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SXTAB16

Signed Extend and Add Byte 16 extracts two 8-bit values from a register, sign-extends them to 16 bits each, adds the results to two 16-bit values from another register, and writes the final results to the destination register. The instruction can specify a rotation by 0, 8, 16, or 24 bits before extracting the 8-bit values.

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

A1

313029282726252423222120191817161514131211109876543210
!= 111101101000!= 1111Rdrotate(0)(0)0111Rm
condRn

A1

SXTAB16{<c>}{<q>} {<Rd>,} <Rn>, <Rm> {, ROR #<amount>}

if Rn == '1111' then SEE "SXTB16";
d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);  rotation = UInt(rotate:'000');
if d == 15 || m == 15 then UNPREDICTABLE;

T1

15141312111098765432101514131211109876543210
111110100010!= 11111111Rd1(0)rotateRm
Rn

T1

SXTAB16{<c>}{<q>} {<Rd>,} <Rn>, <Rm> {, ROR #<amount>}

if Rn == '1111' then SEE "SXTB16";
d = UInt(Rd);  n = UInt(Rn);  m = UInt(Rm);  rotation = UInt(rotate:'000');
if d == 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.

<amount> Is the rotate amount, encoded in rotate:
rotate <amount>
00 (omitted)
01 8
10 16
11 24

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    rotated = ROR(R[m], rotation);
    R[d]<15:0>  = R[n]<15:0> + SignExtend(rotated<7:0>, 16);
    R[d]<31:16> = R[n]<31:16> + SignExtend(rotated<23:16>, 16);

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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