Exceptions and Interrupts

Ibex implements trap handling for interrupts and exceptions according to the RISC-V Privileged Specification, version 1.11.

When entering an interrupt/exception handler, the core sets the mepc CSR to the current program counter and saves mstatus.MIE to mstatus.MPIE. All exceptions cause the core to jump to the base address of the vector table in the mtvec CSR. Interrupts are handled in vectored mode, i.e., the core jumps to the base address plus four times the interrupt ID. Upon executing an MRET instruction, the core jumps to the program counter previously saved in the mepc CSR and restores mstatus.MPIE to mstatus.MIE.

The base address of the vector table is initialized to the boot address (must be aligned to 256 bytes, i.e., its least significant byte must be 0x00) when the core is booting. The base address can be changed after bootup by writing to the mtvec CSR. For more information, see the Control and Status Registers documentation.

The core starts fetching at the address made by concatenating the most significant 3 bytes of the boot address and the reset value (0x80) as the least significant byte. It is assumed that the boot address is supplied via a register to avoid long paths to the instruction fetch unit.

Privilege Modes

Ibex supports operation in Machine Mode (M-Mode) and User Mode (U-Mode). The core resets into M-Mode and will jump to M-Mode on any interrupt or exception. On execution of an MRET instruction, the core will return to the Privilege Mode stored in mstatus.MPP.

Interrupts

Ibex supports the following interrupts.

Interrupt Input Signal	ID	Description
irq_nm_i	31	Non-maskable interrupt (NMI)
irq_fast_i[14:0]	30:16	15 fast, local interrupts
irq_external_i	11	Connected to platform-level interrupt controller
irq_timer_i	7	Connected to timer module
irq_software_i	3	Connected to memory-mapped (inter-processor) interrupt register

All interrupts except for the non-maskable interrupt (NMI) are controlled via the mstatus, mie and mip CSRs. After reset, all interrupts are disabled. To enable interrupts, both the global interrupt enable (MIE) bit in the mstatus CSR and the corresponding individual interrupt enable bit in the mie CSR need to be set. For more information, see the Control and Status Registers documentation.

If multiple interrupts are pending, they are handled in the priority order defined by the RISC-V Privileged Specification, version 1.11 (see Machine Interrupt Registers, Section 3.1.9). The fast interrupts have a platform defined priority. In Ibex they take priority over all other interrupts and between fast interrupts the highest priority is given to the interrupt with the lowest ID.

The NMI is enabled independent of the values in the mstatus and mie CSRs, and it is not visible through the mip CSR. It has interrupt ID 31, i.e., it has the highest priority of all interrupts and the core jumps to the trap-handler base address (in mtvec) plus 0x7C to handle the NMI. When handling the NMI, all interrupts including the NMI are ignored. Nested NMIs are not supported.

All interrupt lines are level-sensitive. It is assumed that the interrupt handler signals completion of the handling routine to the interrupt source, e.g., through a memory-mapped register, which then deasserts the corresponding interrupt line.

In Debug Mode, all interrupts including the NMI are ignored independent of mstatus.MIE and the content of the mie CSR.

Recoverable Non-Maskable Interrupt

To support recovering from an NMI happening during a trap handling routine, Ibex features additional CSRs for backing up mstatus.MPP, mstatus.MPIE, mepc and mcause. These CSRs are not accessible by software running on the core.

These CSRs are nonstandard. For more information, see the corresponding proposal.

Exceptions

Ibex can trigger an exception due to the following exception causes:

Exception Code	Description
1	Instruction access fault
2	Illegal instruction
3	Breakpoint
5	Load access fault
7	Store access fault
8	Environment call from U-Mode (ECALL)
11	Environment call from M-Mode (ECALL)

The illegal instruction exception, instruction access fault, LSU error exceptions and ECALL instruction exceptions cannot be disabled and are always active.

Nested Interrupt/Exception Handling

Ibex does support nested interrupt/exception handling in software. The hardware automatically disables interrupts upon entering an interrupt/exception handler. Otherwise, interrupts/exceptions during the critical part of the handler, i.e. before software has saved the mepc and mstatus CSRs, would cause those CSRs to be overwritten. If desired, software can explicitly enable interrupts by setting mstatus.MIE to 1’b1 from within the handler. However, software should only do this after saving mepc and mstatus. There is no limit on the maximum number of nested interrupts. Note that, after enabling interrupts by setting mstatus.MIE to 1’b1, the current handler will be interrupted also by lower priority interrupts. To allow higher priority interrupts only, the handler must configure mie accordingly.

The following pseudo-code snippet visualizes how to perform nested interrupt handling in software.

isr_handle_nested_interrupts(id) {
  // Save mpec and mstatus to stack
  mepc_bak = mepc;
  mstatus_bak = mstatus;

  // Save mie to stack (optional)
  mie_bak = mie;

  // Keep lower-priority interrupts disabled (optional)
  mie = ~((1 << (id + 1)) - 1);

  // Re-enable interrupts
  mstatus.MIE = 1;

  // Handle interrupt
  // This code block can be interrupted by other interrupts.
  // ...

  // Restore mstatus (this disables interrupts) and mepc
  mstatus = mstatus_bak;
  mepc = mepc_bak;

  // Restore mie (optional)
  mie = mie_bak;
}

Nesting of interrupts/exceptions in hardware is not supported. The purpose of the nonstandard mstack CSRs in Ibex is only to support recoverable NMIs. These CSRs are not accessible by software. While handling an NMI, all interrupts are ignored independent of mstatus.MIE. Nested NMIs are not supported.

Double Fault Detection

Ibex has a mechanism to detect when a double fault has occurred. A double fault is defined as a synchronous exception occurring whilst handling a previous synchronous exception. The cpuctrl custom CSR has fields to provide software visibility and access to this mechanism.

When a synchronous exception occurs, Ibex sets cpuctrl.sync_exception_seen. Ibex clears cpuctrl.sync_exception_seen when mret is executed. If a synchronous exception occurs whilst cpuctrl.sync_exception_seen is set, a double fault has been detected.

When a double fault is detected, the double_fault_seen_o output is asserted for one cycle and cpuctrl.double_fault_seen is set. Note that writing the cpuctrl.double_fault_seen field has no effect on the double_fault_seen_o output.