不知为什么，STM32的中、小容量启动文件中没有发现UART4，UART5的中断处理函数，也就是在HD以下的版本中是没有UART4、UART5中断的喽？但查了下，中等容量的STM32F103RCT6是有UART4、UART5的，并且支持部份中断，以下摘自库函数（@version V3.5.0）说明：
/**
* @brief Enables or disables the specified USART interrupts.
* @param USARTx: Select the USART or the UART peripheral.
* This parameter can be one of the following values:
* USART1, USART2, USART3, UART4 or UART5.
* @param USART_IT: specifies the USART interrupt sources to be enabled or disabled.
* This parameter can be one of the following values:
* @arg USART_IT_CTS: CTS change interrupt (not available for UART4 and UART5)
* @arg USART_IT_LBD: LIN Break detection interrupt
* @arg USART_IT_TXE: Tansmit Data Register empty interrupt
* @arg USART_IT_TC: Transmission complete interrupt
* @arg USART_IT_RXNE: Receive Data register not empty interrupt
* @arg USART_IT_IDLE: Idle line detection interrupt
* @arg USART_IT_PE: Parity Error interrupt
* @arg USART_IT_ERR: Error interrupt(Frame error, noise error, overrun error)
* @param NewState: new state of the specified USARTx interrupts.
* This parameter can be: ENABLE or DISABLE.
* @retval None
*/
很明显，UART4、UART5是具备产生中断的能力的。那么为何没有相应的中断呢？此点暂不得而知。解决办法也是有的，最简单采用startup_stm32f10x_hd.s即可，不过涉及到内存分配，此处不予讨论。也可在对就容量的启动文件中添加代码：
IMPORT UART4_IRQHandler
IMPORT UART5_IRQHandler
DCD UART4_IRQHandler
DCD UART5_IRQHandler
自此算是基本解决了。
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感谢@mcloud ，此处有一个重大的问题我一直没有注意到！那就是对于STM32启动文件的理解。以下贴上STM32启动文件的相关内容：
小容量产品是指闪存存储器容量在16K至32K字节之间的STM32F101xx、STM32F102xx和STM32F103xx微控制器。
中容量产品是指闪存存储器容量在64K至128K字节之间的STM32F101xx、STM32F102xx和STM32F103xx微控制器。
大容量产品是指闪存存储器容量在256K至512K字节之间的STM32F101xx和STM32F103xx微控制器。
互联型产品是指STM32F105xx和STM32F107xx微控制器。
- startup_stm32f10x_ld_vl.s: for STM32 Low density Value line devices
- startup_stm32f10x_ld.s: for STM32 Low density devices
- startup_stm32f10x_md_vl.s: for STM32 Medium density Value line devices
- startup_stm32f10x_md.s: for STM32 Medium density devices
- startup_stm32f10x_hd.s: for STM32 High density devices
- startup_stm32f10x_xl.s: for STM32 XL density devices
- startup_stm32f10x_cl.s: for STM32 Connectivity line devices
cl：互联型产品，stm32f105/107系列
vl：超值型产品，stm32f100系列
xl：超高密度产品，stm32f101/103系列
ld：低密度产品，FLASH小于64K
md：中等密度产品，FLASH=64 or 128
hd：高密度产品，FLASH大于128
● Low-density devices: STM32F101xx, STM32F102xx and STM32F103xx
microcontrollers where the Flash memory density ranges between 16 and 32 Kbytes.
● Medium-density devices: STM32F101xx, STM32F102xx and STM32F103xx
microcontrollers where the Flash memory density ranges between 64 and 128 Kbytes.
● High-density devices: STM32F101xx and STM32F103xx microcontrollers where the
Flash memory density ranges between 256 and 512 Kbytes.
● XL-density devices: STM32F101xx and STM32F103xx microcontrollers where the
Flash memory density ranges between 512 and 1024 Kbytes.
● Medium-density Low-Power devices: STM32L15xx microcontrollers where the Flash
memory density ranges between 64 and 128 Kbytes.
● Low Power Medium-density Plus devices:STM32L15xx and STM32L162xx
microcontrollers where the Flash memory density is 256 Kbytes.
● Low Power High-density devices: STM32L15xx and STM32L162xx microcontrollers
where the Flash memory density is 384 Kbytes.
那么问题来了，我之所以会有如题的疑惑，乃是因为用着STM32F103RCT6这款芯片，却用的是startup_stm32f10x_md.s的启动文件，那自然是没UART4、UART5的。而STM32F103RCT6实则是256K内存，属于 hd：高密度产品，FLASH大于128 。难怪在看芯片文档时，该芯片是有UART4、UART5的。
来源：oschina
链接：https://my.oschina.net/u/1858860/blog/368156