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Cache Poor

Creating hardware to remove a software layer can drastically reduce your cache. By Philip Ling.

General purpose doesn't really go hand in hand with high performance: if it did, there wouldn't be so many variations on a theme and the term application specific wouldn't exist. Tuning a device for maximum performance usually comes at the cost of reducing its scope, hence the application specific tag. But general purpose doesn't have to mean poor performance.

The term 'general purpose' is normally with respect to hardware as opposed to software, but the two are merging. The most general purpose language available today has to be Java. Its portability and object oriented nature makes it attractive to programmers and hardware designers alike. With the flexibility of easy deployment on any processor comes the proviso that it is received by a Java virtual machine (jvm). However, this proviso can impact dramatically on performance and, whilst there are a number of methods emerging to combat that impact, each has its good and bad points.

The absolute minimum requirement of a jvm is the ability to read the class file format (see box) and perform the operations specified within it correctly by interpreting the Java byte code. Execution by interpretation imposes a performance penalty which in many situations is unacceptable, mainly due to the fetch and decode overhead.

One technique commonly used to reduce this run time overhead is a just in time (jit) compiler, which takes a string of byte code and compiles it into the processor's native machine code. While this accelerates execution, it adds another software layer, and synonymous with adding software is losing processor cycles.

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Graham Pitcher

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