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SHADD8

Signed Halving Add 8 performs four signed 8-bit integer additions, halves the results, and writes the results to the destination register.

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

A1

313029282726252423222120191817161514131211109876543210
!= 111101100011RnRd(1)(1)(1)(1)1001Rm
cond

A1

SHADD8{<c>}{<q>} {<Rd>,} <Rn>, <Rm>

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

T1

15141312111098765432101514131211109876543210
111110101000Rn1111Rd0010Rm

T1

SHADD8{<c>}{<q>} {<Rd>,} <Rn>, <Rm>

d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);
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, encoded in the "Rn" field.

<Rm>

Is the second general-purpose source register, encoded in the "Rm" field.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    sum1 = SInt(R[n]<7:0>) + SInt(R[m]<7:0>);
    sum2 = SInt(R[n]<15:8>) + SInt(R[m]<15:8>);
    sum3 = SInt(R[n]<23:16>) + SInt(R[m]<23:16>);
    sum4 = SInt(R[n]<31:24>) + SInt(R[m]<31:24>);
    R[d]<7:0>   = sum1<8:1>;
    R[d]<15:8>  = sum2<8:1>;
    R[d]<23:16> = sum3<8:1>;
    R[d]<31:24> = sum4<8:1>;

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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