rfe

Return From Exception

RFE<c> <Rn>{!}

Loads PC and CPSR from the stack.

Pseudocode Operation

if (P == 0) then address ← Rn
else address ← Rn - 8
if (U == 1) then
  new_pc ← [address]
  new_cpsr ← [address + 4]
  if (W == 1) then Rn ← Rn + 8
else
  new_pc ← [address]
  new_cpsr ← [address + 4]
  if (W == 1) then Rn ← Rn - 8
PC ← new_pc
CPSR ← new_cpsr

Example

RFE r1!

Encoding

Binary Layout

1111100

31:25

19:16

00001010000

15:5

00000

4:0

Format System

Opcode 0xF8100A00

Extension A32 (System)

Operands

Rn
First source / base general-purpose register

Reference

View in Arm A64 ISA Reference ↗ Arm AArch32 ISA

Instruction Forms

Encoding	Instruction	ISA	Bit pattern
0xF8100A00	RFEDA{<c>}{<q>} <Rn>{!}	A32	1111100 \| 0 \| 0 \| 0 \| W \| 1 \| Rn \| 00001010000 \| 00000
0xF9100A00	RFEDB{<c>}{<q>} <Rn>{!}	A32	1111100 \| 1 \| 0 \| 0 \| W \| 1 \| Rn \| 00001010000 \| 00000
0xF8900A00	RFE{IA}{<c>}{<q>} <Rn>{!}	A32	1111100 \| 0 \| 1 \| 0 \| W \| 1 \| Rn \| 00001010000 \| 00000
0xF9900A00	RFEIB{<c>}{<q>} <Rn>{!}	A32	1111100 \| 1 \| 1 \| 0 \| W \| 1 \| Rn \| 00001010000 \| 00000
0xE810C000	RFEDB{<c>}{<q>} <Rn>{!}	T32	1110100 \| 00 \| 0 \| W \| 1 \| Rn \| 1 \| 1 \| 00000000000000
0xE990C000	RFE{IA}{<c>}{<q>} <Rn>{!}	T32	1110100 \| 11 \| 0 \| W \| 1 \| Rn \| 1 \| 1 \| 00000000000000

Description

Return From Exception loads two consecutive memory locations using an address in a base register: An address adjusted by the size of the data loaded can optionally be written back to the base register. The PE checks the value of the word loaded from the higher address for an illegal return event. See Illegal return events from AArch32 state. RFE is undefined in Hyp mode and constrained unpredictable in User mode.

Operation

if ConditionPassed() then
    EncodingSpecificOperations();
    if PSTATE.EL == EL2 then
        UNDEFINED;
    elsif PSTATE.EL == EL0 then
        UNPREDICTABLE;                        // UNDEFINED or NOP
    else
        address = if increment then R[n] else R[n]-8;
        if wordhigher then address = address+4;
        new_pc_value = MemA[address,4];
        spsr = MemA[address+4,4];
        if wback then R[n] = if increment then R[n]+8 else R[n]-8;
        AArch32.ExceptionReturn(new_pc_value, spsr);

rfe