Low-Latency Vectored Interrupt Mode - 2025.1 English - UG1629

A low-latency vectored interrupt mode is available, which allows the Interrupt Controller to directly supply the interrupt vector for each individual interrupt (using the input port Interrupt_Address). The address of all interrupt handlers, including interrupts that are using the trap-vector base-address in mtvec, must be passed to the Interrupt Controller when initializing the interrupt system. When a particular interrupt occurs, this address is supplied by the Interrupt Controller, which allows MicroBlaze V to directly jump to the handler code. The interrupt address must remain valid until MicroBlaze V has taken the interrupt and jumped to the address.

With this mode, MicroBlaze V also directly sends the appropriate interrupt acknowledge to the Interrupt Controller (using the Interrupt_Ack output port), although it is still the responsibility of the interrupt service routine to acknowledge level sensitive interrupts at the source.

This information allows the Interrupt Controller to acknowledge interrupts appropriately, both for level-sensitive and edge-triggered interrupts.

To inform the Interrupt Controller of the interrupt handling events, Interrupt_Ack is set to:

01

When MicroBlaze V jumps to the interrupt handler code.

10

When the MRET instruction is executed.

11

When the global interrupt enable bit MIE in mstatus is changed from 0 to 1, which enables interrupts again.

The Interrupt_Ack output port is active during one clock cycle, and is then reset to 00.

With low-latency interrupt mode, control is directly passed to the interrupt handler for each individual interrupt utilizing this mode. In this case, it is the responsibility of each handler to save and restore used registers. This can be achieved with the RISC-V GCC compiler using the following code:

/* Declare a low-latency interrupt handler */
void interrupt_handler_name() __attribute__((aligned(4), interrupt));

mepc ← PC
if C_USE_INTERRUPT = 2
   PC ← Interrupt_Address
   Interrupt_Ack ← 01
else
   PC ← mtvec
mcause ← 0x8000000B
mstatus.mpie ← mstatus.mie
mstatus.mie ← 0
if C_USE_MMU > 0
   mstatus.mpp ← privilege mode
else
   mstatus.mpp ← 11
mtval ← 0
Reservation ← 0