language and then converted into the p-code, for downloading into a non-volatile read/write memory of a portable combination pager and personal information manager (PIM) device that includes the processor. In this device only 1-2 Kbytes of static random access memory (RAM) are available to load the p-code of applets for execution by the processor. The software components include variables and event handlers that respond to events such as key presses of buttons disposed on a control panel of the device. The variables include persistent variables that are stored in memory between invocations of an applet, invocation variables that are store in RAM during execution of an applet, and state variables that are only stored in RAM during a specific state of the applet. Event variables are available for processing of a specific event, but are replaced by different event variables when a different event must be processed. A kernel comprising a portion of the operating system for the device serves as a p-code interpreter, producing machine instructions from the p-code loaded into RAM, for execution by the processor. Accordingly, applets, which comprise only a few kilobytes of p-code, are executed by loading only a few of the software components into RAM at a time.