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MUL, MULS

Multiply multiplies two register values. The least significant 32 bits of the result are written to the destination register. These 32 bits do not depend on whether the source register values are considered to be signed values or unsigned values.

Optionally, it can update the condition flags based on the result. In the T32 instruction set, this option is limited to only a few forms of the instruction. Use of this option adversely affects performance on many implementations.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 and T2 ) .

A1

313029282726252423222120191817161514131211109876543210
!= 11110000000SRd(0)(0)(0)(0)Rm1001Rn
cond

Flag setting (S == 1)

MULS{<c>}{<q>} <Rd>, <Rn>{, <Rm>}

Not flag setting (S == 0)

MUL{<c>}{<q>} <Rd>, <Rn>{, <Rm>}

d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);  setflags = (S == '1');
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE;

T1

1514131211109876543210
0100001101RnRdm

T1

MUL<c>{<q>} <Rdm>, <Rn>{, <Rdm>} // (Inside IT block)

MULS{<q>} <Rdm>, <Rn>{, <Rdm>} // (Outside IT block)

d = UInt(Rdm);  n = UInt(Rn);  m = UInt(Rdm);  setflags = !InITBlock();

T2

15141312111098765432101514131211109876543210
111110110000Rn1111Rd0000Rm

T2

MUL<c>.W <Rd>, <Rn>{, <Rm>} // (Inside IT block, and <Rd>, <Rn>, <Rm> can be represented in T1)

MUL{<c>}{<q>} <Rd>, <Rn>{, <Rm>}

d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);  setflags = FALSE;
if d == 15 || n == 15 || m == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rdm>

Is the second general-purpose source register holding the multiplier and the destination register, encoded in the "Rdm" field.

<Rd>

Is the general-purpose destination register, encoded in the "Rd" field.

<Rn>

Is the first general-purpose source register holding the multiplicand, encoded in the "Rn" field.

<Rm>

Is the second general-purpose source register holding the multiplier, encoded in the "Rm" field. If omitted, <Rd> is used.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    operand1 = SInt(R[n]);  // operand1 = UInt(R[n]) produces the same final results
    operand2 = SInt(R[m]);  // operand2 = UInt(R[m]) produces the same final results
    result = operand1 * operand2;
    R[d] = result<31:0>;
    if setflags then
        PSTATE.N = result<31>;
        PSTATE.Z = IsZeroBit(result<31:0>);
        // PSTATE.C, PSTATE.V unchanged

Operational information

If CPSR.DIT is 1, this instruction has passed its condition execution check, and does not use R15 as either its source or destination:

  • The execution time of this instruction is independent of:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
  • The response of this instruction to asynchronous exceptions does not vary based on:
    • The values of the data supplied in any of its registers.
    • The values of the NZCV flags.
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