Читать книгу Microprocessor 4 - Philippe Darche - Страница 15

In order to limit the programs' memory footprint for reasons of cost, memory size, performance or, in particular, power saving, one solution is to compress the machine code at compilation and its decompression at execution, for example, when it is loaded in the MPU cache memory (Wolfe and Chanin 1992). One benefit lies in the fact that the compiler has not been modified. For implementation, the Huffman (1952) (de)compression algorithm can be used, for example. Because of its objectives, it is intended especially for embedded systems with an MPU/MCU⁵ RISC. Two industrial examples are Thumb^® and Thumb-2 for which the 16-bit instruction word is a compression of the classic version of Arm® processors, which have a 32-bit format. RISC-V (Waterman 2016) has a compressed version of its code suggested by (Waterman 2011). A comparison between MPUs can be made using a measurement of the code density.

The principle can quite clearly be applied to data and to buses (cf. V2) for the same aims.