11.132 SSUB8

Signed parallel byte-wise subtraction.

Syntax

SSUB8{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 subtracts each byte of the second operand from the corresponding byte of the first operand and writes the results into the corresponding bytes of the destination. The results are modulo 28. 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[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.

It sets a GE flag to 1 to indicate that the corresponding result is greater than or equal to zero. This is equivalent to a SUBS instruction setting the N and V condition flags to the same value, so that the GE condition passes.

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

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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