Operation

The RSB instruction subtracts the value in Rn from the value of Operand2. This is useful because of the wide range of options for Operand2.

In certain circumstances, the assembler can substitute one instruction for another. Be aware of this when reading disassembly listings.

Use of PC and SP in T32 instructions

You cannot use PC (R15) for Rd or any operand.

You cannot use SP (R13) for Rd or any operand.

Use of PC and SP in A32 instructions

You cannot use PC for Rd or any operand in an RSB instruction that has a register-controlled shift.

Use of PC for any operand, in instructions without register-controlled shift, is deprecated.

If you use PC (R15) as Rn or Rm, the value used is the address of the instruction plus 8.

If you use PC as Rd:

• Execution branches to the address corresponding to the result.
• If you use the S suffix, see the SUBS pc,lr instruction.

Use of SP and PC in A32 instructions is deprecated.

Condition flags

If S is specified, the RSB instruction updates the N, Z, C and V flags according to the result.

16-bit instructions

The following forms of this instruction are available in T32 code, and are 16-bit instructions:

RSBS Rd, Rn, #0

Rd and Rn must both be Lo registers. This form can only be used outside an IT block.

RSB{cond} Rd, Rn, #0

Rd and Rn must both be Lo registers. This form can only be used inside an IT block.

Example

    RSB     r4, r4, #1280       ; subtracts contents of R4 from 1280