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The Java Nanokernel - JN The Java Nanokernel runs on evaluation boards assembled by National Semiconductor for their NS486SXF part. National describes this chip as a 32-bit 486-class controller with on-chip peripherals for embedded systems. JN is designed to be as small and efficient as possible while supporting the functionality required to run the Java Virtual Machine. Above JN an application interface (API) has been implemented which provides only: 1) those services that we found necessary to run the JVM as an application; 2) services necessary to port the KA9Q TCP/IP stack. JavaSoft's JavaOS currently implements as much functionality as possible in interpreted Java. Unlike JavaOS, JN, its drivers, and the JN TCP/IP stack are implemented in C. The JVM runs as an application thread. However, unlike Java hosted on Unix or Windows, the only functions included in JN are those required to run Java threads. JVM was originally written using a multi-threading C runtime called Greenthreads. This runtime coordinates activities, such as I/O, between interpreted Java threads and the host system. A large part of porting Java consists of porting or reimplementing this Greenthreads functionality, which was not originally documented or specified. Our initial approach to determining this functionality was to link Java without Greenthreads, locate and inspect the JVM calls to the missing routines, and guess at the required functionality. This reverse engineering approach resulted in a system that almost worked and was not unduly tainted by the Greenthreads implementation. This process also identified a few services that did require inspecting the Greenthread source. We were thus able to implement a small OS providing the Greenthread functionality without ever becoming expert in the existing Greenthreads implementation. Our determination of this API was a significant part of this effort; our document describing the resulting API served as the initial basis for the corresponding JavaSoft specification (not yet available). The current system consists of the JN nanokernel; a modified subset of the KA9Q TCP/IP stack; emulation API's providing the low-level system functionality required by both JVM and KA9Q; a simple utility suite (including the ability to format and load a simple filesystem onto a PCMCIA RAM disk); a modified JVM run-time; a Java environment with all classes needed to run both the Java compiler and compiled Java applications that do not use the Abstract Windowing Toolkit (AWT); a simple Java CLI (Command Line Interpreter, i.e., shell) which uses a VT100 terminal connected via the UART; a very simple Java editor and a simple Java Web server; a test suite (consisting of low-level JN tests, TCP tests, and Java tests); and drivers for the following: clock, calendar, PCMCIA controller, NS16550 serial-port UART, National Semiconductor AT/LANTIC Ethernet chip, and a bidirectional parallel port supporting a 640x480 24-bit Connectix Color QuickCam camera. A detailed report (from which the above text was excerpted) describing JN as of December 1996 is now available. JN: An Operating System for an Embedded Java Network Computer (html) JN: An Operating System for an Embedded Java Network Computer (postscript) A document describing most of the JN API is now available as a postscript document.