编写程序：利用串口采用DMA方式发送数据到主存，统计发送数据中包含的字符数

以下是一个基于STM32的例子程序，可以实现串口采用DMA方式发送数据到主存并统计发送数据中包含的字符数：

#include "stm32f4xx.h"

#define BUFFER_SIZE 100

uint8_t buffer[BUFFER_SIZE];
uint32_t count = 0;

void DMA2_Stream7_IRQHandler(void)
{
    if (DMA_GetITStatus(DMA2_Stream7, DMA_IT_TCIF7) != RESET)
    {
        DMA_ClearITPendingBit(DMA2_Stream7, DMA_IT_TCIF7);
        count = 0;
        for (int i = 0; i < BUFFER_SIZE; i++)
        {
            if (buffer[i] != 0)
                count++;
            else
                break;
        }
    }
}

int main(void)
{
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2 | RCC_AHB1Periph_GPIOA, ENABLE);
    RCC_APB1PeriphClockCmd(RCC_APB1Periph_USART2, ENABLE);

    GPIO_InitTypeDef GPIO_InitStruct;
    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_2 | GPIO_Pin_3;
    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOA, &GPIO_InitStruct);

    GPIO_PinAFConfig(GPIOA, GPIO_PinSource2, GPIO_AF_USART2);
    GPIO_PinAFConfig(GPIOA, GPIO_PinSource3, GPIO_AF_USART2);

    USART_InitTypeDef USART_InitStruct;
    USART_InitStruct.USART_BaudRate = 115200;
    USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
    USART_InitStruct.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
    USART_InitStruct.USART_Parity = USART_Parity_No;
    USART_InitStruct.USART_StopBits = USART_StopBits_1;
    USART_InitStruct.USART_WordLength = USART_WordLength_8b;
    USART_Init(USART2, &USART_InitStruct);

    USART_Cmd(USART2, ENABLE);

    NVIC_InitTypeDef NVIC_InitStruct;
    NVIC_InitStruct.NVIC_IRQChannel = DMA2_Stream7_IRQn;
    NVIC_InitStruct.NVIC_IRQChannelCmd = ENABLE;
    NVIC_InitStruct.NVIC_IRQChannelPreemptionPriority = 0;
    NVIC_InitStruct.NVIC_IRQChannelSubPriority = 0;
    NVIC_Init(&NVIC_InitStruct);

    DMA_InitTypeDef DMA_InitStruct;
    DMA_InitStruct.DMA_Channel = DMA_Channel_4;
    DMA_InitStruct.DMA_DIR = DMA_DIR_PeripheralToMemory;
    DMA_InitStruct.DMA_Memory0BaseAddr = (uint32_t)buffer;
    DMA_InitStruct.DMA_BufferSize = BUFFER_SIZE;
    DMA_InitStruct.DMA_PeripheralBaseAddr = (uint32_t)&USART2->DR;
    DMA_InitStruct.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
    DMA_InitStruct.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
    DMA_InitStruct.DMA_Mode = DMA_Mode_Normal;
    DMA_InitStruct.DMA_Priority = DMA_Priority_VeryHigh;
    DMA_InitStruct.DMA_FIFOMode = DMA_FIFOMode_Disable;
    DMA_InitStruct.DMA_MemoryBurst = DMA_MemoryBurst_Single;
    DMA_InitStruct.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;
    DMA_Init(DMA2_Stream7, &DMA_InitStruct);

    DMA_ITConfig(DMA2_Stream7, DMA_IT_TC, ENABLE);

    while (1)
    {
        USART_SendData(USART2, 'a');
        while (USART_GetFlagStatus(USART2, USART_FLAG_TXE) == RESET);
        DMA_Cmd(DMA2_Stream7, ENABLE);
        while (DMA_GetFlagStatus(DMA2_Stream7, DMA_FLAG_TCIF7) == RESET);
        DMA_Cmd(DMA2_Stream7, DISABLE);
    }
}

程序的基本思路是：在主循环中，通过USART_SendData函数向串口发送数据，然后启动DMA传输，将接收到的数据存储到buffer数组中，当DMA传输完成时，遍历buffer数组，统计其中不为0的元素个数，即为发送数据中包含的字符数。注意，由于使用了DMA传输，因此在中断服务函数中需要清除DMA_IT_TCIF7中断标志位，并在主循环中使用DMA_GetFlagStatus函数检查DMA传输是否完成。

需要注意的是，具体的代码实现可能会因为不同的芯片和开发环境而有所不同。此外，在实际应用中，还需要考虑数据传输的正确性和可靠性，以及串口和DMA的配置和使用方法等问题。