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Reverse Subtract with Carry.


RSC{S}{cond} {Rd}, Rn, Operand2



is an optional suffix. If S is specified, the condition flags are updated on the result of the operation.


is an optional condition code.


is the destination register.


is the register holding the first operand.


is a flexible second operand.


The RSC instruction subtracts the value in Rn from the value of Operand2. If the carry flag is clear, the result is reduced by one.

You can use RSC to synthesize multiword arithmetic.

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

RSC is not available in Thumb code.

Use of PC and SP

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

Use of PC for any operand in RSC 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 in RSC instructions is deprecated.

Note The deprecation of SP and PC is only in ARMv6T2 and above.

Condition flags

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

Correct example

    RSCSLE  r0,r5,r0,LSL r4     ; conditional, flags set

Incorrect example

    RSCSLE  r0,pc,r0,LSL r4    ; PC not permitted with register 
                               ; controlled shift