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REV16

Byte-Reverse Packed Halfword reverses the byte order in each16-bit halfword of a 32-bit register.

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

A1

313029282726252423222120191817161514131211109876543210
!= 111101101011(1)(1)(1)(1)Rd(1)(1)(1)(1)1011Rm
cond

A1

REV16{<c>}{<q>} <Rd>, <Rm>

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

T1

1514131211109876543210
1011101001RmRd

T1

REV16{<c>}{<q>} <Rd>, <Rm>

d = UInt(Rd);  m = UInt(Rm);

T2

15141312111098765432101514131211109876543210
111110101001Rn1111Rd1001Rm

T2

REV16{<c>}.W <Rd>, <Rm> // (<Rd>, <Rm> can be represented in T1)

REV16{<c>}{<q>} <Rd>, <Rm>

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

CONSTRAINED UNPREDICTABLE behavior

If m != n, then one of the following behaviors must occur:

  • The instruction is undefined.
  • The instruction executes as NOP.
  • The instruction executes with the additional decode: m = UInt(Rn);.
  • The instruction executes with the additional decode: m = UInt(Rm);.
  • 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.

<Rd>

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

<Rm>

For encoding A1 and T1: is the general-purpose source register, encoded in the "Rm" field.

For encoding T2: is the general-purpose source register, encoded in the "Rm" field. It must be encoded with an identical value in the "Rn" field.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    bits(32) result;
    result<31:24> = R[m]<23:16>;
    result<23:16> = R[m]<31:24>;
    result<15:8>  = R[m]<7:0>;
    result<7:0>   = R[m]<15:8>;
    R[d] = result;

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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