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This project is a disassembler library designed for emulation or automated analysis of x86 instructions. The primary output format of the disassembly is not text-based like most disassemblers. This library disassembles instructions to a structure containing the instruction mnemonic and the operands, which are in turn broken out into their components. Instruction mnemonics and operands are represented using an enumeration. This allows easier analysis or emulation of the instruction without costly parsing or string compares.

When it is necessary to disassemble to a string to display to a user, functions are provided to easily convert the structure form of an instruction to a string.

Also provided is an assembler library that is specifically designed to aid in the creation of run-time generated code.

No dependencies

The asmx86 library does not have any external dependencies. It is a single C source file that can be included in any C or C++ project.

Open source

Released under the 2-clause BSD license. Use it anywhere, even in a commercial product, for free.

Thread-safe and allocation free

The asmx86 library is designed to be efficient. There are no dynamic allocations anywhere in the code, and it uses a minimal amount of stack space with a completely stateless design. It safe to call directly even from multiprocessor kernel code.

Fast disassembly

A benchmark of text disassembly of 10 million instructions ran over 7 times faster than Capstone Engine. Structure-based disassembly is even faster and better aligned to the needs of emulation and automated analysis, as it provides the components of an instruction with no need for extra parsing.

Disassembler API

Instruction disassembly to structure

Disassembly can be performed using one of the following functions, depending on the target operating mode of the processor:

bool Disassemble16(const uint8_t* opcode,
                   uint64_t addr,
                   size_t maxLen,
                   Instruction* result);
bool Disassemble32(const uint8_t* opcode,
                   uint64_t addr,
                   size_t maxLen,
                   Instruction* result);
bool Disassemble64(const uint8_t* opcode,
                   uint64_t addr,
                   size_t maxLen,
                   Instruction* result);

Pass the bytes of the instruction as opcode and the length of this buffer as maxLen. Pass the address of the instruction on the target as addr. The results of the disassembly will be written to the Instruction structure pointed to by result.

These functions return true if a valid instruction was disassembled, and false otherwise.

Convert structure disassembly to string

A function is also provided to convert an Instruction structure into a human readable string:

size_t FormatInstructionString(char* out,
                               size_t outMaxLen,
                               const char* fmt,
                               const uint8_t* opcode,
                               uint64_t addr,
                               const Instruction* instr);

Pass a pointer to the output buffer as out and the size of this buffer in characters as outMaxLen.

The fmt parameter contains a printf-like format string for displaying the instruction. The following format specifiers are defined:

  • %a: Shows the address of the instruction as passed in the addr parameter.
  • %b: Shows the bytes of the instruction. The opcode parameter should contain the same contents as the call to Disassemble.
  • %i: Shows the instruction mnemonic.
  • %o: Shows the operands.

Width specifiers are allowed on the format specifiers. For example, the fmt string "%8a %7i %o" might show the following output:

08041a98  mov     eax, dword [ebp+0x14]

The opcode and addr parameters may be used by the format specifiers as described above. They are ignored if none of the provided format specifiers access them.

The instr parameter should be a pointer to an Instruction structure that was the result of a call to the Disassemble APIs.

This function returns the number of characters written.

Instruction disassembly to string

Functions are provided to disassemble instructions directly to a human readable string. These functions will output the structure disassembly as well. They are effectively a combined call to the Disassemble and FormatInstructionString APIs.

size_t DisassembleToString16(char* out,
                             size_t outMaxLen,
                             const char* fmt,
                             const uint8_t* opcode,
                             uint64_t addr,
                             size_t maxLen,
                             Instruction* instr);
size_t DisassembleToString32(char* out,
                             size_t outMaxLen,
                             const char* fmt,
                             const uint8_t* opcode,
                             uint64_t addr,
                             size_t maxLen,
                             Instruction* instr);
size_t DisassembleToString64(char* out,
                             size_t outMaxLen,
                             const char* fmt,
                             const uint8_t* opcode,
                             uint64_t addr,
                             size_t maxLen,
                             Instruction* instr);

The out, outMaxLen, and fmt parameters are the same as the FormatInstructionString function, see the above documentation.

The opcode parameter is required and should contain the bytes of the instruction to disassemble, and the length of this buffer should be passed as maxLen. Pass the address of the instruction on the target as addr.

The instr parameter is required and the provided Instruction structure will be written with the results of the disassembly.

These functions return the number of characters written, or zero if the instruction is invalid.

The Instruction structure

Disassembly results are typically written to an Instruction structure, which is defined as follows:

struct Instruction
    InstructionOperation operation;
    InstructionOperand operands[3];
    uint32_t flags;
    SegmentRegister segment;
    size_t length;

The operation member is an enumeration which matches the name of the instruction mnemonic. See the header file for a full listing of the supported instructions.

The operands array contains the operands to the instruction. Each operand is a structure and is defined below.

The flags member is a bit field that may contain one of more of the following flags:

  • X86_FLAG_LOCK: The lock prefix was provided to this instruction.
  • X86_FLAG_REP: This instruction is an unconditional repeated string instruction.
  • X86_FLAG_REPNE: The repeated string instruction is conditional and uses the REPNE prefix.
  • X86_FLAG_REPE: The repeated string instruction is conditional and uses the REPE prefix.
  • X86_FLAG_ANY_REP: The instruction is any repeated string instruction.
  • X86_FLAG_OPSIZE: The operand size prefix was used.
  • X86_FLAG_ADDRSIZE: The address size prefix was used.
  • X86_FLAG_INSUFFICIENT_LENGTH: The instruction may be valid but an insufficient number of bytes were provided. The Disassemble function will return false when this flag is set.

The segment member contains the segment prefix, if any. This will be either SEG_DEFAULT or a segment register (e.g. SEG_ES).

The length member contains the length of the instruction in bytes. This can be used to continue disassembling at the next instruction. Be sure to check the return value of Disassemble as an invalid instruction may leave a zero here.

Each operand is described by the structure below:

struct InstructionOperand
    OperandType operand;
    OperandType components[2];
    uint8_t scale;
    uint16_t size;
    int64_t immediate;
    SegmentRegister segment;

The operand member is an enumeration that contains the operand. This may be a register, or one of the following special enumeration members:

  • NONE: The operand is invalid.
  • IMM: The operand is a constant integer. See the immediate member.
  • MEM: The operand is a memory reference.

If the operand member is NONE, none of the other members are defined. Otherwise, the size member contains the size in bytes of the operand. If the operand member is IMM, the immediate member is also defined. If the operand member is MEM, all other members are defined as below.

The components member contains the address components of a memory operand. The first element is never scaled, and the second element is multiplied by the scale member. Any element can be NONE, which can be represented as a constant zero. If the element is not NONE, it will be a register.

The size member contains the size of the operand in bytes. For register operands, this is the size of the register. For memory operands, this is the size of the memory access.

The immediate member contains the value of a constant integer when the operand is the IMM type. When the operand is a memory reference, the immediate member contains a constant offset for the memory reference.

The segment member contains the segment register that will be used for a memory access. This will always contain a segment register, as SEG_DEFAULT is resolved to the default register.

For memory references, the address of the memory reference is effectively the following formula, where references to the component array should look up the current value of the register or substitute zero if the value is NONE:

address = components[0] + components[1] * scale + immediate

Assembler API

The asmx86 library also provides an assembler library for emitting run-time generated code. It is designed to emit machine code using an easy-to-read API without going through any kind of string parsing. The compiled code is very close to the performance of writing machine code manually into a buffer.

The following is a simple example of how to write code into a buffer:

uint8_t* code = allocate();
code += X86_EMIT64_R(code, push, REG_RBP);
code += X86_EMIT64_RR(code, mov_64, REG_RBP, REG_RSP);
code += X86_EMIT64_RI(code, sub_64, REG_RSP, 0x8);
code += X86_EMIT64_MR(code, mov_32, X86_MEM(REG_RBP, -0x8), REG_EDI);
code += X86_EMIT64_MR(code, mov_32, X86_MEM(REG_RBP, -0x4), REG_ESI);
code += X86_EMIT64_RR(code, xor_32, REG_EAX, REG_EAX);
code += X86_EMIT64_RM(code, mov_32, REG_ECX, X86_MEM(REG_RBP, -0x8));
code += X86_EMIT64_RM(code, mov_32, REG_EDX, X86_MEM(REG_RBP, -0x4));
X86_MARK_JUMP_LABEL_64(code, loop);
code += X86_EMIT64_RR(code, add_32, REG_EAX, REG_EDX);
code += X86_EMIT64_R(code, inc_32, REG_EDX);
code += X86_EMIT64_R(code, dec_32, REG_ECX);
code += X86_EMIT64_T(code, jnz, loop);
code += X86_EMIT64_RR(code, mov_64, REG_RSP, REG_RBP);
code += X86_EMIT64_R(code, pop, REG_RBP);
code += X86_EMIT64(code, retn);

This emits a function to a buffer, which can then be executed by the processor. Often it is not possible to know the maximum size of the code before emitting it, so another set of APIs is available to allow dynamic allocation of buffer space as it is needed.

These APIs are prefixed by DYNALLOC, for example:

X86_DYNALLOC_EMIT64_R(buffer, AllocSpaceForInstr, AdvanceForInstr,
                      push, REG_RBP);

The AllocSpaceForInstr and AdvanceForInstr parameters are to be provided by the caller, and manage the buffer provided. The buffer passed can be any type of object (including a C++ class instance), which the functions provided can use as context to manage the buffer.

The AllocSpaceForInstr function should have the following prototype (the name can be chosen by the caller):

uint8_t* AllocSpaceForInstr(void* buffer, size_t length);

This function will be called with the buffer parameter equal to the EMIT call, and the length of the instruction that is to be written. This function can perform any buffer management necessary, such as allocating extra space and emitting an unconditional jump instruction to the new buffer space. It should return a pointer to where the instruction should be written, and must have at least length bytes available for writing. The actual instruction written may not be as long as the length given, as some instruction lengths are dependent on the address of the instruction. It will never be longer than the length provided.

The AdvanceForInstr function should have the following prototype:

void AdvanceForInstr(void* buffer, size_t length);

This function will be called after an instruction is written to the buffer. The buffer parameter will be equal to the one provided in the EMIT call, and the length parameter is the actual length of the instruction written (which may be less than the length parameter passed to the AllocSpaceForInstr function). The buffer management function can perform any updates necessary to mark the instruction as written and ensure that the next instruction will be written to the correct location following the one written.

Instructions that have been written to buffers should not be moved in any way until all labels have been resolved. The label API internally keeps pointers to the instructions until they are resolved.

It is recommended to define a set of pass-through macros that provide the necessary functions to the API automatically, such that the caller can write code like this:

CodeBufferObject buffer;
EMIT_RR(&buffer, mov_64, REG_RAX, REG_RDI);
EMIT_RR(&buffer, add_64, REG_RAX, REG_RSI);
EMIT(&buffer, retn);

This API is not provided by asmx86, as it is dependent on the host system's memory management system, but demonstrates the intended use of the DYNALLOC set of APIs. The EMIT_RR and EMIT macros shown above should be written to resolve to something like the following:

                       mov_64, REG_RAX, REG_RDI);
                       add_64, REG_RAX, REG_RSI);