BENTLY 3500/62过程变量监测器
在IBM PC/XT系统中，只有八条中断请求（IRQ）线，编号从IRQ0到IRQ7。这些中断线最初包含在8259A优先级中断控制器（PIC）芯片上。IBM PC/AT计算机通过将第二个从属8259APIC级联到原始主8259APIC中，增加了八条IRQx线，编号为IRQ8到IRQ15。主PIC处的中断线IRQ2作为从PIC的级联输入。图中PCI到ISA桥PIIX4E 82371EB部分第121页的图E-2说明了这种主/从架构（标准PC/AT中断映射）。为了保持与PC/XT系统的向后兼容性，IBM选择在从属PIC上使用新的IRQ9输入作为PC/XT扩展总线上的旧IRQ2中断线。因此，在AT系统中，IRQ9中断线连接到AT扩展总线（或ISA总线）上的旧IRQ2引脚。
In the IBM PC/XT systems, only eight Interrupt Request (IRQ) lines exist, numbered from IRQ0 to IRQ7. These interrupt lines were included originally on a 8259A Priority Interrupt Controller (PIC) chip. The IBM PC/AT computer added eight more IRQx lines, numbered IRQ8 to IRQ15, by cascading a second slave 8259A PIC into the original master 8259A PIC. The interrupt line IRQ2 at the master PIC was committed as the cascade input from the slave PIC. This master/slave architecture, the standard PC/AT interrupt mapping, is illustrated in Figure E-2 on page 121 within the PCI-to-ISA Bridge PIIX4E 82371EB section of the diagram. 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 on the AT Expansion Bus (or ISA bus).
