Intel 8259 Programmable Interrupt Controller

Intel 8259 Programmable Interrupt Controller

The chip responsible for handling interrupt requests in the PC is the Intel 8259 Programmable Interrupt Controller. To use interrupts, you need to know how to read and set the Intel 8259’s interrupt mask register (IMR) and how to send the end-of-interrupt (EOI) command to the Intel 8259.

Each bit in the IMR contains the mask status of an IRQ line; bit 0 is for IRQ0, bit 1 is for IRQ1, and so on. If a bit is set (1), then the corresponding IRQ is masked and will not generate an interrupt. If a bit is clear (0), then the corresponding IRQ is unmasked and can generate interrupts. The IMR is programmed through port 21h.

Note When in APIC mode, the PIC is programmed differently, and IRQ routing behaves differently. For more information, refer to the APIC datasheets and specifications provided by Intel.

PCI Interrupts

PCI devices can share interrupts. The BIOS or operating system may assign multiple PCI devices to the same IRQ line. Any interrupt service routine (ISR) written for PCI devices must be able to handle shared interrupts. Refer to Interrupt-Driven PC System Design (ISBN: 0-929392-50-7) for more information on PCI interrupts.

Writing an Interrupt Service Routine (ISR)

The first step in adding interrupts to your software is to write the ISR. This is the routine that will automatically be executed each time an interrupt request occurs on the specified IRQ. An ISR is different than standard routines that you write. First, on entrance, the processor registers should be pushed onto the stack BEFORE you do anything else. Second, just before exiting your ISR, you must clear the interrupt status flag and write an end-of-interrupt command to the Intel 8259 controller. Finally, when exiting the ISR, in addition to popping all the registers you pushed on entrance, you must use the IRET instruction and not a plain RET. The IRET automatically pops the flags, CS, and IP that were pushed when the interrupt was called.

Most C compilers allow you to identify a procedure (function) as an interrupt type and will automatically add these instructions to your ISR, with one important exception: most compilers do not automatically add the end-of-interrupt command to the procedure; you must do this yourself. Other than this and the few exceptions discussed below, you can write your ISR just like any other routine. It can call other functions and procedures in your program and it can access global data. If you are writing your first ISR, RTD recommends focusing on the basics, such as incrementing a global variable.

Most operating systems have restrictions on what instructions can be called in your ISR. Consult your OS documentation for details on writing your ISR.

Note A complete explanation of interrupt programming is beyond the scope of this manual. For more information on interrupts, refer to the Appendix.

Sample Code

RTD’s drivers provide examples of ISR’s and interrupt handling. Refer to them as working examples. These drivers were shipped with your cpuModule, but they they can also be downloaded from RTD’s website (www.rtd.com)..

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