Arm® A32/T32 Instruction Set Architecture: Armv8, for Armv8-A architecture profile Documentation

SBC, SBCS (immediate)

Subtract with Carry (immediate) subtracts an immediate value and the value of NOT (Carry flag) from a register value, and writes the result to the destination register.

If the destination register is not the PC, the SBCS 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:

• The SBC variant of the instruction is an interworking branch, see Pseudocode description of operations on the AArch32 general-purpose registers and the PC.
• The SBCS variant of the instruction performs an exception return without the use of the stack. In this case:
  • The PE branches to the address written to the PC, and restores PSTATE from SPSR_<current_mode>.
  • The PE checks SPSR_<current_mode> for an illegal return event. See Illegal return events from AArch32 state.
  • The instruction is undefined in Hyp mode.
  • The instruction is constrained unpredictable in User mode and System mode.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .

A1

d = UInt(Rd);  n = UInt(Rn);  setflags = (S == '1');  imm32 = A32ExpandImm(imm12);

T1

d = UInt(Rd);  n = UInt(Rn);  setflags = (S == '1');  imm32 = T32ExpandImm(i:imm3:imm8);
if d == 15 || n == 15 then UNPREDICTABLE; // Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

if ConditionPassed() then
    EncodingSpecificOperations();
    (result, nzcv) = AddWithCarry(R[n], NOT(imm32), PSTATE.C);
    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;

Operational information

If CPSR.DIT is 1 and this instruction does not use R15 as either its source or destination:

• The execution time of this instruction is independent of:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.
• The response of this instruction to asynchronous exceptions does not vary based on:
  • The values of the data supplied in any of its registers.
  • The values of the NZCV flags.