MLA

MLA

Instruction	MLA
Function	Multiply Accumulate
Category	Multiply
ARM family	All (except ARMv1)
Notes	-

MLA : Multiply Accumulate

MLA is used to multiply registers (signed or unsigned) to create a 32 bit result, which is then added to another value, which produces the final result.
Operand 2 is multiplied by operand 3, the value of operand 4 is added, and the result is stored in operand 1. All operands are registers.

If the S bit is specified, the flags are N, Z, and C flags are updated as follows: N is the result bit 31, Z is set if the result is zero, and C is undefined. V is not updated.

There is also a Multiply instruction for basic multiplication.

Syntax

  MLA  <op 1>, <op 2>, <op 3>, <op 4>

Function

  op_1 = (op_2 × op_3) + op_4

Example

  MOV    R1, #5
  MOV    R2, #3
  MOV    R3, #18
  MLA    R0, R1, R2, R3
  ; R0 now equals 33

Restrictions

• You cannot specify R15 for any of the operands.
• Having operand 1 and operand 2 be the same register is unpredicable, thus, for example, the following cannot be used:

  MLA   R0, R0, R1, R2

• MLA provides the lower 32 bits of a 64 bit product, therefore the result is the same for both signed and unsigned values.

Technical

The instruction bit pattern is as follows:

31 - 28	27 - 21	20	19 - 16	15 - 12	11 - 8	7 - 4	3 - 0
condition	0 0 0 0 0 0 1	S	op_1	op_4	op_3	1 0 0 1	op_2