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Interrupts are used to demand attention from the CPU. There are processor, hardware and software driven interrupts. The PC was designed as an interrupt driven system. Communications between the CPU, hardware and software occurs by means of of flags or interrupts which signal when a device or piece of software needs attention from the CPU. The PC is limited to 256 interrupts each of which receives a number. The Programmable Interrupt Controller (PIC) controls which interrupt gets the attention of the CPU and when. Basically, the lower numbered interrupts receive attention from the CPU before the higher numbered interrupts. The PC interrupts can be divided into six categories:

1. Processor Interrupts - Interrupts 00h to 07h are called by the processor directly, but can also be called from software using the INT instruction.

2. Hardware Interrupts - The hardware interrupts differ from all the software interrupts in that they have a direct channel to the processor thorough an Interrupt Request Line or IRQ. There are 16 IRQ lines on PC's (there are 8 IRQ lines on 8086/8088 based computers). These lines are hardwired on the motherboard and directly access the processor. The 8259 Programmable Interrupt Controller or PIC on the motherboard manages all the hardware interrupts. These controllers receive the requests from the various devices and pass the request along to the processor after converting them to specific instructions.

This allows a device or subsystem to work in the background until a particular event occurs that demands attention, like a keystroke or sound. An interrupt is created by asserting a logic level on an Interrupt Request (IRQ) line. This lines are accessed from any of the computers bus slots. An XT computer had 8 IRQ's, while the AT provides 16. These lines are controlled through the Programmable Interrupt Controller which in turn triggers the desired interrupt request to the CPU. Below is a listing of the hardware interrupts for a PC compatible computer:

IRQ
Interrupt
Function
0
08h
System timer IC
1
09h
Keyboard controller IC
2
0Ah
Second IRQ controller IC
3
0Bh
Serial port 2 (COM 2: 2F8h-2FFh and COM 4: 2E8h-2EFh) or hardware modem use
4
0Ch
Serial port 1 (COM 1: 3F8h-3FFh and COM 3: 3E8h-3EFh) or serial port mouse use
5
0Dh
Parallel port 2 (LPT 2: 378h or 278h) or general adapter use (typically used for sound cards)
6
0Eh
Floppy disk controller
7
0Fh
Parallel port 1 (LPT 1: 3BCh [mono] or 378h [color]
8
70h
Real time clock or RTC
9
71h
Unused (redirected to IRQ 2)
10
72h
USB on systems so equipped (can be disabled) or general adapter use
11
73h
Windows sound system on systems so equipped (can be disabled) or general adapter use
12
74h
Motherboard mouse port on systems so equipped or general adapter use
13
75h
Math coprocessor
14
76h
Primary AT/IDE hard disk controller
15
77h
Secondary AT/IDE hard disk controller on systems so equipped (can be disabled) or general adapter use

An AT systems uses IRQ 2 on the motherboard so instead of leaving it empty, IRQ 9 now is wired to this IRQ, in effect, cascading, or redirecting, the IRQ. So IRQ 9 is now redirected to IRQ 2. When a interrupt request is made, the CPU saves the current state of registers in a small amount of memory called the stack, then the CPU is directed to the interrupt vector table, which is a list of program locations that represent each interrupt. Here, the CPU locates the interrupt handler for the desired interrupt request and executes the routine. Once the desires task is finished, the prior register contents are popped from the stack they are being held in and the CPU will continue with it's task it was performing prior to the interrupt.

3. Software Interrupts - These interrupts are generated by the ROM BIOS during the start up of the computer. These interrupts are used for general low-level services.

4. DOS Interrupts - These interrupts are available when DOS is running and provide additional routines for enhanced access to devices and other resources.

5. ROM Basic Interrupts - These interrupts are available when Basic is running.

6. General use Interrupts - These interrupts are available for use by other programs.

Interrupt Vector Table - When power is applied to a computer, the POST procedure creates a table of interrupt vectors that is 1024 bytes and contains a maximum of 256 interrupts. This table lists pointers to interrupt service routines. The interrupt vector table starts at memory location 0000:0000h and ends at 0000:03FCh. An interrupt vector is a 4-byte value of the form offset;segment, which represents the address of a routine to be called when the CPU receives an interrupt. The interrupt vector table is first initialized by the start up ROM but changes are made to it's contents as the first ROM Extensions and later the operating system files are loaded. The ability to update the contents of the interrupt vector table provides a means to easily expand operating system services.

 

Interrupt
Address
Type
Function
00h
0000:0000h
Processor
Divide By Zero
01h
0000:0004h
Processor
Single Step
02h
0000:0008h
Processor
Nonmaskable Interrupt (NMI)
03h
0000:000Ch
Processor
Breakpoint Instruction
04h
0000:0010h
Processor
Overflow Instruction
05h
0000:0014h
BIOS/Software
Print Screen
05h
0000:0014h
Hardware
Bounds Exception (80286, 80386)
06h
0000:0018h
Hardware
Invalid Op Code (80286, 80386)
07h
0000:001Ch
Hardware
Math Coprocessor Not Present
08h
0000:0020h
Hardware
Double Exception Error (80286, 80386) (AT Only)
08h
0000:0020h
Hardware
System Timer - IRQ 0
09h
0000:0024h
Hardware
Keyboard - IRQ 1
09h
0000:0024h
Hardware
Math Coprocessor Segment Overrun (80286, 80386) (AT Only)
0Ah
0000:0028h
Hardware
IRQ 2 - Cascade from Second programmable Interrupt Controller
0Ah
Hardware
Invalid Task Segment State (80286, 80286) (AT Only)
0Ah
Hardware
IRQ 2 - General Adapter Use (PC Only)
0Bh
0000:002Ch
Hardware
IRQ 3 - Serial Communications (COM 2)
0Bh
Hardware
Segment Not Present (80286, 80386)
0Ch
0000:0030h
Hardware
IRQ 4 - Serial Communications (COM 1)
0Ch
Hardware
Stack Segment Overflow (80286, 80386)
0Dh
0000:0034h
Hardware
Parallel Printer (LPT 2) (AT Only)
0Dh
Hardware
IRQ 5 - Fixed Disk (XT Only)
0Dh
Software
General Protection Fault (80286, 80386)
0Eh
0000:0038h
Software
IRQ 6- Diskette Drive Controller
0Eh
Software
Page Fault (80386 Only)
0Fh
0000:003Ch
Software
IRQ 7 - Parallel printer (LPT 1)
10h
0000:0040h
Software
Video
10h
Software
Numeric Coprocessor Fault (80286, 80386)
11h
0000:0044h
Software
Equipment List
12h
0000:0048h
Software
Memory Size
13h
0000:004Ch
Software
Fixed Disk/ Diskette
14h
0000:0050h
Software
Serial Communication
15h
0000:0054h
Software
System Services
16h
0000:0058h
Software
Keyboard
17h
0000:005Ch
Software
Parallel Printer
18h
0000:0060h
Software
Load ROM Basic (PC Only)
18h
Software
Process Boot Failure (XT, AT)
19h
0000:0064h
Software
Boot Strap Loader
1Ah
0000:0068h
Software
Time of Day
1Bh
0000:006Ch
Software
Keyboard Break
1Ch
0000:0070h
User
User Timer Tick
1Dh
0000:0074h
BIOS Table
Video Parameter Table
1Eh
0000:006Ch
BIOS Table
Diskette parameter table
1Fh
0000:007Ch
User
Video Graphics Characters
20h-3Fh
0000:0080h - 0000:00FCh
Software
Reserved for DOS
40h
0000:0100h
Software
Diskette Boot Revector
41h
0000:0104h
BIOS Table
Fixed Disk Parameter Table
42h
0000:0108h
BIOS Table
EGA Default Video Driver
43h
0000:010Ch
User
Video Graphics Characters
44h - 45h
0000:0110h - 0000:0114h
Software
Reserved
46h
0000:0118h
BIOS Table
Fixed Disk Parameter Table
47h - 49h
0000:011Ch - 0000:0124h
Software
Reserved
4Ah
0000:0128h
User
User Alarm
4Bh - 6Fh
0000:012Ch - 0000:01BCh
Software
Reserved
70h
0000:01C0h
Hardware
IRQ 8 - Real Time Clock
71h
0000:01C4h
Hardware
IRQ 9 - Redirect Cascade
72h
0000:01C8h
Hardware
IRQ 10 - General Adapter Use
73h
0000:01CCh
Hardware
IRQ 11 - General Adapter Use
74h
0000:01D0h
Hardware
IRQ 12 - General Adapter Use/ PS/2 Mouse
75h
0000:01D4h
Hardware
IRQ 13 - Math Coprocessor Exception
76h
0000:01D8h
Hardware
IRQ 14 - Primary Hard Disk Controller
77h
0000:01DCh
Hardware
IRQ 15 - Secondary Hard Disk Controller / General Adapter Use

 


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