RSB

RSB

Instruction	RSB[S]
Function	Subtraction
Category	Data processing
ARM family	All
Notes	-

RSB[S] : Reverse Subtract

RSB will subtract one value from another. The difference between this and SUB is that operand 2 is now the value being subtracted from. This is explained more below.

Operand 1 is a register, operand 2 can be a register, shifted register, or an immediate value (which may be shifted).

If the S bit is set (RSBS), the N and Z flags are set according to the result, and the C and V flags are set as follows: C if the result generated a borrow (a not carry; unsigned underflow); V if the result generated a signed overflow.

Because operand 2 is able to be a register or immediate with shifting, you can do things such as negation (0 - Op1) or various types of fast multiplication.

There is also RSC to perform subtraction with the Carry flag considered.

Syntax

  RSB<suffix>  <dest>, <op 1>, <op 2>

Function

  dest = op_2 - op_1

Example

   RSB R0, R1, R1, LSL #4    ; R0 = (R1 << 2) - R1
                             ; or in other words: R0 = (R1 * 15)

Technical

The instruction bit pattern is as follows:

31 - 28	27	26	25	24 - 21	20	19 - 16	15 - 12	11 - 0
condition	0	0	I	0 0 1 1	S	op_1	dest	op_2/shift