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A fast and lightweight decoder for x86 and x86-64.
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Fadec — Fast Decoder for x86-32 and x86-64

Fadec is a fast and lightweight decoder for x86-32 and x86-64. To meet the goal of speed, lookup tables are used to map the opcode the (internal) description of the instruction encoding. This table currently has a size of roughly 19.5 kiB (for 32/64-bit combined).

Note: This is not a disassembler, it does not intend to produce valid assembly.

Key features

Q: Why not just just use any other decoder available out there?

A: Because I needed to embed a small and fast decoder in a project which didn't link against a libc.

  • Small size: the compiled library uses only 40 kiB and the main decode routine is only a few hundreds lines of code.
  • Performance: Fadec is significantly faster than libopcodes or Capstone due to the absence of high-level abstractions and the small lookup table.
  • Almost no dependencies: the formatter only uses the function snprintf, the decoder itself has no dependencies, making it suitable for environments without a full libc or malloc-style memory allocation.
  • Correctness: even corner cases should be handled correctly (if not, that's a bug), e.g., the order of prefixes, the presence of the lock prefix, or properly handling VEX.W in 32-bit mode.

Basic Usage

FdInstr instr;
// Decode from buffer in 64-bit mode and virtual address 0x401000
int ret = fd_decode(buffer, sizeof(buffer), 64, 0x401000, &instr);
// ret<0 indicates an error, ret>0 the number of decoded bytes
// Relevant properties of instructions can now be queries using the FD_* macros.


The API consists of two functions to decode and format instructions, as well as several accessor macros (see fadec.h). Direct access of the FdInstr structure is not recommended.

  • int fd_decode(const uint8_t* buf, size_t len, int mode, uintptr_t address, FdInstr* out_instr)
    • Decode a single instruction
    • Return value: number of bytes used, or a negative value in case of an error.
    • buf/len: buffer containing instruction bytes. At most 15 bytes will be read. If the instruction is longer than len, an error value is returned.
    • mode: architecture mode, either 32 or 64.
    • address: virtual address of the decoded instruction. This is used for computing jump targets and segment-offset-relative memory operations (MOV with moffs* encoding) and stored in the instruction.
    • out_instr: Pointer to the instruction buffer, might get written partially in case of an error.
  • void fd_format(const FdInstr* instr, char* buf, size_t len)
    • Format a single instruction to a human-readable format.
    • instr: decoded instruction.
    • buf/len: buffer for formatted instruction string

Intended differences to other decoders

To achieve higher performance, minor differences to other decoders exist, requiring special handling.

  • The decoded operand sizes are not always exact. However, the exact size can be reconstructed in all cases.
    • For instructions with rare memory access sizes (e.g. lgdt), the provided size is zero. These are: cmpxchg16b, cmpxchg8b, fbld (for 80-bit), fbstp (for 80-bit), fldenv, frstor, fsave, fstenv, fstp (for 80-bit), fxrstor, fxsave, lds, lds, lgdt, lidt, lldt, ltr, sgdt, sidt, sldt, str
    • For some SSE/AVX instructions, the operand size is an over-approximation of the real size, e.g. for permutations or extensions.
    • The operand size of segment and FPU registers is always zero.
  • An implicit fwait in FPU instructions is decoded as a separate instruction (matching the opcode layout in machine code). For example:
    • finit is decoded as FD_FWAIT + FD_FINIT
    • fninit is decoded as plain FD_FINIT
  • For scas and cmps, the repz prefix can be queried using FD_HAS_REP (matching prefix byte in machine code).

Known issues

  • The EVEX prefix (AVX-512) is not supported (yet).
  • The layout of entries in the tables can be improved to improve usage of caches. (Help needed.)
  • No Python API.
  • Low test coverage. (Help needed.)
  • No benchmarking has been performed yet. (Help needed.)
  • Prefixes for indirect jumps and calls are not properly decoded, e.g. notrack, bnd. This requires additional information on the prefix ordering, which is currently not decoded. (Analysis of performance impact and help needed.)

If you find any other issues, please report a bug. Or, even better, send a patch fixing the issue.

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