Most of the existing work on verified compilation leaves unverified the translation of assembly programs into binary code in object file formats (e.g., the Executable and Linkable Format or ELF). The challenges of developing verified assemblers come from the intrinsic complexities in low-level assembling processes caused by the need to support different computer architectures and their details, such as encoding a large number of instructions and verifying its correctness. We present a framework that overcomes the above challenges. It works as a template which may be instantiated to generate verified assemblers for different architectures targeting ELF object files. For this, it is parameterized over the implementation and verification of architecture-dependent assembling processes through well-defined interfaces. By plugging the architecture-dependent parts into the template, we get complete verified assemblers. To manage the complexity in developing and verifying encoding of instructions, we integrate into our framework the CSLED framework for automatically generating verified instruction encoders and decoders from declarative instruction specifications. To show the effectiveness of our framework, we have applied it to generate verified assemblers for the complete X86 and RISC-V assembly languages in CompCert.