Whatever their cause, interrupts are essentially requests for attention. In the same way, peripherals in a computer system can request the attention of the processor. The event that makes a microprocessor stop executing one routine to perform some other routine to service a request, is called an INTERRUPT.
Interrupts increase the overall efficiency of a computer system, because the external devices request the attention of the processor as needed. If a system had no interrupts, the processor would have to POLL every device in the system periodically, to see if any of them required attention. Many of the standard I/O provisions of the PC computer generate interrupts when they need the processor to process data being received by, or being sent by, those interfaces.
The processor can process two kinds of interrupts, those it can ignore and those it must service as soon as they occur. Interrupts can be generated by the interfaces to DEVICES, such as disk drives and the parallel or serial ports, or internally, by interrupt-generating machine code INSTRUCTIONS.
The 8086 family of microprocessors can recognise 256 different interrupts, each with a unique TYPE CODE (number) that identifies it to the microprocessor. The processor uses this type code (a number between 0 hex and FF hex) to point to a location in an INTERRUPT VECTOR TABLE. This table is stored in the first 1024 bytes of the memory space, starting at address 00000 hex, (0000:0000 hex in Segment and Offset format).
The Interrupt Vector Table is in the first 1024 bytes of the processors address-able memory space. The POST routines in the BIOS set up an Interrupt Vector Table at the time of boot-up with the addresses of the routines supplied by the code in the BIOS ROM and DOS and our Application programs add there vectors to this table as they are loaded.
The original IBM XT computer had one 8259 device and this provided eight IRQ inputs. The first two of these IRQ lines is wired to functions on the system board and the rest are available on the Bus Connector for use by external devices.
8 bit bus computers (8086/8088 based XT type)
IRQ Used for 0 RAM REFRESH & CLOCK TICK 1 KEYBOARD INTERFACE 2 EGA VIDEO (some interface cards have the option of using an IRQ) 3 COM2 (also COM4) 4 COM1 (also COM3) 5 HARD DISK (PC/XT type computer only) 6 FLOPPY DISK 7 PARALLEL PRINTER PORT LPT 1
ISA Bus (and MCA, EISA) computers (286, 386 and 486 based computers)
The AT computer added a second 8259 device and this provided a further seven IRQ inputs. The second device is cascaded into one of the inputs (IRQ2) on the first device and the Bus connector for the IRQ2 input is wired to IRQ9 on the second 8259 chip.
IRQ Used for 0 System timer 1 Keyboard Interface 2 Cascade for second IRQ controller - BUS connection is to IRQ 9. 3 Serial port, COM2 (may also be COM4) 4 Serial Port COM1 (may also be COM3) 5 Parallel Port LPT2, also the default for many Sound Cards (note 1) 6 Floppy Disk Drive Interface 7 Parallel Port LPT1 (note 2) 8 Real Time Clock (RTC) 9 Available (some Video Display Interface cards have the option of using this IRQ) 10 Available (note 3) 11 Available (note 3) 12 PS/2 - type mouse port (most modern system boards have a PS/2 mouse port fitted) 13 Coprocessor 14 Primary IDE/ATA adaptor 15 Secondary IDE/ATA adaptor
Note 1:- As very few systems have a second Parallel Port fitted, and a Parallel Port does not usually need it's IRQ, it is usually safe to assign IRQ 5 to a Sound Card or a Network Interface card. Along with IRQ10, this is a common default for Sound Cards.
Note 2:- A Parallel Port does not usually need it's IRQ, so IRQ7 can be used for other uses (note 3).
Note 3:- These IRQ lines can be used for:
Tertiary IDE adaptor at I/O address 01E8 to 01EF and using IRQ 11 or 12
Quaternary adaptor at I/O address 0168 to 016F and using IRQ 10 or 11
Some sound cards have a built-in IDE Interface configured as the Tertiary adaptor.
Examine the list of Interrupts in the table and pick out the Hardware and Software Interrupts.
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