Version: (using Devel) Compiler: gcc 4.2.0 OS: Linux Installed from: Compiled sources I encountered two valid x86 instructions that Valgrind could not handle: bytes: 0xdd37 dis: "fsave (108)Byte ptr DS:[edi]" bytes: 0x668d7602 dis: "lea si,Word ptr DS:[esi+02h]" I believe that Valgrind can handle these opcodes, but has a problem decoding the operands. One should be able to reproduce these bugs with a simple asm file.