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 operand.
You can use SP in A32 instructions but this is deprecated. You cannot use SP in T32 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.Availability
The 32-bit instruction is available in A32 and T32.
For the ARMv7-M architecture, the 32-bit T32 instruction is only available in an ARMv7E-M implementation.
There is no 16-bit version of this instruction in T32.