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SMULL, SMULLS

Signed Multiply Long multiplies two 32-bit signed values to produce a 64-bit result.

In A32 instructions, the condition flags can optionally be updated based on the result. Use of this option adversely affects performance on many implementations.

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

A1

313029282726252423222120191817161514131211109876543210
!= 11110000110SRdHiRdLoRm1001Rn
cond

Flag setting (S == 1)

SMULLS{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

Not flag setting (S == 0)

SMULL{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

dLo = UInt(RdLo);  dHi = UInt(RdHi);  n = UInt(Rn);  m = UInt(Rm);  setflags = (S == '1');
if dLo == 15 || dHi == 15 || n == 15 || m == 15 then UNPREDICTABLE;
if dHi == dLo then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If dHi == dLo, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The value in the destination register is unknown.

T1

15141312111098765432101514131211109876543210
111110111000RnRdLoRdHi0000Rm

T1

SMULL{<c>}{<q>} <RdLo>, <RdHi>, <Rn>, <Rm>

dLo = UInt(RdLo);  dHi = UInt(RdHi);  n = UInt(Rn);  m = UInt(Rm);  setflags = FALSE;
if dLo == 15 || dHi == 15 || n == 15 || m == 15 then UNPREDICTABLE;
// Armv8-A removes UNPREDICTABLE for R13
if dHi == dLo then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

If dHi == dLo, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The value in the destination register is unknown.

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.

<RdLo>

Is the general-purpose destination register for the lower 32 bits of the result, encoded in the "RdLo" field.

<RdHi>

Is the general-purpose destination register for the upper 32 bits of the result, encoded in the "RdHi" 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.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    result = SInt(R[n]) * SInt(R[m]);
    R[dHi] = result<63:32>;
    R[dLo] = result<31:0>;
    if setflags then
        PSTATE.N = result<63>;
        PSTATE.Z = IsZeroBit(result<63:0>);
        // 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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