Hardware and Software Interrupts:

Hardware interrupts:

 The nonmaskable interrupt is generated by en external device, trough a rising edge on

the NMI pin
 An external device, trough a high logic level on the INTR pin (the external device has to
specify the interrupt number).
Software interrupts: Software interrupts (exceptions) using the INT instruction (followed by
the interrupt number (type)).
Interrupt Vector Table:

An "interrupt vector table" (IVT) is a data structure that associates a list of interrupt
handlers with a list of interrupt requests in a table of interrupt vectors. An entry in the
interrupt vector is the address of the interrupt handler. While the concept is common across
processor architectures, each IVT may be implemented in an architecture-specific fashion.
For example, a dispatch table is one method of implementing an interrupt vector table.

The first 1Kbyte of memory of 8086 (00000 to003FF) is set aside as a table for storing the
starting addresses of Interrupt Service Procedures (ISP). Since 4-bytes are required for
storing starting addresses of ISPs, the table can hold 256 Interrupt procedures. The starting
address of an ISP is often called the Interrupt Vector or Interrupt Pointer. Therefore the table
is referred as Interrupt Vector Table. In this table, IP value is put in as low word of the vector
& CS is put in high vector.

Dedicated interrupts of 8086:

The following are the various types of interrupts:

- Type 0 interrupts: This interrupt is also known as the divide by zero interrupt. For cases
where the quotient becomes particularly large to be placed / adjusted an error might occur.
- Type 1 interrupts: This is also known as the single step interrupt. This type of interrupt is
primarily used for debugging purposes in assembly language.
- Type 2 interrupts: also known as the non-maskable NMI interrupts. These type of
interrupts are used for emergency scenarios such as power failure.
- Type 3 interrupts: These type of interrupts are also known as breakpoint interrupts. When
this interrupt occurs a program would execute up to its break point.

-Type 4 interrupts: Also known as overflow interrupts is generally existent after an arithmetic
operation was performed.

Interrupt Priority Structure

Bios interrupts:

BIOS interrupt calls are a facility that operating systems and application programs use
to invoke the facilities of the Basic Input/Output System on IBM PC compatible computers.
Traditionally, BIOS calls are mainly used by MS-DOS programs and some other software such
as boot loaders (including, mostly historically, relatively simple application software that
boots directly and runs without an operating system—especially game software.) BIOS only
runs in the real address mode (Real Mode) of the x86 CPU, so programs that call BIOS either
must also run in real mode or must switch from protected mode to real mode before calling
BIOS and then switch back again. For this reason, modern operating systems that use the
CPU in Protected Mode generally do not use the BIOS to support system functions, although
some of them use the BIOS to probe and initialize hardware resources during their early
stages of booting.
In all computers, software instructions control the physical hardware (screen, disk,
keyboard, etc.) from the moment the power is switched on. In a PC, the BIOS, preloaded in
ROM on the main board, takes control immediately after the processor is reset, including
during power-up or when a hardware reset button is pressed. The BIOS initializes the
hardware, finds, loads and runs the boot program (usually, but not necessarily, an OS
loader), and provides basic hardware control to the operating system running on the
machine, which is usually an operating system but may be a directly booting single software
application.
For IBM's part, they provided all the information needed to use their BIOS fully or to
directly utilize the hardware and avoid BIOS completely, when programming the early IBM
PC models (prior to the PS/2). From the beginning, programmers had the choice of using
BIOS or not, on a per-hardware-peripheral basis. Today, the BIOS in a new PC still supports
most, if not all, of the BIOS interrupt function calls defined by IBM for the IBM
AT (introduced in 1984), along with many more newer ones, plus extensions to some of the
originals (e.g. expanded parameter ranges). This, combined with a similar degree of
hardware compatibility, means that most programs written for an IBM AT can still run
correctly on a new PC today, assuming that the faster speed of execution is acceptable
(which it typically is for all but games that use CPU-based timing). Despite the considerable
limitations of the services accessed through the BIOS interrupts, they have proven extremely
useful and durable to technological change.

Interrupt
Description
vector

Executed when Shift-Print screen is pressed, as well as when

05h
the BOUND instruction detects a bound failure.

10h Video Services

AH Description

00h Set Video Mode

01h Set Cursor Shape

02h Set Cursor Position

03h Get Cursor Position And Shape

04h Get Light Pen Position

05h Set Display Page

06h Clear/Scroll Screen Up

07h Clear/Scroll Screen Down

08h Read Character and Attribute at Cursor

09h Write Character and Attribute at Cursor

 0Ah Write Character at Cursor

0Bh Set Border Color

0Ch Write Graphics Pixel

0D
Read Graphics Pixel
h

0Eh Write Character in TTY Mode

0Fh Get Video Mode

10h Set Palette Registers (EGA, VGA, SVGA)

11h Character Generator (EGA, VGA, SVGA)

12h Alternate Select Functions (EGA, VGA, SVGA)

13h Write String

1Ah Get or Set Display Combination Code (VGA, SVGA)

1Bh Get Functionality Information (VGA, SVGA)

1Ch Save or Restore Video State (VGA, SVGA)

4Fh VESA BIOS Extension Functions (SVGA)

11h Returns equipment list

12h Return conventional memory size

 Low Level Disk Services

AH Description

00h Reset Disk Drives

01h Check Drive Status

02h Read Sectors

03h Write Sectors

04h Verify Sectors

05h Format Track

13h
08h Get Drive Parameters

09h Init Fixed Drive Parameters

0Ch Seek To Specified Track

0D
Reset Fixed Disk Controller
h

15h Get Drive Type

16h Get Floppy Drive Media Change Status

17h Set Disk Type

18h Set Floppy Drive Media Type

 41h Extended Disk Drive (EDD) Installation Check

42h Extended Read Sectors

43h Extended Write Sectors

44h Extended Verify Sectors

45h Lock/Unlock Drive

46h Eject Media

47h Extended Seek

48h Extended Get Drive Parameters

49h Extended Get Media Change Status

4Eh Extended Set Hardware Configuration

14h Serial port services

AH Description

00
Serial Port Initialization
h

01
Transmit Character
h

02
Receive Character
h

03 Status
 h

15h Miscellaneous system services

AH AL Description

00h Turn on cassette drive motor

01h Turn off cassette drive motor

02h Read data blocks from cassette

03h Write data blocks to cassette

4Fh Keyboard Intercept

83h Event Wait

84h Read Joystick

85h Sysreq Key Callout

86h Wait

87h Move Block

88h Get Extended Memory Size

89h Switch to Protected Mode

C0h Get System Parameters

C1h Get Extended BIOS Data Area Segment

 C2h Pointing Device Functions

C3h Watchdog Timer Functions - PS/2 systems only

C4h Programmable Option Select - MCA bus PS/2 systems only

D8
EISA System Functions - EISA bus systems only
h

Get Extended Memory Size (Newer function, since 1994). Gives

E8h 01h
results for memory size above 64 Mb.

Query System Address Map. The information returned

E8h 20h from E820 supersedes what is returned from the
older AX=E801h and AH=88hinterfaces.

Keyboard services

AH Description

00
Read Character
h

01
Read Input Status
h

16h
02
Read Keyboard Shift Status
h

05
Store Keystroke in Keyboard Buffer
h

10
Read Character Extended
h

11 Read Input Status Extended

 h

12
Read Keyboard Shift Status Extended
h

Printer services

AH Description

00
Print Character to Printer
h
17h

01
Initialize Printer
h

02
Check Printer Status
h

Execute Cassette BASIC: Very early true IBM computers contain Microsoft
Cassette BASIC in the ROM, to be started by this routine in the event of a failure
18h to boot from disk (called by the BIOS). On virtually all clones and later models in
the PC line from IBM, which lack BASIC in ROM, this interrupt typically displays a
message such as "No ROM BASIC" and halts.

After POST this interrupt is used by BIOS to load the operating system. A
program can call this interrupt to reboot the computer (but must ensure that
19h hardware interrupts or DMA operations will not cause the system to hang or
crash during either the reinitialization of the system by BIOS or the boot
process).

1Ah Real Time Clock Services

AH Description

00
Read RTC
h
 01
Set RTC
h

02
Read RTC Time
h

03
Set RTC Time
h

04
Read RTC Date
h

05
Set RTC Date
h

06
Set RTC Alarm
h

07
Reset RTC Alarm
h

PCI Services - implemented by BIOSes supporting PCI 2.0 or later

AX Description

B101h PCI Installation Check

1Ah B102h Find PCI Device

B103h Find PCI Class Code

B106h PCI Bus-Specific Operations

B108h Read Configuration Byte

 B109h Read Configuration Word

B10Ah Read Configuration Dword

B10Bh Write Configuration Byte

B10Ch Write Configuration Word

B10Dh Write Configuration Dword

B10Eh Get IRQ Routine Information

B10Fh Set PCI IRQ

1Bh Ctrl-Break handler - called by INT 09 when Ctrl-Break has been pressed

1Ch Timer tick handler - called by INT 08

Not to be called; simply a pointer to the VPT (Video Parameter Table), which
1Dh
contains data on video modes

Not to be called; simply a pointer to the DPT (Diskette Parameter Table),

1Eh
containing a variety of information concerning the diskette drives

Not to be called; simply a pointer to the VGCT (Video Graphics Character Table),
1Fh
which contains the data for ASCII characters 80h to FFh

41h Address pointer: FDPT = Fixed Disk Parameter Table (1st hard drive)

46h Address pointer: FDPT = Fixed Disk Parameter Table (2nd hard drive)

4Ah Called by RTC for alarm

MEMORY AND I/O INTERFACING: