以MSP430说明FreeRTOS移植

MSP430RTOS移植函数说明文档

通过MSP430的官网移植函数用来说明在移植时需要实现的函数和原因

FreeRTOS移植需要实现的代码

1，pxPortInitialiseStack()

会在任务创建时调用此函数

参数：

pxTopOfStack:任务私有栈栈顶地址

pxCode:任务指针

pvParameters：传给任务的参数指针

私有栈用来保存任务的状态寄存器，指令指针，通用寄存器，任务内部需要用栈的数据

不同的处理器栈初始化顺序结构不同，但本质是一样的

StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t pxCode, void *pvParameters )

{

/*

Place a few bytes of known values on the bottom of the stack.

This is just useful for debugging and can be included if required.

MSP430特有

*pxTopOfStack = ( StackType_t ) 0x1111;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x2222;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x3333;

pxTopOfStack--;

*/

/* The msp430 automatically pushes the PC then SR onto the stack before

executing an ISR. We want the stack to look just as if this has happened

so place a pointer to the start of the task on the stack first - followed

by the flags we want the task to use when it starts up. */

在初始化私有栈时，会假设任务已经开始运行了，

FreeRTOS基于时间片的任务调度，所以在任务切换时，会进入定时器中断，然后进行任务切换，不同的处理器中断发生时会执行不同的寄存器入栈操作，MSP430会保存PC和SR寄存器，所以在栈顶两个空间用来保存指令指令，SR寄存器，初始化就是函数指针和一个任意数

*pxTopOfStack = ( StackType_t ) pxCode;

pxTopOfStack--;

*pxTopOfStack = portFLAGS_INT_ENABLED;

pxTopOfStack--;

/* Next the general purpose registers.

通用寄存器入栈，根据内核说明入栈

*/

*pxTopOfStack = ( StackType_t ) 0x4444;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x5555;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x6666;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x7777;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x8888;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0x9999;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0xaaaa;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0xbbbb;

pxTopOfStack--;

/* When the task starts is will expect to find the function parameter in

R15.

任务参数，不同编译器使用不同的通用寄存器用来传递参数，参看编译器说明是使用哪一个寄存器

*/

*pxTopOfStack = ( StackType_t ) pvParameters;

pxTopOfStack--;

通用寄存器

*pxTopOfStack = ( StackType_t ) 0xdddd;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0xeeee;

pxTopOfStack--;

*pxTopOfStack = ( StackType_t ) 0xffff;

pxTopOfStack--;

/* A variable is used to keep track of the critical section nesting.

This variable has to be stored as part of the task context and is

initially set to zero.

临界嵌套数

*/

*pxTopOfStack = ( StackType_t ) portNO_CRITICAL_SECTION_NESTING;

/* Return a pointer to the top of the stack we have generated so this can

be stored in the task control block for the task.

返回堆栈，之后的地址用于任务内部栈

*/

return pxTopOfStack;

}

2，void vPortSetupTimerInterrupt( void )

初始化硬件定时器作为系统滴答时钟

3,定时器中断例程

vTickISR:

保存当前任务私有栈

portSAVE_CONTEXT

调用系统滴答时钟处理函数，函数会返回一个BOOL类型数据，当为0时不进行任务切换，为1时进行任务切换

call #xTaskIncrementTick

cmp.w #0x0, R12

jeq SkipContextSwitch

call #vTaskSwitchContext

SkipContextSwitch:

如果不进行任务切换，重新装载任务栈

portRESTORE_CONTEXT

4，vPortYield(),手动实现任务切换

/*

* Manual context switch called by the portYIELD() macro.

*/

vPortYield:

/* Mimic an interrupt by pushing the SR.

手动切换任务，不会自动保存PC和SR值，需要手动入栈，调用函数vPortYield时PC 指令已经入栈了，所以不需要再手动将PC入栈

*/

push S R

/* Now the SR is stacked we can disable interrupts.

关中断

*/

dint

/* Save the context of the current task. */

portSAVE_CONTEXT

/* Switch to the highest priority task that is ready to run. */

call #vTaskSwitchContext

/* Restore the context of the new task. */

portRESTORE_CONTEXT

/*-----------------------------------------------------------*/

5，启动调度器xPortStartScheduler()

* Start off the scheduler by initialising the RTOS tick timer, then restoring * the context of the first task.

*/

xPortStartScheduler:

/* Setup the hardware to generate the tick. Interrupts are disabled when this function is called.

*/

调用定时器初始化函数，使能滴答时钟

call #vPortSetupTimerInterrupt

/* Restore the context of the first task that is going to run.

装载任务栈启动第一个任务

*/

portRESTORE_CONTEXT

/*-----------------------------------------------------------*/

4，保存和装载上下文

portSAVE_CONTEXT macro

IMPORT pxCurrentTCB

IMPORT usCriticalNesting

/* Save the remaining registers. */

push r4

push r5

push r6

push r7

push r8

push r9

push r10

push r11

push r12

push r13

push r14

push r15

mov.w &usCriticalNesting, r14

push r14

mov.w &pxCurrentTCB, r12

mov.w r1, 0(r12) 当前栈顶保存到私有栈

endm

/*-----------------------------------------------------------*/

portRESTORE_CONTEXT macro

mov.w &pxCurrentTCB, r12

mov.w @r12, r1 //R1作为堆栈寄存器使用，这条指令时将堆栈切换至当前任务的私有栈

pop r15

mov.w r15, &usCriticalNesting

pop r15

pop r14

pop r13

pop r12

pop r11

pop r10

pop r9

pop r8

pop r7

pop r6

pop r5

pop r4

/* The last thing on the stack will be the status register.

Ensure the power down bits are clear ready for the next

time this power down register is popped from the stack. */

bic.w #0xf0,0(SP)

reti 中断返回指令，会执行中断POP指令

endm

/*-----------------------------------------------------------*/

以上就是移植需要实现的所有函数

现在来看一看系统在启动时是如何实现第一个任务启动的并实现任务切换的

首先调用xPortStartScheduler()函数启动调度器。

函数初始化了滴答时钟，并调用portRESTORE_CONTEXT切换到第一个任务。

注意此时的堆栈还是系统堆栈，要实现任务切换就需要将指令指针指向任务，

之后执行的pop指令就是将任务私有栈中的数据装载到通用寄存器中，SR和PC的出栈操作会在调用RETI指令时实现，所以RETI后CPU进入第一个任务空间中运行，任务切换就通过定时器中断函数实现

以上可以看出系统任务切换就是任务之间的的状态寄存器，指令指针，通用寄存器切换的过程，移植要做的工作就是针对所使用的处理器，实现就寄存器保护、转载的过程，系统是通过定时器中断来实现任务切换的，所以需要一个定时器，并在定时器中断例程中实现上下文保护，任务切换，上下文装载。