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SSAX

Signed Subtract and Add with Exchange exchanges the two halfwords of the second operand, performs one 16-bit integer subtraction and one 16-bit addition, and writes the results to the destination register. It sets PSTATE.GE according to the results.

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

A1

313029282726252423222120191817161514131211109876543210
!= 111101100001RnRd(1)(1)(1)(1)0101Rm
cond

A1

SSAX{<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
111110101110Rn1111Rd0000Rm

T1

SSAX{<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();
    sum  = SInt(R[n]<15:0>) + SInt(R[m]<31:16>);
    diff = SInt(R[n]<31:16>) - SInt(R[m]<15:0>);
    R[d]<15:0>  = sum<15:0>;
    R[d]<31:16> = diff<15:0>;
    PSTATE.GE<1:0> = if sum  >= 0 then '11' else '00';
    PSTATE.GE<3:2> = if diff >= 0 then '11' else '00';

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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