mnepc

Machine Exception Program Counter

Written with the PC of an instruction on an exception or interrupt taken in M-mode.

Also controls where the hart jumps on an exception return from M-mode.

Attributes

Requirement

Smrnmi

Defining extensions

Smrnmi

Resumable Non-Maskable Interrupts

CSR Address

0x741

Length

* 32 when CSR[misa].MXL == 0 * 64 when CSR[misa].MXL == 1

Privilege Mode

M

Format

This CSR format changes dynamically.

mnepc Format when CSR[misa].MXL == 0
Figure 1. mnepc Format when CSR[misa].MXL == 0
mnepc Format when CSR[misa].MXL == 1
Figure 2. mnepc Format when CSR[misa].MXL == 1

Field Summary

Name Location Type Reset Value

mnepc.PC

* 31:0 when CSR[misa].MXL == 0 * 63:0 when CSR[misa].MXL == 1

RW-RH

UNDEFINED_LEGAL

Fields

PC

Location

  • 31:0 when CSR[misa].MXL == 0

  • 63:0 when CSR[misa].MXL == 1

Description

When a NMI / double trap is taken into M-mode, mnepc.PC is written with the virtual address of the instruction that was interrupted or that encountered the exception. Otherwise, mnepc.PC is never written by the implementation, though it may be explicitly written by software.

On an exception return from M-mode NMI / double trap (from the MNRET instruction), control transfers to the virtual address read out of mnepc.PC.

Because PCs are always halfword-aligned, bit 0 of mnepc.PC is always read-only 0.

Because PCs are always word-aligned, bits 1:0 of mnepc.PC are always read-only 0.

When misa.C is clear, bit 1 is masked to zero. Writes to bit 1 are still captured, and may be visible on the next read with misa.C is set.

Type

RW-RH

Reset value

UNDEFINED_LEGAL

Software write

This CSR may store a value that is different from what software attempts to write.

When a software write occurs (e.g., through csrrw), the following determines the written value:

PC = return csr_value.PC & ~64'b1;

Software read

This CSR may return a value that is different from what is stored in hardware.

if (implemented?(ExtensionName::C) && CSR[misa].C == 1'b1) {
  return CSR[mnepc].PC & ~64'b1;
} else {
  return CSR[mnepc].PC & ~64'b11;
}