Chapter 13

ARM Exception Handling

(Real-Time Embedded Multithreading : Using ThreadX® and ARM®)

Department of Computer Science Hsu Hao ChenProfessor Hsung-Pin Chang

Outline

Exception Resulting for ARM Exception (Actions) Reset Vector Initialization ThreadX Initialization Thread Scheduling ThreadX Interrupt Handling Internal Interrupt Processing

Exception Resulting for ARM

Exceptions resulting for the direct effect of executing an instruction

Exceptions resulting as a side effect of executing an instruction

Exceptions resulting from external interrupts, unrelated to instruction execution

Exception (Actions) (1/2)

Step 1: Save CPSR -> SPSR

Step 2: Change to the operating mode

corresponding to the exception Step 3:

Modify the CPSR of the new operating mode Step 4:

Save r15(PC) register -> r14(LR) register Step 5:

Change the PC to the appropriate exception vector

Exception (Actions) (2/2)

Reset Vector Initialization(1/2)

Reset Vector Initialization(2/2)

LDR pc,=__my_low_level_init

branch (jump) or load PC

ThreadX Initialization(1/2)

Note: any code after tx_kernel_enter will never be executed

ThreadX Initialization(2/2)

tx_kernel_enter

_tx_initialize_low_level

tx_application_define

_tx_thread_schedule

Thread Scheduling

Recovering thread context Solicited context Interrupt context

Saving thread context _tx_thread_system_return

Solicited Context

extremely small(48 bytes of stack space)

Interrupt Context

Example :ARM code fragment(1/6)

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Example :ARM code fragment(3/6)

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Example :ARM code fragment(4/6)

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Example :ARM code fragment(5/6)

LDMIA sp!, {r0,r1} //r0= *sp!//r1= *(sp!

+4)

IA: Increment AfterIB: Increment BeforeDA: Decrement AfterDB: Decrement Before

Example :ARM code fragment(6/6)

MSRNE SPSR_cxsf, r1 //copy r1->SPSR_cxsf

NE: Not equal

BX lr //branch (jump) to lr

Mov pc, lr //copy lr->pc

ThreadX Interrupt Handling IRQ interrupt handling

LDR pc, =__tx_irq_handler FIQ interrupt handling

LDR pc, =__tx_fiq_handler

Example Of A ThreadX IRQ Handler

Example Of A ThreadX FIQ Handler

Internal Interrupt Processing

Idle system Saving solicited thread contexts

_tx_thread_context_save Saving interrupt thread contexts Nested interrupt handling

Enable and disable nesting for IRQ interrupt handlers

Enable and disable nesting for FIQ interrupt handlers

Enable And Disable Nesting For IRQ Interrupt Handlers

EXPORT __tx_irq_handlerEXPORT __tx_irq_processing_return__tx_irq_handler

B _tx_thread_context_save__tx_irq_processing_returnBL _tx_thread_irq_nesting_start

BL application_irq_handler

BL _tx_thread_irq_nesting_endB _tx_thread_context_restore

Enable And Disable Nesting For FIQ Interrupt Handlers

EXPORT __tx_fiq_handlerEXPORT __tx_fiq_processing_return__tx_fiq_handler

B _tx_thread_fiq_context_save__tx_fiq_processing_returnBL _tx_thread_fiq_nesting_start

BL application_fiq_handler

BL _tx_thread_fiq_nesting_endB _tx_thread_fiq_context_restore