RSB, RSBS (register-shifted register)

Reverse Subtract (register-shifted register) subtracts a register value from a register-shifted register value, and writes the result to the destination register. It can optionally update the condition flags based on the result.

A1

313029282726252423222120191817161514131211109876543210
!= 11110000011SRnRdRs0stype1Rm
cond

Encoding for the Flag setting variant

Applies when (S == 1)

RSBS{<c>}{<q>} {<Rd>,} <Rn>, <Rm>, <shift> <Rs>

Encoding for the Not flag setting variant

Applies when (S == 0)

RSB{<c>}{<q>} {<Rd>,} <Rn>, <Rm>, <shift> <Rs>

Decode for all variants of this encoding

constant d = UInt(Rd); constant n = UInt(Rn); constant m = UInt(Rm); constant s = UInt(Rs); constant setflags = (S == '1'); constant shift_t = DecodeRegShift(stype); if d == 15 || n == 15 || m == 15 || s == 15 then UNPREDICTABLE;

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

Assembler Symbols

<c>

See Standard assembler syntax fields.

<q>

See Standard assembler syntax fields.

<Rd>

Is the general-purpose destination register, encoded in the "Rd" field.

<Rn>

Is the first general-purpose source register, encoded in the "Rn" field.

<Rm>

Is the second general-purpose source register, encoded in the "Rm" field.

<shift>

Is the type of shift to be applied to the second source register, encoded in stype:

stype <shift>
00 LSL
01 LSR
10 ASR
11 ROR
<Rs>

Is the third general-purpose source register holding a shift amount in its bottom 8 bits, encoded in the "Rs" field.

Operation

if ConditionPassed() then EncodingSpecificOperations(); constant shift_n = UInt(R[s]<7:0>); constant shifted = Shift(R[m], shift_t, shift_n, PSTATE.C); constant (result, nzcv) = AddWithCarry(NOT(R[n]), shifted, '1'); R[d] = result; if setflags then PSTATE.<N,Z,C,V> = nzcv;

Operational information

If CPSR.DIT is 1, this instruction has passed its condition execution check, and does not use R15 as either its source or destination:


Internal version only: isa v01_32, pseudocode v2024-12_rel ; Build timestamp: 2024-12-16T10:54

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