中断优先级
AXI Interrupt Controller支持中断优先级。 在Vivado Block Design中， bit-0连接的中断优先级最高， 越靠近bit-0的中断优先级最高。
AXI Interrupt Controller的手册pg099中的描述如下：
代码语言：javascript复制Priority between interrupt requests is determined by vector position. The least significant bit (LSB, in this case bit 0) has the highest priority.
Intr(0) is always the highest priority interrupt and each successive bit to the left has a corresponding lower interrupt priority.中断嵌套
要使用中断优先级，通常也要使能中断嵌套（Nested Interrupts）。在Vivado Block Design中， 配置AXI Interrupt Controller时，在“advanced”选项中，要选择“Interrupt Level Register” 。
AXI Interrupt Controller的手册pg099中的描述如下：
代码语言：javascript复制Nested Interrupts
The core provides support for nested interrupts, by implementing an Interrupt Level
Register. This can be used by software to prevent lower priority interrupts from occurring
when handling an interrupt, thus allowing interrupts to be enabled during interrupt
handling to immediately take a higher priority interrupt. Software must save and restore
the Interrupt Level Register and return address.
Because the processor jumps directly to the unique Interrupt vector address to service a
particular interrupt when using fast interrupt mode, the user interrupt service routine code itself must save and restore the Interrupt Level Register and Return Address in this case. In
normal interrupt mode, this is handled by the software driver.选择“Interrupt Level Register” 后，中断处理代码会保存和恢复r14，并且在进入设备的ISR之前会使能中断。 具体代码请查看XIntc_DeviceInterruptHandler( )。
Xintc.h中的说明如下：
代码语言：javascript复制For nested interrupts, XIntc_DeviceInterruptHandler saves
microblaze r14 register on entry and restores on exit. This is
required since compiler does not support nesting. It enables
Microblaze interrupts after blocking further interrupts from
the current interrupt number and interrupts below current
interrupt priority by writing to Interrupt Level Register of
INTC on entry. On exit, it disables microblaze interrupts and
restores ILR register default value(0xFFFFFFFF)back. It is
recommended to increase STACK_SIZE in linker script for nested
interrupts.
