11.179 USAX

Unsigned parallel subtract and add halfwords with exchange.

Syntax

USAX{cond} {Rd}, Rn, Rm

where:

cond

is an optional condition code.

Rd

is the destination register.

Rm, Rn

are the ARM registers holding the operands.

Operation

This instruction exchanges the two halfwords of the second operand, then performs a subtraction on the two top halfwords of the operands and an addition on the bottom two halfwords. It writes the results into the corresponding halfwords of the destination. The results are modulo 216. It sets the APSR GE flags.

Register restrictions

You cannot use PC for any register.

You can use SP in ARM instructions but this is deprecated in ARMv6T2 and above. You cannot use SP in Thumb instructions.

GE flags

This instruction does not affect the N, Z, C, V, or Q flags.

It sets the GE flags in the APSR as follows:

GE[1:0]

for bits[15:0] of the result.

GE[3:2]

for bits[31:16] of the result.

It sets GE[1:0] to 1 to indicate that the addition overflowed, generating a carry. This is equivalent to an ADDS instruction setting the C condition flag to 1.

It sets GE[3:2] to 1 to indicate that the subtraction gave a result greater than or equal to zero, meaning a borrow did not occur. This is equivalent to a SUBS instruction setting the C condition flag to 1.

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

Note:

GE[1:0] are set or cleared together, and GE[3:2] are set or cleared together.

Architectures

This ARM instruction is available in ARMv6 and above.

This 32-bit Thumb instruction is available in ARMv6T2 and above. For the ARMv7-M architecture, it is only available in an ARMv7E-M implementation.

There is no 16-bit version of this instruction in Thumb.

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