How is MRET different from an ordinary JALR return?
MRET takes its target from mepc and also restores privilege plus interrupt-enable state; JALR only computes a register-plus-immediate PC target and may write a link register.
MRET
RISC-V MRET Instruction Details
Instruction ManualI-typeMachine trap return: pc<-mepc, privilege<-mstatus.MPP, MIE<-MPIE, with MPIE, MPP, and related state updated as specified by the privileged architecture.
Instruction Syntax
mret
Operand Breakdown
Destination rd: general-purpose register receiving the result.
Source rs1: register holding the first operand.
Immediate imm: 12-bit signed value, sign-extended before operation with rs1.
PrivilegedSystem & Privileged
Instruction Behavior
MRET is the privileged return instruction used by a machine-mode trap handler. It sets pc to mepc, restores the current privilege mode from mstatus.MPP, restores MIE from MPIE, sets MPIE to 1, and sets MPP to the least-privileged supported mode; if the return mode is below M, MPRV is also cleared. MRET is legal only in M-mode, and lower-privilege execution raises an illegal-instruction exception.
MRET Decode And Return Animation
Starts from the fixed SYSTEM encoding, then shows xRET permission checks, EPC-to-PC restoration, and previous interrupt-enable / previous privilege field updates.
no explicit operands
mretmret
Execution Contextmode=M
31..20
19..15
14..12
11..7
6..0
001100000010
funct12
00000
rs1=x0
000
funct3
00000
rd=x0
1110011
SYSTEM
instruction
0x30200073
opcode
1110011 -> SYSTEM
funct3
000 -> privileged return form
fixed
funct12=001100000010, rs1=x0, rd=x0
permission
M-mode may execute MRET
mepc
mepc=0x0000000000008040 -> pc
MPP
MPP=S -> privilege=S
MPIE
MIE=MPIE=1; MPIE=1
final
0x0000000000008040; mode=S
This animation shows only privileged-architecture ISA-visible return state; it does not simulate OS scheduling, handler body, pipeline behavior, latency, or platform policy.
Quick Understanding & Search Notes
MRET restores the return context saved for a machine-mode trap handler: PC comes from mepc, return privilege from MPP, and interrupt enable from MPIE. It is not a function return or ordinary jump.
In the SYSTEM encoding, funct12=001100000010, rs1=x0, rd=x0, and funct3=000.
After execution, pc<-mepc, privilege<-MPP, MIE<-MPIE, MPIE<-1, and MPP<-least-privileged supported mode.
If returning to a mode below M, MPRV is cleared; xRET may but need not clear an LR/SC reservation.
Common Usage Scenarios
M-mode handler return
Understand this scenario with real code like «mret # Return from M-mode trap».
Firmware/SEE return
Understand this scenario with real code like «mret # Return from M-mode trap».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is I-type.
- Confirm the operand order matches the example.
Semantic Check
- Ensure the destination register usage is compatible with the calling convention.
- Confirm this is not the lower-level form of a pseudo-instruction expansion.
Pitfalls / Common Confusions
Legal only in M-mode; execution in S/U-mode raises an illegal-instruction exception.
MRET is not an ordinary jump: the target PC comes from mepc, and return privilege plus interrupt enable come from mstatus.MPP/MPIE.
When returning to a mode below M, MPRV is cleared; do not assume the handler keeps its prior MPRV data-access setting.
xRET may clear an LR/SC reservation but is not required to; handlers should explicitly clear it if needed.
FAQ
Does MRET write an integer register?
No. Its encoding fixes rd to x0; its architectural effects are updates to PC, current privilege, and relevant mstatus fields.