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Lesson 7. Interruptions.

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Internal hardware interruptions
External hardware interruptions
Software interruptions
Most common interruptions:

INT 21H (DOS interruption)

Multiple calls to DOS functions.

INT 10H (BIOS interruption)

Video input/output.

INT 16H (BIOS interruption)

Keyboard input/output.

INT 17H (BIOS interruption)

Printer input/output.

Internal interruptions are generated by certain events which come up during the execution of a program.

This type of interruptions are managed on their total
A clear example of this type of interruptions is the one which actualizes the
counter of the computer internal clock, the hardware makes the call to this
interruption several times during a second in order to maintain the time up to

Even though we cannot directly manage this interruption, since we cannot control the time updating by means of software, it is
possible to use its effects on the computer to our benefit, for example to create a "virtual clock" updated
continuously thanks to the clock's internal counter. We only have to write a
program which reads the actual value of the counter and to translates it into an understandable format for the user.
External interruptions are generated by peripheral devices, such as keyboards,
printers, communication cards, etc. They are also generated by coprocessors.

It is not possible to deactivate external interruptions.

These interruptions are not sent directly to the CPU, but rather they are sent
to an integrated circuit whose function is to exclusively handle this type of
interruptions. The circuit, called PIC8259A, is controlled by the CPU using for
this control a series of communication ways called paths.

Software interruptions can be directly activated by the assembler invoking the
number of the desired interruption with the INT instruction.

The use of interruptions helps us in the creation of programs, and by using them our programs are shorter, it is easier to
understand them and they usually have a better performance mostly due to their smaller size.

This type of interruptions can be separated in two categories: the operative
system DOS interruptions and the BIOS interruptions.

The difference between the two is that the operative system interruptions are
easier to use but they are also slower since these interruptions make use of the BIOS to achieve their goal, on the other hand
the BIOS interruptions are much faster but they have the disadvantage that since they are part of the hardware, they are very
specific and can vary depending even on the brand of the maker of the circuit.

The election of the type of interruption to use will depend solely on the
characteristics you want to give your program: speed, using the BIOS ones, or
portability, using the ones from the DOS.

Purpose: To call on diverse DOS functions.



Note: When we work in MASM it is necessary to specify that the value we are
using is hexadecimal.

This interruption has several functions, to access each one of them it is
necessary that the function number which is required at the moment of calling the interruption is in the AH register.

Functions to display information to the video.

02H Exhibits output
09H Chain Impression (video)
40H Writing in device/file

Functions to read information from the keyboard.

01H Input from the keyboard
0AH Input from the keyboard using buffer
3FH Reading from device/file

Functions to work with files.

In this section only the specific task of each function is exposed, for a
reference about the concepts used, refer to unit 7, titled : "Introduction to
file handling".

FCB Method

0FH Open file
14H Sequential reading
15H Sequential writing
16H Create file
21H Aleatory reading
22H Aleatory writing


3CH Create file
3DH Open file
3EH Close file driver
3FH Reading from file/device
40H Writing in file/device
42H Move pointer of reading/writing in file


It displays one character to the screen.

Calling registers:

AH = 02H
DL = Value of the character to display.

Return registers:


This function displays the character whose hexadecimal code corresponds to the value stored in the DL register, and no register
is modified by using this

The use of the 40H function is recommended instead of this function.


It displays a chain of characters on the screen.

Call registers:

AH = 09H
DS:DX = Address of the beginning of a chain of characters.

Return registers:


This function displays the characters, one by one, from the indicated address in
the DS:DX register until finding a $ character, which is interpreted as the end
of the chain.

It is recommended to use the 40H function instead of this one.


To write to a device or a file.

Call registers:

AH = 40H
BX = Path of communication
CX = Quantity of bytes to write
DS:DX = Address of the beginning of the data to write

Return registers:

CF = 0 if there was no mistake

AX = Number of bytes written

CF = 1 if there was a mistake

AX = Error code

The use of this function to display information on the screen is done by giving
the BX register the value of 1 which is the preassigned value to the video by
the operative system MS-DOS.


To read a keyboard character and to display it.

Call registers

AH = 01H

Return registers:

AL = Read character

It is very easy to read a character from the keyboard with this function, the
hexadecimal code of the read character is stored in the AL register. In case it
is an extended register the AL register will contain the value of 0 and it will
be necessary to call on the function again to obtain the code of that character.


To read keyboard characters and store them on the buffer.

Call registers:

AH = 0AH
DS:DX = Area of storage address
BYTE 0 = Quantity of bytes in the area
BYTE 1 = Quantity of bytes read
from BYTE 2 till BYTE 0 + 2 = read characters

Return characters:


The characters are read and stored in a predefined space on memory.
The structure of this space indicate that in the first byte are indicated how many characters will be read. On the second byte
the number of characters already read are stored, and from the third byte on the read characters are written.

When all the indicated characters have been stored the speaker sounds and any additional character is ignored. To end the capture
of the chain it is necessary to hit [ENTER].


To read information from a device or file.

Call registers:

AH = 3FH
BX = Number assigned to the device
CX = Number of bytes to process
DS:DX = Address of the storage area

Return registers:

CF = 0 if there is no error and AX = number of read bytes.
CF = 1 if there is an error and AX will contain the e

To open an FCB file

Call registers:

AH = 0FH
DS:DX = Pointer to an FCB

Return registers:

AL = 00H if there was no problem, otherwise it returns to 0FFH


To sequentially read an FCB file.

Call registers:

AH = 14H
DS:DX = Pointer to an FCB already opened.

Return registers:

AL = 0 if there were no errors, otherwise the corresponding error cod
What this function does is that it reads the next block of information from the
address given by DS:DX, and updates this register.


To sequentially write and FCB file.

Call registers:

AH = 15H
DS:DX = Pointer to an FCB already opened.

Return registers:

AL = 00H if there were no errors, otherwise it will contain the error code: 1 full disk or read-only file, 2 error on the
formation or on the specification of the FCB.

The 15H function updates the FCB after writing the register to the present


To create an FCB file. Call registers:

AH = 16H
DS:DX = Pointer to an already opened FCB.

Return registers:

AL = 00H if there were no errors, otherwise it will contain the 0FFH value.

It is based on the information which comes on an FCB to create a file on a disk.


To read in an Aleatory manner an FCB file.

Call registers:

AH = 21H
DS:DX = Pointer to and opened FCB.

Return registers:

A = 00H if there was no error, otherwise AH will contain the code of the error: 1 if it is the end of file, 2 if there is an
FCB specification error and 3 if a partial register was read or the file pointer is at the end of the same.

This function reads the specified register by the fields of the actual block and
register of an opened FCB and places the information on the DTA, Disk Transfer


To write in an Aleatory manner an FCB file.

Call registers:

AH = 22H
DS:DX = Pointer to an opened FCB.

Return registers:

AL = 00H if there was no error, otherwise it will contain the error code: 1 if the disk is full or the file is an only read
and 2 if there is an error on the FCB specification.

It writes the register specified by the fields of the actual block and register
of an opened FCB. It writes this information from the content of the DTA.


To create a file if it does not exist or leave it on 0 length if it exists,

Call registers:

AH = 3CH
CH = File attribute
DS:DX = Pointer to an ASCIIZ specification.
Return registers:

CF = 0 and AX the assigned number to handle if there is no error, in case there is, CF will be 1 and AX will contain the error
code: 3 path not found, 4 there are no handles available to assign and 5 access denied.

This function substitutes the 16H function. The name of the file is specified on
an ASCIIZ chain, which has as a characteristic being a conventional chain of
bytes ended with a 0 character.

The file created will contain the attributes defined on the CX register in the
following manner:
Value Attributes
00H Normal
02H Hidden
04H System
06H Hidden and of system
The file is created with the reading and writing permissions. It is not possible
to create directories using this function.


It opens a file and returns a handle.

Call registers:

AH = 3DH
AL = manner of access
DS:DX = Pointer to an ASCIIZ specification

Return registers:

CF = 0 and AX = handle number if there are no errors, otherwise CF = 1 and AX = error code: 01H if the function is not valid,
02H if the file was not found, 03H if the path was not found, 04H if there are no available handles, 05H in case access is
denied, and 0CH if the access code is not valid.

The returned handled is 16 bits.

The access code is specified in the following way:

7 6 5 4 3 2 1

. . . . 0 0 0 Only reading
. . . . 0 0 1 Only writing
. . . . 0 1 0 Reading/Writing
. . . x . . . RESERVED


Close file (handle).

Call registers:

AH = 3EH
BX = Assigned handle

Return registers:

CF = 0 if there were no mistakes, otherwise CF will be 1 and AX will contain the error code: 06H if the handle is invalid.

This function updates the file and frees the handle it was using.


To read a specific quantity of bytes from an open file and store them on a
specific buffer.

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