BENTLY 3500/62過程變量監測器
在IBM PC/XT系統中，只有八條中斷請求（IRQ）線，編號從IRQ0到IRQ7。這些中斷線最初包含在8259A優先級中斷控制器（PIC）芯片上。IBM PC/AT計算機通過將第二個從屬8259APIC級聯到原始主8259APIC中，增加了八條IRQx線，編號為IRQ8到IRQ15。主PIC處的中斷線IRQ2作為從PIC的級聯輸入。圖中PCI到ISA橋PIIX4E 82371EB部分第121頁的圖E-2說明了這種主/從架構（標準PC/AT中斷映射）。為了保持與PC/XT系統的向后兼容性，IBM選擇在從屬PIC上使用新的IRQ9輸入作為PC/XT擴展總線上的舊IRQ2中斷線。因此，在AT系統中，IRQ9中斷線連接到AT擴展總線（或ISA總線）上的舊IRQ2引腳。
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).
