5. x86 ADD Instruction Opcode

Bit number zero marked s specifies the size of the operands the ADD instruction operates upon:

• If s = 0 then the operands are 8-bit registers and memory locations.

• If s = 1 then the operands are either 16-bits or 32-bits:

  • Under 32-bit operating systems the default is 32-bit operands if s = 1.

  • To specify a 16-bit operand (under Windows or Linux) you must insert a special operand-size prefix byte in front of the instruction (example of this later.)

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You'll soon see that this direction bit d creates a problem that results in one instruction have two different possible opcodes.

x86 ADD instruction opcode :

Bit number one, marked d, specifies the direction of the data transfer:

• If d = 0 then the destination operand is a memory location, e.g.

          add [ebx], al
  

• If d = 1 then the destination operand is a register, e.g.

          add al, [ebx]