8259A
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The 8259A is an improved version of the 8259 programmable interrupt controller. Both provide eight IRQ (interrupt request) lines, numbered 0–7. The device was originally designed by Intel and meant to work with the 8-bit Intel 8085 and 16-bit Intel 8086 microprocessors. NEC apparently created the 8259A, although the design has been widely duplicated. The 8259(A) was used in the original IBM PC, and provided the standard used for the next 20 years (and beyond). In 1984, a second controller, commonly called the 8259B to differentiate it, was added in the IBM AT, extending the ISA bus architecture. The IRQ2 pins on each chip were connected together, so a device on that bus wire could be configured as either operating on IRQ2 or IRQ9 (the second set of IRQ lines were numbered 8–15).
This convoluted arrangement finally began to disappear in the late 1990s and early 2000s, initially on multiprocessor systems. It has been succeeded by Intel's IO-APIC (advanced programmable interrupt controller) standard for PC compatible systems, though IO-APIC systems still emulate an 8259 environment by default and retain a certain level of compatibility while in advanced mode. Non-PC compatible systems never had this arrangement.
IRQs in the PC AT environment
- IRQ0 – 8253/8254 interval timer (System Timer)
- IRQ1 – keyboard
- IRQ2 – reserved for the 8259B (Tied to IRQ 9)
- IRQ3 – COM2 and COM4
- IRQ4 – COM1 and COM3
- IRQ5 – LPT2
- IRQ6 – Floppy controller
- IRQ7 – LPT1
- IRQ8 – Real-time clock (RTC)
- IRQ9 – Bridged to IRQ2
- IRQ10 – Not Assigned
- IRQ11 – Not Assigned
- IRQ12 – PS/2 mouse
- IRQ13 – Math coprocessor
- IRQ14 – Primary IDE controller
- IRQ15 – Secondary IDE controller
IRQ7 was initially a common choice for use of a sound card, but it was found that this would interfere with the printer port (LPT1), so IRQ5 later became the most prevalent choice. Serial ports (COMx) could frequently be disabled in order to make another port available. However, the common literature which says "COM1 and COM3" (for instance) turned out to be rather misleading, as it was nearly impossible to operate two serial ports on a single IRQ.
IRQ sharing became more practical on PCI-based systems, but it could still be problematic in some cases. This resulted in a few interrupts being very heavily used, while others would see very little use. In modern systems, IRQ selection is completely automatic, and users can only make manual alterations on some systems (many manufacturers hide such configuration options).
External links
- 8259(A) Programmable Interrupt Controller (PIC) (http://www.xs4all.nl/~ganswijk/chipdir/reg/8259.txt)
- 8259 Programmable Interrupt Controller (http://satyap.csoft.net/8259.html)