Introduction to Embedded Systems

ARM Exception Handling andARM Exception Handling andSoftware Interrupts (SWI)Software Interrupts (SWI)

Introduction to Embedded Systems

Outline of this presentationOutline of this presentation• Normal program flow vs. exceptions

– Exceptions vs. interrupts• Software Interrupts

– What is an SWI?– What happens on an SWI?– Vectoring SWIs– What happens on SWI completion?– What do SWIs do?– A Complete SWI Handler– A C_SWI_Handler (written in C)

• Loading the Software Interrupt Vector Table

Introduction to Embedded Systems

Normal Program Flow Normal Program Flow vs.vs. Exceptions Exceptions • Normally, programs execute sequentially in user mode• Exceptions and interrupts break the sequential flow of a program.• Most exceptions have an associated software exception handler- a

software routine that executes when an exception occurs.• In ARM, Software Interrupt (SWI) is the “system call” exception. • Types of ARM exceptions

– reset when CPU reset pin is asserted – undefined instruction when the processor cannot decode an instruction– software interrupt when CPU executes the SWI instruction – Prefetch abort when the processor attempts to fetch an instruction from

an address without the correct access permissions– data abort when an instruction attempts to access data memory

without the correct access permission – IRQ when CPU's external interrupt request pin is asserted – FIQ when CPU's external fast interrupt request pin is asserted

Introduction to Embedded Systems

TerminologyTerminologyThe terms exception and interrupt are often confused

• Exception – They can happen because of some kind of exceptional condition during execution mode. usually refers to an internal CPU event such as – floating point overflow – MMU fault (e.g., page fault)

• Interrupt - occurs because of external devices outside the CPU core.usually refers to an external I/O event such as

• I/O device request • Reset

It is a special type of exception• SWI – Explicit instruction as a part of code.

Introduction to Embedded Systems

Exceptions and associated modesExceptions and associated modes

Introduction to Embedded Systems

When exception occurs:When exception occurs:• The core automatically

- saves the cpsr to the spsr of the exception mode- saves the pc to the lr of the exception mode- sets the cpsr to the exception mode- set pc to the address of the exceptional handler

Introduction to Embedded Systems

Vector tableVector table

It is a table of addresses that the ARM core branches to when an exception is raised

Introduction to Embedded Systems

Vector TableVector Table• Service routine cannot be put at these addresses• Theses addresses commonly contain branch instructions of one of the

following forms:• B<address> - This branch instruction provides a branch relative from

the pc• LDR pc, [pc, #offset] – This instruction loads the handler address from

memory to pc-This results in slight delay due to extra memory

access.-Any address in the memory can be used for branch

• LDR pc, [pc, #-0xff0] – This instruction is used when a vector interrupt controller is present( VIC PL190)

• MOV pc, #immediate – This copies an immediate value into the pc. The address must be an 8-bit immediate rotated right by an even number of bits

Introduction to Embedded Systems

Exception priority levelsException priority levels

Introduction to Embedded Systems

Entering and exiting an exception handler Entering and exiting an exception handler

Introduction to Embedded Systems

Controlling the interruptsControlling the interrupts

• Enabling an interrupt

• Disabling an interrupt

Introduction to Embedded Systems

Interrupt latencyInterrupt latency• It is the interval of time from an external interrupt request signal being

raised to the first fetch of an instruction of a specific interrupt service routine.

• Interrupt latency depends on a combination of hardware and software.

Introduction to Embedded Systems

Interrupt latencyInterrupt latency

Introduction to Embedded Systems

Link register offsetsLink register offsets

Introduction to Embedded Systems

How the interrupts are designedHow the interrupts are designed

• SWI are normally used to call privileged operating systems

• IRQ is normally assigned to general purpose interrupts

• FIQ is reserved for one single interrupt source that requires fast response time.

Introduction to Embedded Systems

Interrupt handling schemesInterrupt handling schemes• Non nested interrupt handler- it handles and services individual

interrupts sequentially• Nested interrupt handler- handles multiple interrupts without a priority

assignment• Reentrant interrupt handler- handles multiple interrupts that can be

prioritized• Prioritized simple interrupt handler- handles prioritized interrupts• Prioritized standard interrupt handler- handles higher priority interrupts

in a shorter time than lower priority interrupts• Prioritized direct interrupt handler- handles higher-priority interrupts in a

shorter time and goes directly to a specific service routine• Prioritized grouped interrupt handler- is a mechanism for handling

interrupts that are grouped into different priority levels

Introduction to Embedded Systems

Simple non nested interrupt handlerSimple non nested interrupt handler

Introduction to Embedded Systems

Interrupt handlerInterrupt handler

Introduction to Embedded Systems

Nested interrupt handlerNested interrupt handler