Java in 4Kbyte of rom

Electronics Times,  March 6, 2000  

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Californian 8bit microcontroller supplier Scenix has built a version of Java that uses up just 4Kbyte of on-chip rom.

The virtual machine is derived from the JavaCard version developed for smartcards. The company has stripped the design down a little further to get it below the 4Kbyte limit, without class libraries. Most JavaCard implementations are designed for 16Kbyte of program memory.

The Scenix virtual machine restricts data types to 16bit integers. There are no floating-point or larger data types. Like JavaCard, it has no garbage collection and each application can have only one thread of control.

Chris Waters, principal software engineer, said: "Having multi- threading would be nice but unrealistic. The motivation behind this was twofold: writing in Java is much simpler; and to open up access to larger programs."

Derived from the PIC16 design, the Scenix microcontroller uses a Harvard architecture that splits the program and data memory buses. This limits program space to on-chip memory although the parts can access additional off-chip data memory. Because the code for a Java program is treated as data by the microcontroller, it is possible to get the devices to run much larger applications.

The company has developed its own debugger for Java applications and a linker. The compiler is based on IBM's open-source software. To save space, the linker only includes classes actually used by an application, even if others are declared in the source. It also optimises the method tables to save space.

Waters said: "We will also put together a JavaCard-compliant version."

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