rsb

Reverse Subtract (A32)

RSB{S}<c> <Rd>, <Rn>, <Operand2>

Calculates Rd = Operand2 - Rn.

Details

Computes the reverse subtraction Rd = Operand2 − Rn and stores the result in Rd. When S=1, updates condition flags: N and Z flags set according to result, C flag set to the borrow (NOT of the borrow-out), V flag set on signed overflow. This is an A32 data-processing instruction available in all ARM implementations.

Pseudocode Operation

result ← Operand2 - Rn
Rd ← result
if S == 1 then
  N ← result[31]
  Z ← (result == 0)
  C ← NOT(Borrow)
  V ← (Operand2[31] != Rn[31]) AND (Operand2[31] != result[31])
endif

Example

RSB r0, r1, r2

Encoding

Binary Layout
cond
0010
011
0
Rn
Rd
imm12
 
Format Data Proc
Opcode 0x02600000
Extension A32 (Base)

Operands

  • Rd
    Destination general-purpose register
  • Rn
    First source / base general-purpose register
  • Operand2
    Flexible second operand (register or shifted register)

Reference (Arm AArch32 ISA)

Instruction Forms

Encoding Instruction ISA Bit pattern
0x02600000 RSB{<c>}{<q>} {<Rd>,} <Rn>, #<const> A32 cond | 0010 | 011 | 0 | Rn | Rd | imm12
0x4240 RSB<c>{<q>} {<Rd>, }<Rn>, #0 T32 010000 | 1001 | Rn | Rd
0xF1C00000 RSB<c>.W {<Rd>,} <Rn>, #0 T32 11110 | i | 0 | 1110 | 0 | Rn | 0 | imm3 | Rd | imm8
0x00600060 RSB{<c>}{<q>} {<Rd>,} <Rn>, <Rm>, RRX A32 cond | 0000 | 011 | 0 | Rn | Rd | 00000 | 11 | 0 | Rm
0x00600000 RSB{<c>}{<q>} {<Rd>,} <Rn>, <Rm> {, <shift> #<amount>} A32 cond | 0000 | 011 | 0 | Rn | Rd | imm5 | stype | 0 | Rm
0xEBC00030 RSB{<c>}{<q>} {<Rd>,} <Rn>, <Rm>, RRX T32 1110101 | 1110 | 0 | Rn | 0 | 000 | Rd | 00 | 11 | Rm
0xEBC00000 RSB{<c>}{<q>} {<Rd>,} <Rn>, <Rm> {, <shift> #<amount>} T32 1110101 | 1110 | 0 | Rn | 0 | imm3 | Rd | imm2 | stype | Rm
0x00600010 RSB{<c>}{<q>} {<Rd>,} <Rn>, <Rm>, <shift> <Rs> A32 cond | 0000 | 011 | 0 | Rn | Rd | Rs | 0 | stype | 1 | Rm

Description

Reverse Subtract (immediate) subtracts a register value from an immediate value, and writes the result to the destination register. If the destination register is not the PC, the RSBS variant of the instruction updates the condition flags based on the result. The field descriptions for <Rd> identify the encodings where the PC is permitted as the destination register. ARM deprecates any use of these encodings. However, when the destination register is the PC:

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    (result, nzcv) = AddWithCarry(NOT(R[n]), imm32, '1');
    if d == 15 then          // Can only occur for A32 encoding
        if setflags then
            ALUExceptionReturn(result);
        else
            ALUWritePC(result);
    else
        R[d] = result;
        if setflags then
            PSTATE.<N,Z,C,V> = nzcv;