用rt_memheap_init分配堆内存初始化失败是何原因？怎么解决？

然后在初始化的时候先调用
rt_system_heap_init((void *)HEAP_BEGIN, (void *)HEAP_END);
初始化内部SRAM，然后调用
rt_memheap_init(&system_heap,"sdram",(void *)0x68000000,0x100000);
将外部SDRAM挂载到heap_map里面。编译下载后，就可以用rt_malloc等接口了。
FSMC_SRAM_Init()如下：
void FSMC_SRAM_Init(void)
{
    /*-- GPIOs Configuration -----------------------------------------------------*/
    /* Enable GPIOD, GPIOE, GPIOF and GPIOG interface clock */
    RCC->AHB1ENR   |= 0x00000078;
    //PD0,1,4,5,8~15    100M AF OUT_PP UP
    //PE0,1,7~15        100M AF OUT_PP UP
    //PF0~5,12~15       100M AF OUT_PP UP
    //PG0~5,10          100M AF OUT_PP UP
    /* Connect PDx pins to FMC Alternate function */
    GPIOD->AFR[0]  = 0x00CC00CC;
    GPIOD->AFR[1]  = 0xCCCCCCCC;
    /* Configure PDx pins in Alternate function mode */  
    GPIOD->MODER   = 0xAAAA0A0A;
    /* Configure PDx pins speed to 100 MHz */  
    GPIOD->OSPEEDR = 0xFFFF0F0F;
    /* Configure PDx pins Output type to push-pull */  
    GPIOD->OTYPER  = 0x00000000;
    /* pull-up for PDx pins */
    GPIOD->PUPDR   = 0x55550505;
    /* Connect PEx pins to FMC Alternate function */
    GPIOE->AFR[0]  = 0xC00000CC;
    GPIOE->AFR[1]  = 0xCCCCCCCC;
    /* Configure PEx pins in Alternate function mode */
    GPIOE->MODER   = 0xAAAA800A;
    /* Configure PEx pins speed to 100 MHz */
    GPIOE->OSPEEDR = 0xFFFFC00F;
    /* Configure PEx pins Output type to push-pull */  
    GPIOE->OTYPER  = 0x00000000;
    /* pull-up for PEx pins */
    GPIOE->PUPDR   = 0x55554005;
    /* Connect PFx pins to FMC Alternate function */
    GPIOF->AFR[0]  = 0x00CCCCCC;
    GPIOF->AFR[1]  = 0xCCCC0000;
    /* Configure PFx pins in Alternate function mode */   
    GPIOF->MODER   = 0xAA000AAA;
    /* Configure PFx pins speed to 100 MHz */
    GPIOF->OSPEEDR = 0xFF000FFF;
    /* Configure PFx pins Output type to push-pull */  
    GPIOF->OTYPER  = 0x00000000;
    /* pull-up for PFx pins */
    GPIOF->PUPDR   = 0x55000555;
    /* Connect PGx pins to FMC Alternate function */
    GPIOG->AFR[0]  = 0x00CCCCCC;
    GPIOG->AFR[1]  = 0x00000C00;
    /* Configure PGx pins in Alternate function mode */
    GPIOG->MODER   = 0x00200AAA;
    /* Configure PGx pins speed to 100 MHz */
    GPIOG->OSPEEDR = 0x00300FFF;
    /* Configure PGx pins Output type to push-pull */  
    GPIOG->OTYPER  = 0x00000000;
    /* pull-up for PGx pins */
    GPIOG->PUPDR   = 0x00100555;
    /*-- FMC/FSMC Configuration --------------------------------------------------*/
    /* Enable the FMC/FSMC interface clock */
    RCC->AHB3ENR         |= 0x00000001;
    //寄存器清零
    //bank1有NE1~4,每一个有一个BCR+TCR，所以总共八个寄存器。
    //这里我们使用NE3 ，也就对应BTCR[4],[5]。        
    FSMC_Bank1->BTCR[4]=0X00000000;
    FSMC_Bank1->BTCR[5]=0X00000000;
    FSMC_Bank1E->BWTR[4]=0X00000000;
    //操作BCR寄存器 使用异步模式,模式A(读写共用一个时序寄存器)
    //BTCR[偶数]:BCR寄存器;BTCR[奇数]:BTR寄存器
    FSMC_Bank1->BTCR[4]|=1<<12;//存储器写使能
    FSMC_Bank1->BTCR[4]|=1<<4; //存储器数据宽度为16bit      
    //操作BTR寄存器   （HCLK=168M, 1个HCLK=6ns      
    FSMC_Bank1->BTCR[5]|=8<<8; //数据保持时间（DATAST）为9个HCLK 6*9=54ns   
    FSMC_Bank1->BTCR[5]|=0<<4; //地址保持时间（ADDHLD）未用到     
    FSMC_Bank1->BTCR[5]|=0<<0; //地址建立时间（ADDSET）为0个HCLK 0ns   
    //闪存写时序寄存器  
    FSMC_Bank1E->BWTR[4]=0x0FFFFFFF;//默认值
    //使能BANK1区域3
    FSMC_Bank1->BTCR[4]|=1<<0;
}