4.6.1. Parallel add and subtract

Various byte-wise and halfword-wise additions and subtractions.

Syntax

<prefix>op{cond} {Rd}, Rn, Rm

where:

<prefix>

is one of:

S

Signed arithmetic modulo 28 or 216. Sets APSR GE flags.

Q

Signed saturating arithmetic.

SH

Signed arithmetic, halving the results.

U

Unsigned arithmetic modulo 28 or 216. Sets APSR GE flags.

UQ

Unsigned saturating arithmetic.

UH

Unsigned arithmetic, halving the results.

op

is one of:

ADD8

Byte-wise Addition

ADD16

Halfword-wise Addition.

SUB8

Byte-wise Subtraction.

SUB16

Halfword-wise Subtraction.

ASX

Exchange halfwords of Rm, then Add top halfwords and Subtract bottom halfwords.

SAX

Exchange halfwords of Rm, then Subtract top halfwords and Add bottom halfwords.

cond

is an optional condition code (see Conditional execution).

Rd

is the destination register. Do not use r15 for Rd.

Rm, Rn

are the ARM registers holding the operands. Do not use r15 for Rm or Rn.

Operation

These instructions perform arithmetic operations separately on the bytes or halfwords of the operands. They perform two or four additions or subtractions, or one addition and one subtraction.

You can choose various kinds of arithmetic:

  • Signed or unsigned arithmetic modulo 28 or 216. This sets the APSR GE flags (see Condition flags).

  • Signed saturating arithmetic to one of the signed ranges -215x ≤ 215 -1 or -27x ≤ 27 -1. The Q flag is not affected even if these operations saturate.

  • Unsigned saturating arithmetic to one of the unsigned ranges 0 ≤ x ≤ 216 -1 or 0 ≤ x ≤ 28 -1. The Q flag is not affected even if these operations saturate.

  • Signed or unsigned arithmetic, halving the results. This cannot cause overflow.

Condition flags

These instructions do not affect the N, Z, C, V, or Q flags.

The Q, SH, UQ and UH prefix variants of these instructions do not change the flags.

The S and U prefix variants of these instructions set the GE flags in the APSR as follows:

  • For byte-wise operations, the GE flags are used in the same way as the C (Carry) flag for 32-bit SUB and ADD instructions:

    GE[0]

    for bits[7:0] of the result

    GE[1]

    for bits[15:8] of the result

    GE[2]

    for bits[23:16] of the result

    GE[3]

    for bits[31:24] of the result.

  • For halfword-wise operations, the GE flags are used in the same way as the C (Carry) flag for normal word-wise SUB and ADD instructions:

    GE[1:0]

    for bits[15:0] of the result

    GE[3:2]

    for bits[31:16] of the result.

You can use these flags to control a following SEL instruction, see SEL.

Note

For halfword-wise operations, GE[1:0] are set or cleared together, and GE[3:2] are set or cleared together.

Architectures

These ARM instructions are available in ARMv6 and above.

These 32-bit Thumb instructions are available in ARMv6T2 and above, except the ARMv7-M profile.

There are no 16-bit Thumb versions of these instructions.

Examples

    SHADD8      r4, r3, r9
    USAXNE      r0, r0, r2

Incorrect examples

    QHADD       r2, r9, r3    ; No such instruction, should be QHADD8 or QHADD16
    SAX         r10, r8, r5   ; Must have a prefix.
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