sret

Supervisor Exception Return

This instruction is defined by:

Encoding

Synopsis

Returns from an exception.

When sret is allowed to execute, its behavior depends on whether or not the current privilege mode is virtualized.

When the current privilege mode is (H)S-mode or M-mode

sret sets hstatus = 0, mstatus.SPP = 0, mstatus.SIE = mstatus.SPIE, and mstatus.SPIE = 1, changes the privilege mode according to the table below, and then jumps to the address in sepc.

Table 1. Next privilege mode following an sret in (H)S-mode or M-mode
mstatus.SPP	hstatus.SPV	Mode after sret
0	0	U-mode
0	1	VU-mode
1	0	(H)S-mode
1	1	VS-mode

When the current privilege mode is VS-mode

sret sets vsstatus.SPP = 0, vsstatus.SIE = vstatus.SPIE, and vsstatus.SPIE = 1, changes the privilege mode according to the table below, and then jumps to the address in vsepc.

Table 2. Next privilege mode following an sret in (H)S-mode or M-mode
vsstatus.SPP	Mode after sret
0	VU-mode
1	VS-mode

Access

M	HS	U	VS	VU
Always	Sometimes	Never	Sometimes	Never

Always

Sometimes

Never

Sometimes

Never

Access is determined as follows:

mstatus.TSR	hstatus.VTSR	Behavior when executed from:
M-mode	U-mode	(H)S-mode	VU-mode	VS-mode
0	0	executes	`Illegal Instruction`	executes	`Virtual Instruction`	executes
0	1	executes	`Illegal Instruction`	executes	`Virtual Instruction`	`Virtual Instruction`
1	0	executes	`Illegal Instruction`	`Illegal Instruction`	`Virtual Instruction`	executes
1	1	executes	`Illegal Instruction`	`Illegal Instruction`	`Virtual Instruction`	`Virtual Instruction`

mstatus.TSR

hstatus.VTSR

Behavior when executed from:

M-mode

U-mode

(H)S-mode

VU-mode

VS-mode

executes

Illegal Instruction

executes

Virtual Instruction

executes

Illegal Instruction

executes

Virtual Instruction

executes

Illegal Instruction

Virtual Instruction

executes

Illegal Instruction

Virtual Instruction

Decode Variables

Execution

Pruned, XLEN == 64
Original

if (implemented?(ExtensionName::H)) {
  if (mstatus.TSR == 1'b0 && hstatus.VTSR == 1'b0) {
    if (mode() == PrivilegeMode::U) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  } else if (mstatus.TSR == 1'b0 && hstatus.VTSR == 1'b1) {
    if (mode() == PrivilegeMode::U) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU || mode() == PrivilegeMode::VS) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  } else if (mstatus.TSR == 1'b1 && hstatus.VTSR == 1'b0) {
    if (mode() == PrivilegeMode::U || mode() == PrivilegeMode::S) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  } else if (mstatus.TSR == 1'b1 && hstatus.VTSR == 1'b1) {
    if (mode() == PrivilegeMode::U || mode() == PrivilegeMode::S) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU || mode() == PrivilegeMode::VS) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  }
} else {
  if (mode() == PrivilegeMode::U) {
    raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
  }
}
if (!virtual_mode?()) {
  if (implemented?(ExtensionName::H)) {
    if (hstatus.SPV == 1'b1) {
      if (mstatus.SPP == 1'b1) {
        set_mode(PrivilegeMode::VS);
      } else {
        set_mode(PrivilegeMode::VU);
      }
    } else {
      if (mstatus.SPP == 1'b1) {
        set_mode(PrivilegeMode::S);
      } else {
        set_mode(PrivilegeMode::U);
      }
    }
    hstatus.SPV = 0;
  } else {
    if (mstatus.SPP == 1'b1) {
      set_mode(PrivilegeMode::S);
    } else {
      set_mode(PrivilegeMode::U);
    }
  }
  mstatus.SIE = mstatus.SPIE;
  mstatus.SPIE = 1;
  mstatus.SPP = 2'b00;
  $pc = $bits(sepc);
} else {
  if (mstatus.TSR == 1'b1) {
    raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
  }
  vsstatus.SPP = 0;
  vsstatus.SIE = vsstatus.SPIE;
  vsstatus.SPIE = 1;
  $pc = $bits(vsepc);
}

if (implemented?(ExtensionName::H)) {
  if (mstatus.TSR == 1'b0 && hstatus.VTSR == 1'b0) {
    if (mode() == PrivilegeMode::U) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  } else if (mstatus.TSR == 1'b0 && hstatus.VTSR == 1'b1) {
    if (mode() == PrivilegeMode::U) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU || mode() == PrivilegeMode::VS) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  } else if (mstatus.TSR == 1'b1 && hstatus.VTSR == 1'b0) {
    if (mode() == PrivilegeMode::U || mode() == PrivilegeMode::S) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  } else if (mstatus.TSR == 1'b1 && hstatus.VTSR == 1'b1) {
    if (mode() == PrivilegeMode::U || mode() == PrivilegeMode::S) {
      raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
    } else if (mode() == PrivilegeMode::VU || mode() == PrivilegeMode::VS) {
      raise(ExceptionCode::VirtualInstruction, mode(), $encoding);
    }
  }
} else {
  if (mode() == PrivilegeMode::U) {
    raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
  }
}
if (!virtual_mode?()) {
  if (implemented?(ExtensionName::H)) {
    if (hstatus.SPV == 1'b1) {
      if (mstatus.SPP == 1'b1) {
        set_mode(PrivilegeMode::VS);
      } else {
        set_mode(PrivilegeMode::VU);
      }
    } else {
      if (mstatus.SPP == 1'b1) {
        set_mode(PrivilegeMode::S);
      } else {
        set_mode(PrivilegeMode::U);
      }
    }
    hstatus.SPV = 0;
  } else {
    if (mstatus.SPP == 1'b1) {
      set_mode(PrivilegeMode::S);
    } else {
      set_mode(PrivilegeMode::U);
    }
  }
  mstatus.SIE = mstatus.SPIE;
  mstatus.SPIE = 1;
  mstatus.SPP = 2'b00;
  $pc = $bits(sepc);
} else {
  if (mstatus.TSR == 1'b1) {
    raise(ExceptionCode::IllegalInstruction, mode(), $encoding);
  }
  vsstatus.SPP = 0;
  vsstatus.SIE = vsstatus.SPIE;
  vsstatus.SPIE = 1;
  $pc = $bits(vsepc);
}

Exceptions

This instruction may result in the following synchronous exceptions:

IllegalInstruction
VirtualInstruction