Crusoe – Transmeta Corps’ x86 compatible VLIW mobile CPU

In early 2000, chip designer Transmeta Corporation unveiled its innovative Crusoe family of x86-compatible processors aimed specifically at the mobile computing arena and designed with the objective of maximizing the battery life of mobile devices. In development since the company’s foundation in 1995, the technology underlying the Crusoe processor solution is fundamentally software-based: the power savings come from replacing large numbers of transistors with software. The hardware component is a very long instruction word (VLIW) CPU capable of executing up to four operations in each clock cycle. The VLIW’s native instruction set bears no resemblance to the x86 instruction set; it has been designed purely for fast low-power implementation using conventional CMOS fabrication. The surrounding software layer gives x86 programs the impression that they are running on x86 hardware. The software layer is called Code Morphing software because it dynamically morphs (that is, translates) x86 instructions into VLIW instructions. The Code Morphing software includes a number of advanced features to achieve good system-level performance. Code Morphing support facilities are also built into the underlying CPUs. In other words, the Transmeta designers have judiciously rendered some functions in hardware and some in software, according to the product design goals and constraints. Transmeta’s software translates blocks of x86 instructions once, saving the resulting translation in a translation cache. The next time the (now translated) code is executed, the system skips the translation step and directly executes the existing optimized translation at full speed. This unique approach to executing x86 code eliminates millions of transistors, replacing them with software. The initial implementation of the Crusoe processor uses roughly one-quarter of the logic...

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