Interrupt handling (ARM)

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This page provides an overview of how Embedded Xinu performs interrupt handling on ARM architectures. This only concerns ARM-specific details; in particular it must be understood that the actual meaning prescribed to interrupts is determined using a board-specific mechanism, such as the BCM2835 Interrupt Controller on the Raspberry Pi. Furthermore, note that the ARM architecture and its exception/interrupt handling mechanisms are well documented by ARM Ltd., especially in various versions of the ARM Architecture Reference Manual[1]. This page is only intended to give an overview of relevant details in the context of Embedded Xinu.

Contents

IRQs and FIQs

Overview

ARM processors define two types of "interrupts":

  • IRQs (Interrupt Requests). These are the "normal" type of interrupt.
  • FIQs (Fast Interrupt Requests). These are an feature that software can optionally use to increase the speed and/or priority of interrupts from a specific source. For simplicity, Embedded Xinu does not use FIQs. However, FIQs could be useful for those looking to design real-time and embedded software on top of or instead of the base Embedded Xinu kernel.

Both IRQs and FIQs are examples of exceptions supported by the ARM. Beware that the term "IRQ" is often used generically, whereas here it specifically refers to the ARM-architecture IRQ exception.

Receiving an IRQ or FIQ

When the ARM receives an IRQ, it will enter a special IRQ mode and, by default, begin execution at physical memory address 0x18. Similarly, when the ARM receives a FIQ, it will enter a special FIQ mode and, by default, begin execution at physical memory address 0x1C. Before enabling IRQs or FIQs, software is expected to copy ARM instructions to the appropriate address. In the case of IRQs, there is only room for one ARM instruction, so it needs to be a branch instruction to a place where the full handler is stored. In Embedded Xinu, these special "glue" instructions, or exception vectors, are set in loader/platforms/arm-rpi/start.S. The "full" IRQ handler is located in system/arch/arm/irq_handler.S.

Banked registers

In IRQ mode and FIQ modes, some registers are banked, meaning that their contents are dependent on the current processor mode. The advantage of such registers is that their original values do not need to be explicitly saved by the interrupt handling code. FIQ mode banks more registers than IRQ mode, but both IRQ mode and FIQ mode bank the stack pointer (sp), which essentially means that each mode can use its own stack. However, for simplicity and consistency with other CPU architectures, Embedded Xinu does not use this capability. Instead, the interrupt handling code immediately switches the processor from IRQ mode to "System" mode, which is the mode in which Embedded Xinu normally operates the ARM CPU. This means that the interrupt handling code uses the stack of the currently executing thread, so perhaps the main disadvantage of this approach is that it increases the stack size required by each thread.

Managing interrupts

The ARM responds to IRQs and FIQs if and only if bits 7 and 6, respectively, of the Current Program Status Register (cpsr) are 0. By default (after reset) these bits are both 1, so software must initially set them to 0 to enable IRQs and FIQs. Similarly, software can set them to 1 if it needs to disable IRQs and FIQs. However, software does not necessarily need to explicitly manipulate these bits because an alternate instruction named cps (Change Program State) is available that can handle changing these bits, as well as changing processor modes.

Below we explain the enable(), disable(), and restore() functions used by Embedded Xinu to manage interrupts. These are all implemented in the ARM assembly language file system/arch/arm/intutils.S.

enable()

enable() allows the processor to receive IRQ exceptions.

enable:
	cpsie i
	mov pc, lr

enable() executes the cpsie ("Change Program State Interrupt Enable") instruction to enable IRQs. (Recall that FIQs are not used by Embedded Xinu.) It then overwrites the program counter (pc) with the link register (lr) to return from the function. Note that since the second instruction is merely overhead of a function call, enable() could instead be efficiently implemented as an inline function containing inline assembly.

disable()

disable() blocks IRQ exceptions and returns a value that can be passed to restore() to restore the previous state. The previous state may be either IRQs disabled or IRQs enabled. Note that an IRQ exception received during a region of code where interrupts are disable()d is not lost; instead, it remains pending until IRQs are re-enabled.

disable:
	mrs r0, cpsr
	cpsid i
	mov pc, lr

disable() copies the cpsr (Current Program Status Register) into r0, which as per the ARM calling convention[2] is the return value of the function. Therefore, the cpsr is treated as the value that can be passed to restore() to restore the previous interrupt state. The code then executes the cpsid (Change Program State Interrupt Disable) instruction to actually disable the IRQ exception.

restore()

restore() restores the IRQ exceptions disabled/enabled state to the state before a previous call to disable().

restore:
	msr cpsr_c, r0
	mov pc, lr

As per the ARM calling convention[3], the argument to restore() (the previous state value--- in the code this is often stored in a variable named im, for "interrupt mask") is passed in r0. r0 is then copied to the cpsr (Current Program Status Register), which is the opposite of what disable() does. restore() then overwrites the program counter with the link register to return from the function. Note that since the second instruction is merely overhead of a function call, restore() could instead be efficiently implemented as an inline function containing inline assembly.

Further reading

As mentioned in the introduction, this page deals with ARM-architecture details only and therefore does not provide a full explanation of interrupt handling on any specific platform, which typically requires the use of some interrupt controller to actually assign meaning to IRQ exceptions.

Notes

  1. http://infocenter.arm.com/help/index.jsp
  2. http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042e/IHI0042E_aapcs.pdf
  3. http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042e/IHI0042E_aapcs.pdf
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