本帖最后由 时飞 于 2017-2-28 07:15 编辑
描述完毕“内存管理”的整个轮廓之后，我们来查看具体的内存管理函数是怎么实现的；
首先我们查看一下内存初始化函数OSMemInit，这个函数很简单，主要为全局变量gpOSMemBegin，gpOSMemEnd与gpOSMemLFree分配初始数值；
/*****************************************************************************
Function : OSMemInit
Description : Zero the heap and initialize start, end and lowest-free pointer.
Input : None
Output : None
Return : None
*****************************************************************************/
void OSMemInit(void)
{
tOSMem_t *ptOSMemTemp;
// align the heap
gpOSMemBegin = (uOS8_t *)OSMEM_ALIGN_ADDR(OSRAM_HEAP_POINTER);
// initialize the start of the heap
ptOSMemTemp = (tOSMem_t *)(void *)gpOSMemBegin;
ptOSMemTemp->NextMem = OSMEM_SIZE_ALIGNED;
ptOSMemTemp->PrevMem = 0;
ptOSMemTemp->Used = 0;
// initialize the end of the heap
gpOSMemEnd = (tOSMem_t *)(void *)&gpOSMemBegin[OSMEM_SIZE_ALIGNED];
gpOSMemEnd->Used = 1;
gpOSMemEnd->NextMem = OSMEM_SIZE_ALIGNED;
gpOSMemEnd->PrevMem = OSMEM_SIZE_ALIGNED;
// initialize the lowest-free pointer to the start of the heap
gpOSMemLFree = (tOSMem_t *)(void *)gpOSMemBegin;
}
接着是内存分配函数。
/*****************************************************************************
Function : OSMemMalloc
Description : Allocate a block of memory with a minimum of 'size' bytes.
Input : size -- the minimum size of the requested block in bytes.
Output : None
Return : pointer to allocated memory or OS_NULL if no free memory was found.
the returned value will always be aligned (as defined by OSMEM_ALIGNMENT).
*****************************************************************************/
void* OSMemMalloc(uOSMemSize_t size)
{
uOS8_t * pResult = OS_NULL;
uOSMemSize_t ptr, ptr2;
tOSMem_t *ptOSMemTemp, *ptOSMemTemp2;
if(gpOSMemEnd==OS_NULL)
{
OSMemInit();
if(gpOSMemEnd==OS_NULL)
{
return pResult;
}
}
if (size == 0)
{
return pResult;
}
// Expand the size of the allocated memory region so that we can
// adjust for alignment.
size = OSMEM_ALIGN_SIZE(size);
if(size < OSMIN_SIZE_ALIGNED)
{
// every data block must be at least OSMIN_SIZE_ALIGNED long
size = OSMIN_SIZE_ALIGNED;
}
if (size > OSMEM_SIZE_ALIGNED)
{
return pResult;
}
// protect the heap from concurrent access
OSIntLock();
// Scan through the heap searching for a free block that is big enough,
// beginning with the lowest free block.
for (ptr = (uOSMemSize_t)((uOS8_t *)gpOSMemLFree - gpOSMemBegin); ptr < OSMEM_SIZE_ALIGNED - size;
ptr = ((tOSMem_t *)(void *)&gpOSMemBegin[ptr])->NextMem)
{
ptOSMemTemp = (tOSMem_t *)(void *)&gpOSMemBegin[ptr];
if ((!ptOSMemTemp->Used) && (ptOSMemTemp->NextMem - (ptr + SIZEOF_OSMEM_ALIGNED)) >= size)
{
// ptOSMemTemp is not Used and at least perfect fit is possible:
// ptOSMemTemp->NextMem - (ptr + SIZEOF_OSMEM_ALIGNED) gives us the 'user data size' of ptOSMemTemp
if (ptOSMemTemp->NextMem - (ptr + SIZEOF_OSMEM_ALIGNED) >= (size + SIZEOF_OSMEM_ALIGNED + OSMIN_SIZE_ALIGNED))
{
// (in addition to the above, we test if another tOSMem_t (SIZEOF_OSMEM_ALIGNED) containing
// at least OSMIN_SIZE_ALIGNED of data also fits in the 'user data space' of 'ptOSMemTemp')
// -> split large block, create empty remainder,
// remainder must be large enough to contain OSMIN_SIZE_ALIGNED data: if
// ptOSMemTemp->NextMem - (ptr + (2*SIZEOF_OSMEM_ALIGNED)) == size,
// tOSMem_t would fit in but no data between ptOSMemTemp2 and ptOSMemTemp2->NextMem
ptr2 = ptr + SIZEOF_OSMEM_ALIGNED + size;
// create ptOSMemTemp2 struct
ptOSMemTemp2 = (tOSMem_t *)(void *)&gpOSMemBegin[ptr2];
ptOSMemTemp2->Used = 0;
ptOSMemTemp2->NextMem = ptOSMemTemp->NextMem;
ptOSMemTemp2->PrevMem = ptr;
// and insert it between ptOSMemTemp and ptOSMemTemp->NextMem
ptOSMemTemp->NextMem = ptr2;
ptOSMemTemp->Used = 1;
if (ptOSMemTemp2->NextMem != OSMEM_SIZE_ALIGNED)
{
((tOSMem_t *)(void *)&gpOSMemBegin[ptOSMemTemp2->NextMem])->PrevMem = ptr2;
}
}
else
{
// (a ptOSMemTemp2 struct does no fit into the user data space of ptOSMemTemp and ptOSMemTemp->NextMem will always
// be Used at this point: if not we have 2 unused structs in a row, OSMemCombine should have
// take care of this).
// -> near fit or excact fit: do not split, no ptOSMemTemp2 creation
// also can't move ptOSMemTemp->NextMem directly behind ptOSMemTemp, since ptOSMemTemp->NextMem
// will always be Used at this point!
ptOSMemTemp->Used = 1;
}
if (ptOSMemTemp == gpOSMemLFree)
{
// Find next free block after ptOSMemTemp and update lowest free pointer
while (gpOSMemLFree->Used && gpOSMemLFree != gpOSMemEnd)
{
gpOSMemLFree = (tOSMem_t *)(void *)&gpOSMemBegin[gpOSMemLFree->NextMem];
}
}
pResult = (uOS8_t *)ptOSMemTemp + SIZEOF_OSMEM_ALIGNED;
break;
}
}
OSIntUnlock();
return pResult;
}
内存释放函数。
/*****************************************************************************
Function : OSMemFree
Description : Put a tOSMem_t back on the heap.
Input : pMem -- the data portion of a tOSMem_t as returned by a previous
call to OSMemMalloc()
Output : None
Return : None
*****************************************************************************/
void OSMemFree(void *pMem)
{
tOSMem_t *ptOSMemTemp;
if (pMem == OS_NULL)
{
return;
}
if ((uOS8_t *)pMem < (uOS8_t *)gpOSMemBegin || (uOS8_t *)pMem >= (uOS8_t *)gpOSMemEnd)
{
return;
}
// protect the heap from concurrent access
OSIntLock();
// Get the corresponding tOSMem_t ...
ptOSMemTemp = (tOSMem_t *)(void *)((uOS8_t *)pMem - SIZEOF_OSMEM_ALIGNED);
//ptOSMemTemp->Used must be 1
if( ptOSMemTemp->Used==1 )
{
// now set it unused.
ptOSMemTemp->Used = 0;
if (ptOSMemTemp < gpOSMemLFree)
{
// the newly freed struct is now the lowest
gpOSMemLFree = ptOSMemTemp;
}
// finally, see if prev or next are free also
OSMemCombine(ptOSMemTemp);
}
OSIntUnlock();
return;
}
代码已经更新到Github上了，感兴趣的朋友请移步到Github查看更多源代码。
到现在，我们已经实现了上面提到的内存管理方面的第3和第4两项要求了。这样，嵌入式实时操作系统AIOS的内存管理已经基本实现了；
由于大部分的微控制器芯片资源非常有限，部分嵌入式操作系统在设计时没有单独的内存管理模块，而是在使用时临时分配。例如ucos嵌入式操作系统，在创建任务时，总是通过一个数组的形式分配一段内存空间，然后把数组的首地址及长度传递给待创建的任务使用。这样相当于把内存管理分散化，但是如果要整体设置系统的内存位置就不那么方便了。
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