x64-mul-and-div.md

Implicit Operands

Some instructions act on registers or even memory locations that are not stated in a list of operands. These instruction does do in fact have operands, but they represent assumptions made by the instruction. Implicit operands dose not change and cannot be changed. Most of the instruction that have implicit opeands dose have explicit operands as well.

MUL

The MUL instruction multiplies two values and returns the product. However, multiplication has a special problem: It generates output values that are often hugely lager than the input values. think of 0xffffffff * 0xffffffff. So to solve this problem the MUL instructison uses an implicit operands to store the product: by using two registers to hold our product.

Note

The implicit operands for MUL instruction depend on the explicit operand (size)

Instruction	Explicit operand (a.k.a. factor 1)	Implicit operand (a.k.a factor 2)	Implicit operand (a.k.a product)
MUL r/m8	r/m8	AL	AX
MUL r/m16	r/m16	AX	DX:AX
MUL r/m32	r/m32	EAX	EDX:EAX
MUL r/m64	r/m64	RAX	RDX:RAX

The first factor is given in the single explicit operand, which can be either in a register or in a memory location. The second foctor is implicit and always in the A general-purpose register appropriate to the size of the first operand.

Note

DX registers are drafted to hold the high-ordr portion of the product. I.e. if you multiply two 16-bit values and the product is 0x2A46FH, register AX will contain 0x456FH, and DX will contain 0x2AH.

Note

Even in cases when the product is small enough to fit in entirely in the first emplicit operand(A GPR), the high-orrder register whether AH, DX, EDX, or RDX) is zerod out. So you can't use them even if you'r sure that MUL instruction will not need them.

Examples

  mov bl, 21
  mov al, 2
  mul bl ; al = 0x2a

Note

You cannot use the immediate vales as an operand.

The MUL instruction will sets the Carry flag CF when the value of the product overflowss the low-order register.so you can ignore the high-order register when CF isn't set.

  mov eax, 0ffff_ffffh
  mov ebx, 03b72h
  mul ebx ; OF will set

DIV

The DIV instruction divides one unsigned value by another and gives you a quotient and a remainder. In division, you don't have the problem that multiplication has, of generating largo output value. However, it would be useful to be able to divide very large numbers, so we use the same trick as MUL but in reverse.

Instruction	Explicit operand(a.k.a. Divisor)	Implicit operand(a.k.a. Dividend)	Result(Quotient)	Result(Remainder)
DIV r/m8	r/m8	AX	AL	AH
DIV r/m16	r/m16	DX:AX	AX	DX
DIV r/m32	r/m32	EDX:EAX	EAX	EDX
DIV r/m64	r/m64	RDX:RAX	RAX	RDX

Note

DIV instruction dose not place any useful data in any of the flags. And it will leave OF, SF, ZF, AF , PF, AND CF in undifined statate.

Examples

  mov rax, 327
  mov rbx, 2
  div rbx ; sets RAX to 0xA3, and RDX to 0x1

Warning

If you endup divideng by zero, the kernel will send a SIGFPE signel to your process and termnate it.

  mov rax, 525
  mov rbx, 0
  mul rbx