CS 452/652 Spring 2026 - Lecture 8

Interrupts and Timer

Jun 8, 2026 prev next

BCM - Broadcom BCM2711 data sheet
GIC - ARM Generic Interrupt Controller

Interrupts

• Motivation
  • avoid busy waiting for devices
  • instead, device signals to get attention
  • edge vs. level semantics
• Interrupts start with signals asserted by devices
  • each device defines the signals it can generate
  • example: system timer
    • four signals, corresponding to 4 System Timer Compare registers (C0-C3)
    • when CLO matches value in Cx, signal is asserted
  • example of what ARM refers to as a Shared Peripheral Interrupt (SPI)
    • generated by a device, routed to some processor(s)
    • also Private Peripheral Interrupts (PPI) and Software Generated Interrupts (SGIs)
      • PPI specific to a single core (not used here)
      • SGI generated by software - can be used to signal other cores
• Interrupts result in an asynchronous exception at a processor
  • mapped to one of two signals (FIQ and IRQ) at each processor core
  • Reminder: exception vector
    • groups identify execution state when exception occurred:
      • Current EL with SP0/SPx, Lower EL using 64/32 bit mode
    • within each group, entries for different types of exceptions
      • synchronous, IRQ, FIQ, SError
  • If IRQ signal is asserted
    • CPU transfers control to exception handler after execution of current instruction
    • pipeline flush
  • Handler/Kernel
    • saves current application context
    • figures out the reason for the interrupt
    • handle the interrupt
      • includes device control and interrupt acknowledgement (more later)
    • choose next task to run
    • restore chosen task context, return from exception (eret)
  • Masking
    • Interrupts can be masked at processor
      • IRQ/FIQ still get asserted, but processor ignores them
      • DAIF bits in pstate
        • I is mask for IRQ, F is mask for FIQ
    • When exception occurs, interrupts get masked automatically
      • kernel unmasks by ensuring I and F bits are cleared in SPSR_EL1 prior to context switch
      • possible to unmask within the kernel, but we don't do this!

Interrupt Controller

• connects the device signals to FIQ/IRQ at each core
  • interrupt propagation: device → controller → CPU
• BCM 2711 has:
  • GIC-400 (GIC v2)
  • Legacy Interrupt Controller - not used here
  • see BCM Page 85
• functions of the controller:
  • assigns ID to each input signal
  • config interrupt from each signal
    • enable/disable
    • edge-triggered, level-sensitive (query)
  • route input signals to outputs
  • prioritize and schedule interrupts
  • manage state of each interrupt
• GIC-400 split into parts
  • Distributor (CICD)
  • CPU Interface (GICC)
  • GICD and GICC each have control registers that kernel can use to control interrupts
    • GIC_BASE = 0xFF840000 (offset 0x1840000 from 0xFE000000)
    • GICD_BASE = GIC_BASE + 0x1000
    • GICC_BASE = GIC_BASE + 0x2000
  • Control Register Maps:
    • GICD: see GIC, Table 4.1, page 4-75
    • GICC: see GIC, Table 4.2, page 4-76

• Interrupt State managed by the GIC

  • see GIC, Section 3.2.4

  • GIC tracks interrupt state along two dimensions

    • Pending/Not Pending
      • level-sensitive: is the device asserting?
      • edge-triggered: has the device asserted?

    • Active/Not Active - is the CPU handling the interrupt

      	Not Pending	Pending
      Not Active	Inactive	Pending
      Active	Active	Active+Pending

  • Inactive → Pending when device asserts interrupt signal

    • level-sensitive: pending as long as device signal is asserted
    • edge-triggered: pending until interrupt is handled
  • Pending → Active+Pending
    • for level-sensitive, when CPU reads GICC_IAR to find cause of interrupt
  • Pending → Active
    • for edge-triggered, when CPU reads GICC_IAR to find cause of interrupt
  • remove Active state
    • when handler writes to GICC_EOIR to notify GICC of end of processing

System at Boot Time

• GIC distribution and signalling enabled (GICD_CTLR, GICC_CTLR)
• all interrupts disabled at GIC
• interrupt routing ?
• interrupts masked in EL1 (via boot.S)

System Timer Interrupt

• Timer has 4 interrupt signals, corresponding to C0-C3
  • C0 and C2 reserved by VC, so use C1 and/or C3
  • BCM Chap 6 says system timer interrupts are first 4 VC (video core) interrupts
  • Interrupt IDs for video core start at 96 (Section 6.3), so
    • C1 interrupt has InterruptID 97
    • C3 interrupt has InterruptID 99
• route the interrupt to IRQ on CPU 0
  • use GICD_ITARGETSRn
    • each register defines targets for 4 interrupts
    • 1024 interrupts, 8 bits = 8 cores
• make sure IRQ handler is set up in your kernel's exception vector
• enable the interrupt
  • use GICD_ISENABLERn
    • 4-byte registers, with 1 bit per InterruptID
    • 1024 interrupts, 1 bit
  • use GICD_ICENABLERn to disable interrupt
• write values into system timer C1/C3 to trigger interrupt assertion at correct time
• might have to clear interrupt via CS status register (see below)
• enable IRQ handling in CPU via pstate

Timer Interrupt Handling

• save running task state
  • including FP state?
• read GICC_IAR
  • returns interruptID (should be 97/99)!
  • sets interrupt state to Active in GIC
• if system timer interrupt
  • update Cn compare register - overflow?
  • clear/re-arm interrupt via CS status register
  • write InterruptID to GICC_EOIR
    • marks interrupt as not active in GIC
• perform interrupt handling work
  • unblock task(s) waiting for event
• loop: read GICC_IAR again?
• schedule next task
• resume task
• note: system timer interrupts cannot be explicitly disabled at device

Note: Interrupt Handling Overheads

• direct: pipeline flush, handler execution
• indirect: cache disturbance
• high-rate of interrupts? use hybrid approach (Interrupt Mitigation):
  • poll device and deliver event
  • only enable interrupt, if poll (or several polls) unsuccessful
  • disable interrupt after it is triggered
  • real-world example: Linux NAPI
• similar strategy useful for CS 452 microkernel
• excursion: Linux IRQ Suspend Mechanism