x86 Instructions

In the lists in this section, instructions marked with an asterisk (*) are particularly important. Instructions not so marked are not critical.

On the x86 processor, instructions are variable-sized, so disassembling backward is an exercise in pattern matching. To disassemble backward from an address, you should start disassembling at a point further back than you really want to go, then look forward until the instructions start making sense. The first few instructions may not make any sense because you may have started disassembling in the middle of an instruction. There is a possibility, unfortunately, that the disassembly will never synchronize with the instruction stream and you will have to try disassembling at a different starting point until you find a starting point that works.

For well-packed switch statements, the compiler emits data directly into the code stream, so disassembling through a switch statement will usually stumble across instructions that make no sense (because they are really data). Find the end of the data and continue disassembling there.

Instruction Notation

The general notation for instructions is to put the destination register on the left and the source on the right. However, there can be some exceptions to this rule.

Arithmetic instructions are typically two-register with the source and destination registers combining. The result is stored into the destination.

Some of the instructions have both 16-bit and 32-bit versions, but only the 32-bit versions are listed here. Not listed here are floating-point instructions, privileged instructions, and instructions that are used only in segmented models (which Microsoft Win32 does not use).

To save space, many of the instructions are expressed in combined form, as shown in the following example.

means that the first parameter must be a register, but the second can be a register, a memory reference, or an immediate value.

To save even more space, instructions can also be expressed as shown in the following.

which means that the first parameter can be a register or a memory reference, and the second can be a register, memory reference, or immediate value.

Unless otherwise noted, when this abbreviation is used, you cannot choose memory for both source and destination.

Furthermore, a bit-size suffix (8, 16, 32) can be appended to the source or destination to indicate that the parameter must be of that size. For example, r8 means an 8-bit register.

Memory, Data Transfer, and Data Conversion

Memory and data transfer instructions do not affect flags.

Effective Address

For example, LEA eax, [esi+4] means eax = esi + 4. This instruction is often used to perform arithmetic.

Data Transfer

MOVSX and MOVZX are special versions of the mov instruction that perform sign extension or zero extension from the source to the destination. This is the only instruction that allows the source and destination to be different sizes. (And in fact, they must be different sizes.

Stack Manipulation

The stack is pointed to by the esp register. The value at esp is the top of the stack (most recently pushed, first to be popped); older stack elements reside at higher addresses.

The C/C++ compiler does not use the enter instruction. (The enter instruction is used to implement nested procedures in languages like Algol or Pascal.)

The leave instruction is equivalent to:

mov esp, ebp
pop ebp

Data Conversion

All conversions perform sign extension.

Arithmetic and Bit Manipulation

All arithmetic and bit manipulation instructions modify flags.

Arithmetic

The cmp instruction computes the subtraction and sets flags according to the result, but throws the result away. It is typically followed by a conditional jump instruction that tests the result of the subtraction.

Unsigned and signed multiplication. The state of flags after multiplication is undefined.

Unsigned and signed division. The first register in the pseudocode explanation receives the remainder and the second receives the quotient. If the result overflows the destination, a division overflow exception is generated.

The state of flags after division is undefined.

If the condition cc is true, then the 8-bit value is set to 1. Otherwise, the 8-bit value is set to zero.

Binary-coded Decimal

You will not see these instructions unless you are debugging code written in COBOL.

These instructions adjust the al register after performing a packed binary-coded decimal operation.

These instructions adjust the al register after performing an unpacked binary-coded decimal operation.

These instructions adjust the al and ah registers after performing an unpacked binary-coded decimal operation.

Bits

The test instruction computes the logical AND operator and sets flags according to the result, but throws the result away. It is typically followed by a conditional jump instruction that tests the result of the logical AND.

The last bit shifted out is placed in the carry.

Shift r1 left by cl/#n, filling with the top bits of r2/m. The last bit shifted out is placed in the carry.

Shift r1 right by cl/#n, filling with the bottom bits of r2/m. The last bit shifted out is placed in the carry.

Rotation is like shifting, except that the bits that are shifted out reappear as the incoming fill bits. The C-language version of the rotation instructions incorporate the carry bit into the rotation.

Control Flow

The call instruction pushes the return address onto the stack then jumps to the destination.

The ret instruction pops and jumps to the return address on the stack. A nonzero #n in the RET instruction indicates that after popping the return address, the value #n should be added to the stack pointer.

These instructions are remnants of the x86's CISC heritage and in recent processors are actually slower than the equivalent instructions written out the long way.

String Manipulation

After performing the operation, the source and destination register are incremented or decremented by sizeof(T), according to the setting of the direction flag (up or down).

The instruction can be prefixed by REP to repeat the operation the number of times specified by the ecx register.

The rep mov instruction is used to copy blocks of memory.

The rep stos instruction is used to fill a block of memory with accT.

Flags

Interlocked Instructions

The cmpxchg instruction is the atomic version of the following:

   cmp     accT, r/m
   jz      match
   mov     accT, r/m
   jmp     done
match:
   mov     r/m, r1
done:

Miscellaneous

Here is a special case of the int instruction.

The opcode for INT 3 is 0xCC. The opcode for NOP is 0x90.

When debugging code, you may need to patch out some code. You can do this by replacing the offending bytes with 0x90.

Idioms