the IBM PC/XT computers, only eight interrupt request lines exist, numbered from
IRQ0 to IRQ7 at the PIC. The IBM PC/AT computer added eight more IRQx lines,
numbered IRQ8 to IRQ15, by cascading a second slave PIC into the original master
PIC. IRQ2 at the master PIC was committed as the cascade input from the slave PIC.
This architecture is represented in Figure 2-1 on page 51.
To maintain backward compatibility with PC/XT systems, IBM chose to use the new
IRQ9 input on the slave PIC to operate as the old IRQ2 interrupt line on the PC/XT
Expansion Bus. Thus, in AT systems, the IRQ9 interrupt line connects to the old IRQ2
pin (pin B4) on the AT Expansion Bus (or ISA bus).

Table 2-3 PC Hardware Interrupt Line Assignments
IRQ AT FUNCTION COMMENTS
NMI Parity Errors
(Must be enabled in BIOS Setup)

在IBM PC/XT计算机中，只有八条中断请求行，从
图片上的IRQ0到IRQ7。IBM PC/AT计算机又增加了八条IRQx线路，
通过将第二个从属图片级联到原始主图片中，将IRQ8编号为IRQ15
照片。主PIC上的IRQ2作为从PIC的级联输入提交。
该体系结构如第51页图2-1所示。
为了保持与PC/XT系统的向后兼容性，IBM选择使用新的
从PIC上的IRQ9输入作为PC/XT上的旧IRQ2中断线运行
扩展总线。因此，在AT系统中，IRQ9中断线连接到旧的IRQ2
AT扩展总线（或ISA总线）上的引脚（引脚B4）。
表2-3 PC硬件中断线分配
函数注释处的IRQ
NMI奇偶校验错误
（必须在BIOS设置中启用）