void vPortDefineHeapRegions( const HeapRegion_t * const pxHeapRegions )
{
    BlockLink_t * pxFirstFreeBlockInRegion = NULL, * pxPreviousFreeBlock;
    size_t xAlignedHeap;
    size_t xTotalRegionSize, xTotalHeapSize = 0;
    BaseType_t xDefinedRegions = 0;
    size_t xAddress;
    const HeapRegion_t * pxHeapRegion;

    /* Can only call once! */
    configASSERT( pxEnd == NULL );

    pxHeapRegion = &( pxHeapRegions[ xDefinedRegions ] );

    while( pxHeapRegion->xSizeInBytes > 0 )
    {
        xTotalRegionSize = pxHeapRegion->xSizeInBytes;

        /* Ensure the heap region starts on a correctly aligned boundary. */
        xAddress = ( size_t ) pxHeapRegion->pucStartAddress;

        if( ( xAddress & portBYTE_ALIGNMENT_MASK ) != 0 )
        {
            xAddress += ( portBYTE_ALIGNMENT - 1 );
            xAddress &= ~portBYTE_ALIGNMENT_MASK;

            /* Adjust the size for the bytes lost to alignment. */
            xTotalRegionSize -= xAddress - ( size_t ) pxHeapRegion->pucStartAddress;
        }

        xAlignedHeap = xAddress;

        /* Set xStart if it has not already been set. */
        if( xDefinedRegions == 0 )
        {
            /* xStart is used to hold a pointer to the first item in the list of
             *  free blocks.  The void cast is used to prevent compiler warnings. */
            xStart.pxNextFreeBlock = ( BlockLink_t * ) xAlignedHeap;
            xStart.xBlockSize = ( size_t ) 0;
        }
        else
        {
            /* Should only get here if one region has already been added to the
             * heap. */
            configASSERT( pxEnd != NULL );

            /* Check blocks are passed in with increasing start addresses. */
            configASSERT( xAddress > ( size_t ) pxEnd );
        }

        /* Remember the location of the end marker in the previous region, if
         * any. */
        pxPreviousFreeBlock = pxEnd;

        /* pxEnd is used to mark the end of the list of free blocks and is
         * inserted at the end of the region space. */
        xAddress = xAlignedHeap + xTotalRegionSize;
        xAddress -= xHeapStructSize;
        xAddress &= ~portBYTE_ALIGNMENT_MASK;
        pxEnd = ( BlockLink_t * ) xAddress;
        pxEnd->xBlockSize = 0;
        pxEnd->pxNextFreeBlock = NULL;

        /* To start with there is a single free block in this region that is
         * sized to take up the entire heap region minus the space taken by the
         * free block structure. */
        pxFirstFreeBlockInRegion = ( BlockLink_t * ) xAlignedHeap;
        pxFirstFreeBlockInRegion->xBlockSize = xAddress - ( size_t ) pxFirstFreeBlockInRegion;
        pxFirstFreeBlockInRegion->pxNextFreeBlock = pxEnd;

        /* If this is not the first region that makes up the entire heap space
         * then link the previous region to this region. */
        if( pxPreviousFreeBlock != NULL )
        {
            pxPreviousFreeBlock->pxNextFreeBlock = pxFirstFreeBlockInRegion;
        }

        xTotalHeapSize += pxFirstFreeBlockInRegion->xBlockSize;

        /* Move onto the next HeapRegion_t structure. */
        xDefinedRegions++;
        pxHeapRegion = &( pxHeapRegions[ xDefinedRegions ] );
    }

    xMinimumEverFreeBytesRemaining = xTotalHeapSize;
    xFreeBytesRemaining = xTotalHeapSize;

    /* Check something was actually defined before it is accessed. */
    configASSERT( xTotalHeapSize );

    /* Work out the position of the top bit in a size_t variable. */
    xBlockAllocatedBit = ( ( size_t ) 1 ) << ( ( sizeof( size_t ) * heapBITS_PER_BYTE ) - 1 );
}