版本 5b5e3435ed5c2fdbfd451e76dc703c78aa306172
Changes from 5b5e3435ed5c2fdbfd451e76dc703c78aa306172 to current
---
title: nuttx
categories: embedded, arm, stm32, stm32f429
toc: no
...
組員
----
* 翁振育
* 黃亮穎
* 蘇偉嘉
* 李英碩
hackpad : https://stm32f429.hackpad.com/NuttX--hqco2YSgAcJ
| hackpad : https://stm32f429.hackpad.com/NuttX--hqco2YSgAcJ
| github : https://github.com/gototop/nuttx-stm32f4disc-bb
Nuttx 參考應用
---------------
* `PX4 autopilot<http://pixhawk.org/dev/nuttx/start>`_
* [PX4 autopilot](http://pixhawk.org/dev/nuttx/start)
NuttX 架構
------------
＊ 可混用task跟thread。
＊ 支援FPU。
＊ 可混用FIFO跟RR。
與平台相關
------------
Run Nuttx on STM32F429-Disco
----------------------------------------------
| (1) 從 https://github.com/deadbeefcafe/nuttx-stm32f4disc-bb 複製Nuttx到workspace
| (2) cd workspace/nuttx-stm32f4disc-bb/nuttx/tools
| (3) ./configure.sh stm32f429i-disco/nsh
| (4) cd ..
| (5) make CROSSDEV=arm-none-eabi-
| (6) st-flash write nuttx.bin 0x8000000
| (7) sudo screen /dev/ttyUSB0 115200 8n1
設定相關應用程式與功能
------------------
| at stm32f429i-disco/nsh/defconfig
| 120 CONFIG_ARCH_FPU=n
| 465 CONFIG_RTC=y
| 466 CONFIG_RTC_DATETIME=y
| 653 CONFIG_EXAMPLES_HELLO=y
| 676 CONFIG_EXAMPLES_OSTEST=y
Task Control Interfaces
-----------------------
- task_create : 產生一個activates的新task，並回傳系統指派的pid
- task_init : 將task的TCB初始化，但並不activate task
- task_activate : activate task，否則scheduler沒辦法執行它
- task_delete : 將task關掉，並deallocate它的TCB與stack
- task_restart : 將task的children砍掉，deallocate TCB，reset priority
- task_exit : 關掉目前在運行的task，不能在user mode被呼叫
- task_exit : 關掉目前在運行的task
Task state
------------
include/nuttx/sched.h
.. code-block:: c
include/nuttx/sched.h
```
enum tstate_e
{
TSTATE_TASK_INVALID = 0, /* INVALID - The TCB is uninitialized */
TSTATE_TASK_PENDING, /* READY_TO_RUN - Pending preemption unlock */
TSTATE_TASK_READYTORUN, /* READY-TO-RUN - But not running */
TSTATE_TASK_RUNNING, /* READY_TO_RUN - And running */
TSTATE_TASK_INACTIVE, /* BLOCKED - Initialized but not yet activated */
TSTATE_WAIT_SEM, /* BLOCKED - Waiting for a semaphore */
#ifndef CONFIG_DISABLE_SIGNALS
TSTATE_WAIT_SIG, /* BLOCKED - Waiting for a signal */
#endif
#ifndef CONFIG_DISABLE_MQUEUE
TSTATE_WAIT_MQNOTEMPTY, /* BLOCKED - Waiting for a MQ to become not empty. */
TSTATE_WAIT_MQNOTFULL, /* BLOCKED - Waiting for a MQ to become not full. */
#endif
#ifdef CONFIG_PAGING
TSTATE_WAIT_PAGEFILL, /* BLOCKED - Waiting for page fill */
#endif
NUM_TASK_STATES /* Must be last */
};
```
Task list
------------
sched/os_internal.h
Register
---------
.. code-block:: c
typedef struct tasklist_s tasklist_t;
extern volatile dq_queue_t g_pendingtasks;
extern volatile dq_queue_t g_waitingforsemaphore;
#ifndef CONFIG_DISABLE_SIGNALS
extern volatile dq_queue_t g_waitingforsignal;
#endif
#ifndef CONFIG_DISABLE_MQUEUE
extern volatile dq_queue_t g_waitingformqnotempty;
#endif
#ifndef CONFIG_DISABLE_MQUEUE
extern volatile dq_queue_t g_waitingformqnotfull;
#endif
#ifdef CONFIG_PAGING
extern volatile dq_queue_t g_waitingforfill;
#endif
extern volatile dq_queue_t g_inactivetasks;
extern volatile sq_queue_t g_delayed_kufree;
#if defined(CONFIG_NUTTX_KERNEL) && defined(CONFIG_MM_KERNEL_HEAP)
extern volatile sq_queue_t g_delayed_kfree;
#endif
extern volatile pid_t g_lastpid;
extern pidhash_t g_pidhash[CONFIG_MAX_TASKS];
extern const tasklist_t g_tasklisttable[NUM_TASK_STATES];
TCB
------------
include/nuttx/sched.h
.. code-block:: c
Communication
---------------------
| Semaphore
|
| sem_wait
| sem_wait的主程式碼分為兩個部份，如果counter > 0，代表semaphore可以被hold，sem_wait會將task加入holder list
| 否則sem_wait會block task，等待sem_hold unblock task，然後sem_wait再將task加入holder list
struct tcb_s
```
if (sem->semcount > 0)
{
/* Fields used to support list management *************************************/
FAR struct tcb_s *flink; /* Doubly linked list */
FAR struct tcb_s *blink;
/* Task Management Fields *****************************************************/
pid_t pid; /* This is the ID of the thread */
start_t start; /* Thread start function */
entry_t entry; /* Entry Point into the thread */
uint8_t sched_priority; /* Current priority of the thread */
sem->semcount--;
sem_addholder(sem);
rtcb->waitsem = NULL;
ret = OK;
}
else
{
sem->semcount--;
rtcb->waitsem = sem;
errno = 0;
up_block_task(rtcb, TSTATE_WAIT_SEM);
uint8_t task_state; /* Current state of the thread */
uint16_t flags; /* Misc. general status flags */
int16_t lockcount; /* 0=preemptable (not-locked) */
if (errno != EINTR && errno != ETIMEDOUT)
{
sem_addholder(sem);
ret = OK;
}
}
```
#if CONFIG_RR_INTERVAL > 0
int timeslice; /* RR timeslice interval remaining */
#endif
------------------------------------------------------------
/* Stack-Related Fields *******************************************************/
| sem_post
| 一般而言，sem_post會在一個task要離開critical section的時候被呼叫
| 它會先呼叫sem_releaseholder，然後檢查counter是否<= 0，若是，代表還有task在等semaphore，sem_post會找到waiting list的下一個task，unblock它
```
if (sem)
{
sem_releaseholder(sem);
sem->semcount++;
if (sem->semcount <= 0)
{
for (stcb = (FAR struct tcb_s*)g_waitingforsemaphore.head; (stcb && stcb->waitsem != sem); stcb = stcb->flink);
if (stcb)
{
stcb->waitsem = NULL;
up_unblock_task(stcb);
}
}
}
```
#ifndef CONFIG_CUSTOM_STACK
size_t adj_stack_size; /* Stack size after adjustment */
/* for hardware, processor, etc. */
/* (for debug purposes only) */
FAR void *stack_alloc_ptr; /* Pointer to allocated stack */
/* Need to deallocate stack */
FAR void *adj_stack_ptr; /* Adjusted stack_alloc_ptr for HW */
/* The initial stack pointer value */
#endif
/* POSIX Semaphore Control Fields *********************************************/
sem_t *waitsem; /* Semaphore ID waiting on */
/* POSIX Signal Control Fields ************************************************/
#ifndef CONFIG_DISABLE_SIGNALS
sigset_t sigprocmask; /* Signals that are blocked */
sigset_t sigwaitmask; /* Waiting for pending signals */
sq_queue_t sigactionq; /* List of actions for signals */
sq_queue_t sigpendactionq; /* List of pending signal actions */
sq_queue_t sigpostedq; /* List of posted signals */
siginfo_t sigunbinfo; /* Signal info when task unblocked */
#endif
/* POSIX Named Message Queue Fields *******************************************/
#ifndef CONFIG_DISABLE_MQUEUE
FAR msgq_t *msgwaitq; /* Waiting for this message queue */
#endif
/* The form and content of these fields are processor-specific. */
struct xcptcontext xcp; /* Interrupt register save area */
#if CONFIG_TASK_NAME_SIZE > 0
char name[CONFIG_TASK_NAME_SIZE]; /* Task name */
#endif
:
:
:
:
:
:
};
----------------------------------------------------------
| Mutex
| nuttx的mutex實作是建立在semaphore上面
| 在使用mutex時，會先init一個binary semaphore，然後呼叫pthread_mutex_lock，pthread_mutex_lock會呼叫pthread_takesemaphore
| pthread_takesemaphore會跑一個判別式是sem_wait的while loop
| 當owner要交出使用權的時候，會呼叫pthread_mutex_unlock，pthread_mutex_unlock會將mutex->pid設成0，然後呼叫pthread_givesemaphore
| pthread_givesemaphore會呼叫sem_post
|
|
| pthread_mutexinit.c
Register
---------
```
int pthread_mutex_init(FAR pthread_mutex_t *mutex, FAR pthread_mutexattr_t *attr)
{
:
:
status = sem_init((sem_t*)&mutex->sem, pshared, 1);
:
:
}
```
.. image:: /ARM_register.jpg
------------------------------------------------
| phread_mutexlock.c
```
int pthread_mutex_lock(FAR pthread_mutex_t *mutex)
{
:
:
:
ret = pthread_takesemaphore((sem_t*)&mutex->sem);
if (!ret)
{
mutex->pid = mypid;
}
}
```
Communication
---------------------
-------------------------------------------------
| pthread_initalize.c
```
int pthread_takesemaphore(sem_t *sem)
{
if (sem)
{
while (sem_wait(sem) != OK)
{
:
:
:
}
return OK;
}
}
```
```
int pthread_givesemaphore(sem_t *sem)
{
if (sem)
{
if (sem_post(sem) == OK)
{
return OK;
}
else
{
set_errno(EINVAL);
return ERROR;
}
}
else
{
set_errno(EINVAL);
return ERROR;
}
}
```
---------------------------------------------------------
| pthread_mutexunlock.c
```
int pthread_mutex_unlock(FAR pthread_mutex_t *mutex)
{
:
:
:
{
{
mutex->pid = 0;
:
:
:
ret = pthread_givesemaphore((sem_t*)&mutex->sem);
}
sched_unlock();
}
return ret;
}
```
Scheduling
----------------
include/sys/types.h
.. code-block:: c
```
/* HPUX-like MIN/MAX value */
#define PRIOR_RR_MIN 0
#define PRIOR_RR_MAX 255
#define PRIOR_FIFO_MIN 0
#define PRIOR_FIFO_MAX 255
#define PRIOR_OTHER_MIN 0 /* Not use */
#define PRIOR_OTHER_MAX 255 /* Not use */
/* Scheduling Priorities. NOTE: Only the idle task can take
* the true minimum priority. */
#define SCHED_PRIORITY_MAX 255
#define SCHED_PRIORITY_DEFAULT 100
#define SCHED_PRIORITY_MIN 1
#define SCHED_PRIORITY_IDLE 0
```
| 1. RR 時間間隔是 20 ticks 做一次 context switch。
| 觀看每個設定time slice的相關檔案其運算是皆為 timeslice = CONFIG_RR_INTERVAL / MSEC_PER_TICK
| 在 nuttx/include/nuttx/config.h 中
| CONFIG_RR_INTERVAL = 200
| MSEC_PER_TICK = 10
| 2. 若要禁止 context switch，可用sched_lock()
| 此區禁止 context switch
| sched_unlock()用在修改scheduler和priority。
| 3. 若要先將 timer interrupt 暫停，可用saved_state = irqsave();
| 此區暫停 timer interrupt
| irqrestore(saved_state);
| 用在修改 scheduler。
| 注意：timer interrupt 跟 context switch 是不同系統。
| 4. Nuttx 只有兩個 scheduler policy，其 tcb->timeslice = 0 則表示FIFO, 非0則為RR。
Timer interrupt
----------------------
| /nuttx/arch/arm/src/stm32/stm32_vectors.S
|
| processor exception
.. code-block:: c
.word IDLE_STACK /* Vector 0: Reset stack pointer */
.word __start /* Vector 1: Reset vector */
.word stm32_nmi /* Vector 2: Non-Maskable Interrupt (NMI) */
.word stm32_hardfault /* Vector 3: Hard fault */
.word stm32_mpu /* Vector 4: Memory management (MPU) */
.word stm32_busfault /* Vector 5: Bus fault */
.word stm32_usagefault /* Vector 6: Usage fault */
.word stm32_reserved /* Vector 7: Reserved */
.word stm32_reserved /* Vector 8: Reserved */
.word stm32_reserved /* Vector 9: Reserved */
.word stm32_reserved /* Vector 10: Reserved */
.word stm32_svcall /* Vector 11: SVC call */
.word stm32_dbgmonitor /* Vector 12: Debug monitor */
.word stm32_reserved /* Vector 13: Reserved */
.word stm32_pendsv /* Vector 14: Pendable system service request */
.word stm32_systick /* Vector 15: System tick */
/nuttx/arch/arm/src/stm32/stm32_timerisr.c (in line 109 )
.. code-block:: c
int up_timerisr(int irq, uint32_t *regs)
{
/* Process timer interrupt */
sched_process_timer();
return 0;
}
Interrupt handler
Interrupt Handler
------------------------
IRQ numbers
在nuttx/arch/arm/include/stm32/irq.h和nuttx/arch/arm/include/stm32/stm32f40xxx_irq.h中分別定義了可用的ISR name和對應的vector name
.. code-block:: c
Processor Exceptions (vectors 0-15)
包括Internal和Software Interrupts，設定在nuttx/arch/arm/src/chip/stm32_irq.c:void up_irqinitialize(void)
```
//nuttx/arch/arm/include/stm32/irq.h
#define STM32_IRQ_RESERVED (0) /* Reserved vector (only used with CONFIG_DEBUG) */
/* Vector 0: Reset stack pointer value */
/* Vector 1: Reset (not handler as an IRQ) */
#define STM32_IRQ_NMI (2) /* Vector 2: Non-Maskable Interrupt (NMI) */
#define STM32_IRQ_HARDFAULT (3) /* Vector 3: Hard fault */
#define STM32_IRQ_MEMFAULT (4) /* Vector 4: Memory management (MPU) */
#define STM32_IRQ_BUSFAULT (5) /* Vector 5: Bus fault */
#define STM32_IRQ_USAGEFAULT (6) /* Vector 6: Usage fault */
#define STM32_IRQ_SVCALL (11) /* Vector 11: SVC call */
#define STM32_IRQ_DBGMONITOR (12) /* Vector 12: Debug Monitor */
/* Vector 13: Reserved */
#define STM32_IRQ_PENDSV (14) /* Vector 14: Pendable system service request */
#define STM32_IRQ_SYSTICK (15) /* Vector 15: System tick */
```
External interrupts (vectors >= 16)
外部中斷，設定在nuttx/arch/arm/src/stm32/chip/stm32f40xxx_vectors.h
```
//nuttx/arch/arm/include/stm32/stm32f40xxx_irq.h
#define STM32_IRQ_WWDG (STM32_IRQ_INTERRUPTS+0) /* 0: Window Watchdog interrupt */
#define STM32_IRQ_PVD (STM32_IRQ_INTERRUPTS+1) /* 1: PVD through EXTI Line detection interrupt */
#define STM32_IRQ_TAMPER (STM32_IRQ_INTERRUPTS+2) /* 2: Tamper and time stamp interrupts */
#define STM32_IRQ_TIMESTAMP (STM32_IRQ_INTERRUPTS+2) /* 2: Tamper and time stamp interrupts */
#define STM32_IRQ_RTC_WKUP (STM32_IRQ_INTERRUPTS+3) /* 3: RTC global interrupt */
#define STM32_IRQ_FLASH (STM32_IRQ_INTERRUPTS+4) /* 4: Flash global interrupt */
.
```
如何設定Interrupt Handler?
```
//irq_attach(STM32_IRQ_SVCALL, up_svcall); //System Call
//nuttx/sched/irq_attach.c
int irq_attach(int irq, xcpt_t isr)
{
state = irqsave(); //disables all interrupts
if(isr == NULL)
{
up_disable_irq(irq);
isr = irq_unexpected_isr;
}
g_irqvector[irq] = isr; //Vector Table
irqrestore(state);
ret = ok;
}
```
Interrupt Handling的分層架構
1. low-level logic, arch/arm/src/armv7-m/up_exception.S, 接收Interrupt並決定IRQ number
2. That low-level logic than calls some MCU-specific, intermediate level function up_doirq(), mask and acknowledge the interrupt, dispatch, unmask
3. That MCU-specific function then calls the NuttX common interrupt dispatching logic irq_dispatch()
STM32_IRQ_SYSTICK
```
//(void)irq_attach(STM32_IRQ_SYSTICK, (xcpt_t)up_timerisr);
//up_exception.S -> up_doirq(int irq, uint32_t *regs)-> irq_dispatch(int irq, FAR void *context) -> up_timerisr(int irq, uint32_t *regs) -> sched_process_timer(void) -> sched_process_timeslice()
static void sched_process_timeslice(void)
{
if((rtcb->flags & TCB_FLAG_ROUND_ROBIN) != 0)
{
if(rtcb->timeslice <= 1)
{
if(!rtcb->lockcount)
{
rtcb->timeslice = CONFIG_RR_INTERVAL/MSEC_PER_TICK;
if (rtcb->flink && rtcb->flink->sched_priority >= rtcb->sched_priority)
{
up_reprioritize_rtr(rtcb, rtcb->sched_priority);
}
}
}
else
{
rtcb->timeslice--;
}
}
}
```
Configuring High Priority Interrupts
How do you specify a high priority interrupt? if CONFIG_ARCH_RAMVECTORS=y, then vectors will be kept in RAM and the system will support the interface
int up_ramvec_attach(int irq, up_vector_t vector). That interface can be used to attach your C interrupt handler to the vector at run time.
```
//up_vector_t g_ram_vectors[ARMV7M_VECTAB_SIZE] __attribute__((section(".ram_vectors"))); //Using a RAM-based vector table
//void up_ramvec_initialize(void) //Copy vectors to RAM an configure the NVIC to use the RAM vectors.
int up_ramvec_attach(int irq, up_vector_t vector) //Configure the ram vector table so that IRQ number 'irq' will be dipatched by hardware to 'vector'
{
int ret = ERROR;
if ((unsigned)irq < ARMV7M_PERIPHERAL_INTERRUPTS)
{
irqstate_t flags;
flags = irqsave();
if (vector == NULL)
{
up_disable_irq(irq);
vector = exception_common;
}
g_ram_vectors[irq] = vector;
irqrestore(flags);
ret = OK;
}
return ret;
}
```
FPU
-------
| arch/arm/src/armv7-m/up_fpu.S
| 儲存 fpu 的 context switch
.. code-block:: c
```
up_savefpu:
add r1, r0, #(4*REG_S0) /* R1=Address of FP register storage */
/* Some older GNU assemblers don't support all the newer UAL mnemonics. */
#if 1 /* Use UAL mnemonics */
/* Store all floating point registers. Registers are stored in numeric order,
* s0, s1, ... in increasing address order.
*/
vstmia r1!, {s0-s31} /* Save the full FP context */
/* Store the floating point control and status register. At the end of the
* vstmia, r1 will point to the FPCSR storage location.
*/
vmrs r2, fpscr /* Fetch the FPCSR */
str r2, [r1], #4 /* Save the floating point control and status register */
#else
/* Store all floating point registers */
#if 1 /* Use store multiple */
fstmias r1!, {s0-s31} /* Save the full FP context */
#else
vmov r2, r3, d0 /* r2, r3 = d0 */
str r2, [r1], #4 /* Save S0 and S1 values */
str r3, [r1], #4
vmov r2, r3, d1 /* r2, r3 = d1 */
str r2, [r1], #4 /* Save S2 and S3 values */
str r3, [r1], #4
vmov r2, r3, d2 /* r2, r3 = d2 */
str r2, [r1], #4 /* Save S4 and S5 values */
str r3, [r1], #4
vmov r2, r3, d3 /* r2, r3 = d3 */
str r2, [r1], #4 /* Save S6 and S7 values */
str r3, [r1], #4
vmov r2, r3, d4 /* r2, r3 = d4 */
str r2, [r1], #4 /* Save S8 and S9 values */
str r3, [r1], #4
vmov r2, r3, d5 /* r2, r3 = d5 */
str r2, [r1], #4 /* Save S10 and S11 values */
str r3, [r1], #4
vmov r2, r3, d6 /* r2, r3 = d6 */
str r2, [r1], #4 /* Save S12 and S13 values */
str r3, [r1], #4
vmov r2, r3, d7 /* r2, r3 = d7 */
str r2, [r1], #4 /* Save S14 and S15 values */
str r3, [r1], #4
vmov r2, r3, d8 /* r2, r3 = d8 */
str r2, [r1], #4 /* Save S16 and S17 values */
str r3, [r1], #4
vmov r2, r3, d9 /* r2, r3 = d9 */
str r2, [r1], #4 /* Save S18 and S19 values */
str r3, [r1], #4
vmov r2, r3, d10 /* r2, r3 = d10 */
str r2, [r1], #4 /* Save S20 and S21 values */
str r3, [r1], #4
vmov r2, r3, d11 /* r2, r3 = d11 */
str r2, [r1], #4 /* Save S22 and S23 values */
str r3, [r1], #4
vmov r2, r3, d12 /* r2, r3 = d12 */
str r2, [r1], #4 /* Save S24 and S25 values */
str r3, [r1], #4
vmov r2, r3, d13 /* r2, r3 = d13 */
str r2, [r1], #4 /* Save S26 and S27 values */
str r3, [r1], #4
vmov r2, r3, d14 /* r2, r3 = d14 */
str r2, [r1], #4 /* Save S28 and S29 values */
str r3, [r1], #4
vmov r2, r3, d15 /* r2, r3 = d15 */
str r2, [r1], #4 /* Save S30 and S31 values */
str r3, [r1], #4
#endif
/* Store the floating point control and status register */
fmrx r2, fpscr /* Fetch the FPCSR */
str r2, [r1], #4 /* Save the floating point control and status register */
#endif
bx lr
.size up_savefpu, .-up_savefpu
```
| 載回 fpu 的 context switch
.. code-block:: c
```
up_restorefpu:
add r1, r0, #(4*REG_S0) /* R1=Address of FP register storage */
/* Some older GNU assemblers don't support all the newer UAL mnemonics. */
#if 1 /* Use UAL mnemonics */
/* Load all floating point registers. Registers are loaded in numeric order,
* s0, s1, ... in increasing address order.
*/
vldmia r1!, {s0-s31} /* Restore the full FP context */
/* Load the floating point control and status register. At the end of the
* vstmia, r1 will point to the FPCSR storage location.
*/
ldr r2, [r1], #4 /* Fetch the floating point control and status register */
vmsr fpscr, r2 /* Restore the FPCSR */
#else
/* Load all floating point registers Registers are loaded in numeric order,
* s0, s1, ... in increasing address order.
*/
#if 1 /* Use load multiple */
fldmias r1!, {s0-s31} /* Restore the full FP context */
#else
ldr r2, [r1], #4 /* Fetch S0 and S1 values */
ldr r3, [r1], #4
vmov d0, r2, r3 /* Save as d0 */
ldr r2, [r1], #4 /* Fetch S2 and S3 values */
ldr r3, [r1], #4
vmov d1, r2, r3 /* Save as d1 */
ldr r2, [r1], #4 /* Fetch S4 and S5 values */
ldr r3, [r1], #4
vmov d2, r2, r3 /* Save as d2 */
ldr r2, [r1], #4 /* Fetch S6 and S7 values */
ldr r3, [r1], #4
vmov d3, r2, r3 /* Save as d3 */
ldr r2, [r1], #4 /* Fetch S8 and S9 values */
ldr r3, [r1], #4
vmov d4, r2, r3 /* Save as d4 */
ldr r2, [r1], #4 /* Fetch S10 and S11 values */
ldr r3, [r1], #4
vmov d5, r2, r3 /* Save as d5 */
ldr r2, [r1], #4 /* Fetch S12 and S13 values */
ldr r3, [r1], #4
vmov d6, r2, r3 /* Save as d6 */
ldr r2, [r1], #4 /* Fetch S14 and S15 values */
ldr r3, [r1], #4
vmov d7, r2, r3 /* Save as d7 */
ldr r2, [r1], #4 /* Fetch S16 and S17 values */
ldr r3, [r1], #4
vmov d8, r2, r3 /* Save as d8 */
ldr r2, [r1], #4 /* Fetch S18 and S19 values */
ldr r3, [r1], #4
vmov d9, r2, r3 /* Save as d9 */
ldr r2, [r1], #4 /* Fetch S20 and S21 values */
ldr r3, [r1], #4
vmov d10, r2, r3 /* Save as d10 */
ldr r2, [r1], #4 /* Fetch S22 and S23 values */
ldr r3, [r1], #4
vmov d11, r2, r3 /* Save as d11 */
ldr r2, [r1], #4 /* Fetch S24 and S25 values */
ldr r3, [r1], #4
vmov d12, r2, r3 /* Save as d12 */
ldr r2, [r1], #4 /* Fetch S26 and S27 values */
ldr r3, [r1], #4
vmov d13, r2, r3 /* Save as d13 */
ldr r2, [r1], #4 /* Fetch S28 and S29 values */
ldr r3, [r1], #4
vmov d14, r2, r3 /* Save as d14 */
ldr r2, [r1], #4 /* Fetch S30 and S31 values */
ldr r3, [r1], #4
vmov d15, r2, r3 /* Save as d15 */
#endif
/* Load the floating point control and status register. r1 points t
* the address of the FPCSR register.
*/
ldr r2, [r1], #4 /* Fetch the floating point control and status register */
fmxr fpscr, r2 /* Restore the FPCSR */
#endif
bx lr
.size up_restorefpu, .-up_restorefpu
#endif /* CONFIG_ARCH_FPU */
```
| /nuttx/arch/arm/include/armv7-m/irq.h
| struct xcptcontext
| /nuttx/arch/arm/include/armv7-m/irq_lazyfpu.h
| XCPTCONTEXT_REGS = HW_XCPT_REGS + SW_XCPT_REGS
| XCPTCONTEXT_SIZE = 4 * XCPTCONTEXT_REGS
| page 40 ARM (2)
| 共會存入51個暫存器
| 其中8個為硬體一般暫存器包含R0 to R3, R12, R14 = LR, R15 = PC, XPSR
| 其中軟體暫存器浮點數FP共33個 S0 to S31, FPSCR
| 原本10個軟體一般暫存器 R13 = SP, PRIMASK, R4 to R11
| 依序存入XCPT由software registers SP開始到R11，浮點數S0到FPSCR，再來是 hardware registers R0到XPSR
測試環境架設
------------
硬體驅動原理
------------
* `UART<embedded/USART>`_
* [UART](embedded/USART)
* FPU (Floating Point Unit)
效能表現
--------
參考資料
--------
* https://github.com/deadbeefcafe/nuttx-stm32f4disc-bb
* http://bibliodigital.itcr.ac.cr/xmlui/bitstream/handle/2238/3051/FinalReport.pdf
