Résumé : On critical applications, particularly embedded systems, the performance tuning requires multiple passes. SALTO (System for Assembly Language Transformation and Optimization) is a retargetable framework for developing all the spectrum of tools that are needed for performance tuning on low-level codes (assembly-languages) on uniprocessors. SALTO enables the building of profiling, tracing and optimization tools. The user is responsible for giving a machine description of the target architecture, which includes instruction-set of the processor, precise hardware configuration and reservation-tables for all instructions, but high-level functions are provided to him for writing any tool corresponding to his needs. Moreover SALTO will be a part of a global solution for manipulating assembly-code to implement low-level code restructuration as well as to provide a high-level code restructurer with useful information collected from the assembler code and from instruction profiling. SALTO has been tested on Intel platforms running Linux (i486) and Solaris (PentiumPro) and on a Sparcstation running SunOs 4.1. A machine description for the Sparc v7 architecture is currently available. Two examples, a basic block instrumentation and a local reordering optimization, are given in the paper as illustration.