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SMLABB, SMLABT, SMLATB, SMLATT

Signed Multiply Accumulate (halfwords) performs a signed multiply accumulate operation. The multiply acts on two signed 16-bit quantities, taken from either the bottom or the top half of their respective source registers. The other halves of these source registers are ignored. The 32-bit product is added to a 32-bit accumulate value and the result is written to the destination register.

If overflow occurs during the addition of the accumulate value, the instruction sets PSTATE.Q to 1. It is not possible for overflow to occur during the multiplication.

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

A1

313029282726252423222120191817161514131211109876543210
!= 111100010000RdRaRm1MN0Rn
cond

SMLABB (M == 0 && N == 0)

SMLABB{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

SMLABT (M == 1 && N == 0)

SMLABT{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

SMLATB (M == 0 && N == 1)

SMLATB{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

SMLATT (M == 1 && N == 1)

SMLATT{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);  a = UInt(Ra);
n_high = (N == '1');  m_high = (M == '1');
if d == 15 || n == 15 || m == 15 || a == 15 then UNPREDICTABLE;

T1

15141312111098765432101514131211109876543210
111110110001Rn!= 1111Rd00NMRm
Ra

SMLABB (N == 0 && M == 0)

SMLABB{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

SMLABT (N == 0 && M == 1)

SMLABT{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

SMLATB (N == 1 && M == 0)

SMLATB{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

SMLATT (N == 1 && M == 1)

SMLATT{<c>}{<q>} <Rd>, <Rn>, <Rm>, <Ra>

if Ra == '1111' then SEE "SMULBB, SMULBT, SMULTB, SMULTT";
d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);  a = UInt(Ra);
n_high = (N == '1');  m_high = (M == '1');
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.

<Rd>

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

<Rn>

Is the first general-purpose source register holding the multiplicand in the bottom or top half (selected by <x>), encoded in the "Rn" field.

<Rm>

Is the second general-purpose source register holding the multiplier in the bottom or top half (selected by <y>), encoded in the "Rm" field.

<Ra>

Is the third general-purpose source register holding the addend, encoded in the "Ra" field.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    operand1 = if n_high then R[n]<31:16> else R[n]<15:0>;
    operand2 = if m_high then R[m]<31:16> else R[m]<15:0>;
    result = SInt(operand1) * SInt(operand2) + SInt(R[a]);
    R[d] = result<31:0>;
    if result != SInt(result<31:0>) then  // Signed overflow
        PSTATE.Q = '1';
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