Most common interruptions
21H Interruption
Purpose: To call on diverse DOS functions.
A D V E R T I S E M E N T
Syntax:
Int 21H
Note: When we work in TASM program 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 Random reading
22H Random writing
Handles
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
02H FUNCTION
Use:
It displays one character to the screen.
Calling registers:
AH = 02H
DL = Value of the character to display.
Return registers:
None.
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 command.
The use of the 40H function is recommended instead of this function.
09H FUNCTION
Use:
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:
None.
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.
40H FUNCTION
Use:
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.
01H FUNCTION
Use:
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.
0AH FUNCTION
Use:
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:
None.
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].
3FH FUNCTION
Use:
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 error code.
0FH FUNCTION
Use:
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
14H FUNCTION
Use:
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 code will
be returned: 1 error at the end of the file, 2 error on the FCB structure and 3
pa
What this function does is that it reads the next block of information from
the address given by DS:DX, and dates this register.
15H FUNCTION
Use:
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 15H function dates the FCB after writing the register to the present
block.
16H FUNCTION
Use:
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.
21H FUNCTION
Use:
To read in an random 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
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 Area.
22H FUNCTION
Use:
To write in an random 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
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.
3CH FUNCTION
Use:
To create a file if it does not exist or leave it on 0 length if it exists,
Handle.
Call registers:
AH = 3CH
CH = File attribute
DS:DX = Pointer to an ASCII specification.
Return registers:
CF = 0 and AX the assigned number to handle if there is no error, in case there
is, CF
ill be 1 and AX will contain the error code: 3 path not found, 4 there
This function substitutes the 16H function. The name of the file is
specified on an ASCII 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.
3DH FUNCTION
Use:
It opens a file and returns a handle.
Call registers:
AH = 3DH
AL = manner of access
DS:DX = Pointer to an ASCII 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, 03
The returned handled is 16 bits.
The access code is specified in the following way:
BITS
7 6 5 4 3 2 1
. . . . 0 0 0 Only reading
. . . . 0 0 1 Only writing
. . . . 0 1 0 Reading/Writing
. . . x . . . RESERVED
3EH FUNCTION
Use:
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 dates the file and frees the handle it was using.
3FH FUNCTION
Use:
To read a specific quantity of bytes from an open file and store them on a
specific buffer.
5.4.2 10h Interruption
Purpose: To call on diverse BIOS video function
Syntax:
Int 10H
This interruption has several functions, all of them control the video
input/output, 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.
In this tutorial we will see some functions of the 10h interruption.
Common functions of the 10h interruption
02H Function, select the cursor position
09H Function, write attribute and character of the cursor
0AH Function, write a character in the cursor position
0EH Function, Alphanumeric model of the writing characters
02h Function
Use:
Moves the cursor on the computer screen using text model.
Call registers:
AH = 02H
BH = Video page where the cursor is positioned.
DH = row
DL = Column
Return Registers:
None.
The cursor position is defined by its coordinates, starting from the
position 0,0 to position 79,24. This means from the left per computer
screen corner to right lower computer screen. Therefore the numeric values
that the DH and DL registers get in text model are: from 0 to 24 for rows
and from 0 to 79 for columns.
09h Function
Use:
Shows a defined character several times on the computer screen with a
defined attribute, starting with the actual cursor position.
Call registers:
AH = 09H
AL = Character to display
BH = Video page, where the character will display it;
BL = Attribute to use
number of repetition.
Return registers:
None
This function displays a character on the computer screen several times,
using a specified number in the CX register but without changing the cursor
position on the computer screen.
0Ah Function
Use:
Displays a character in the actual cursor position.
Call registers:
AH = 0AH
AL = Character to display
BH = Video page where the character will display it
BL = Color to use (graphic mode only).
CX = number of repetitions
Return registers:
None.
The main difference between this function and the last one is that this one
doesn't allow modifications on the attributes neither does it change the
cursor position.
0EH Function
Use:
Displays a character on the computer screen dates the cursor position.
Call registers:
AH = 0EH
AL = Character to display
BH = Video page where the character will display it
BL = Color to use (graphic mode only).
Return registers:
None
5.4.3 16H interruption
We will see two functions of the 16 h interruption, these functions are
called by using the AH register.
Functions of the 16h interruption
00H Function, reads a character from the keyboard.
01H Function, reads the keyboard state.
00H Function Use:
Reads a character from the keyboard.
Call registers:
AH = 00H
Return registers:
AH = Scan code of the keyboard
AL = ASCII value of the character
When we use this interruption, the program executing is halted until a
character is typed, if this is an ASCII value; it is stored in the Ah
register, Else the scan code is stored in the AL register and the AH
register contents the value 00h.
The proposal of the scan code is to use it with the keys without ASCII
representation as [ALT][CONTROL], the function keys and so on.
01h function
Use:
Reads the keyboard state
Call registers:
AH = 01H
Return registers:
If the flag register is zero, this means, there is information on the
buffer memory, else, there is no information in the buffer memory.
Therefore the value of the Ah register will be the value key stored in the
buffer memory.
5.4.4 17H Interruption
Purpose: Handles the printer input/output.
Syntax:
Int 17H
This interruption is used to write characters on the printer, sets printer
and reads the printer state.
Functions of the 16h interruptions
00H Function, prints value ASCII out
01H Function, sets printer
02H Function, the printer state
00H Function
Use:
Writes a character on the printer.
Call registers:
AH = 00H
AL = Character to print.
DX = Port to use.
Return registers:
AH = Printer device state.
The port to use is in the DX register, the different values are: LPT1 = 0,
LPT2 = 1, LPT3 = 2 ...
The printer device state is coded bit by bit as follows:
BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use
1 and 2 bits are not relevant
Most BIOS sport 3 parallel ports, although there are BIOS which sport 4
parallel ports.
01h Function
Use:
Sets parallel port.
Call registers:
AH = 01H
DX = Port to use
Return registers:
AH = Printer status
Port to use is defined in the DX register, for example: LPT=0, LPT2=1, and
so on.
The state of the printer is coded bit by bit as follows:
BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use
1 and 2 bits are not relevant
Most BIOS sport 3 parallel ports, although there are BIOS which sport 4
parallel ports.
02h Function
Uses:
Gets the printer status.
Call registers:
AH = 01H
DX = Port to use
Return registers
AH = Printer status.
Port to use is defined in the DX register, for example: LPT=0, LPT2=1, and
so on
The state of the printer is coded bit by bit as follows:
BIT 1/0 MEANING
----------------------------------------
0 1 The waited time is over
1 -
2 -
3 1 input/output error
4 1 Chosen printer
5 1 out-of-paper
6 1 communication recognized
7 1 The printer is ready to use
1 and 2 bits are not relevant
Most BIOS sport 3 parallel ports, although there are BIOS which sport 4
parallel ports.
Ways of working with files
There are two ways to work with files, the first one is by means of file
control blocks or "FCB" and the second one is by means of communication
channels, also known as "handles".
The first way of file handling has been used since the CPM operative
system, predecessor of DOS, thus it assures certain compatibility with very
old files from the CPM as well as from the 1.0 version of the DOS, besides
this method allows us to have an unlimited number of open files at the same
time. If you want to create a volume for the disk the only way to achieve
this is by using this method.
Even after considering the advantages of the FCB, the use of the
communication channels it is much simpler and it allows us a better
handling of errors, besides, since it is much newer it is very probable
that the files created this way maintain themselves compatible through
later versions of the operative system.
For a greater facility on later explanations I will refer to the file
control blocks as FCBs and to the communication channels as handles.
FCB method
Introduction
There are two types of FCB, the normal, whose length is 37 bytes and the
extended one of 44 bytes.
On this tutorial we will only deal with the first type, so from now on when
I refer to an FCB, I am really talking about a 37 bytes FCB.
The FCB is composed of information given by the programmer and by
information which it takes directly from the operative system.
When thesetypes of files are used it is only possible to work on the current
directory since the FCBs do not provide sport for the use of the organization by
directories of DOS.
The FCB is formed by the following fields:
POSITION LENGTH MEANING
00H 1 Byte Drive
01H 8 Bytes File name
09H 3 Bytes Extension
0CH 2 Bytes Block number
0EH 2 Bytes Register size
10H 4 Bytes File size
14H 2 Bytes Creation date
16H 2 Bytes Creation hour
18H 8 Bytes Reserved
20H 1 Bytes Current register
21H 4 Bytes Random register
To select the work drive the next format is followed: drive A = 1; drive B
= 2; etc. If 0 is used the drive being used at that moment will be taken as
option.
The name of the file must be justified to the left and in case it is
necessary the remaining bytes will have to be filled with spaces, and the
extension of the file is placed the same way.
The current block and the current register tell the computer which register
will be accessed on reading or writing operations. A block is a gro of
128 registers. The first block of the file is the block 0. The first
register is the register 0, therefore the last register of the first block
would be the 127, since the numbering started with 0 and the block can
contain 128 registers in total.
Opening files
To open an FCB file the 21H interruption, 0FH function is used. The unit,
the name and extension of the file must be initialized before opening it.
The DX register must point to the block. If the value of FFH is returned on
the AH register when calling on the interruption then the file was not
found, if everything came out well a value of 0 will be returned.
If the file is opened then DOS initializes the current block to 0, the size
of the register to 128 bytes and the size of the same and its date are
filled with the information found in the directory.
Creating a new file
For the creation of files the 21H interruption 16H function is used.
DX must point to a control structure whose requirements are that at least
the logic unit, the name and the extension of the file be defined.
In case there is a problem the FFH value will be returned on AL, otherwise
this register will contain a value of 0.
Sequential writing
Before we can perform writing to the disk it is necessary to define the
data transfer area using for this end the 1AH function of the 21H
interruption.
The 1AH function does not return any state of the disk nor or the
operation, but the 15H function, which is the one we will use to write to
the disk, does it on the AL register, if this one is equal to zero there
was no error and the fields of the current register and block are dated.
Sequential reading
Before anything we must define the file transfer area or DTA.
In order to sequentially read we use the 14H function of the 21H
interruption.
The register to be read is the one which is defined by the current block
and register. The AL register returns to the state of the operation, if AL
contains a value of 1 or 3 it means we have reached the end of the file. A
value of 2 means that the FCB is wrongly structured.
In case there is no error, AL will contain the value of 0 and the fields of
the current block and register are dated.
Random reading and writing
The 21H function and the 22H function of the 21H interruption are the ones
in charge of realizing the random readings and writings respectively.
The random register number and the current block are used to calculate
the relative position of the register to read or write.
The AL register returns the same information for the sequential reading of
writing. The information to be read will be returned on the transfer area
of the disk, likewise the information to be written resides on the DTA.
Closing a file
To close a file we use the 10H function of the 21H interruption.
If after invoking this function, the AL register contains the FFH value,
this means that the file has changed position, the disk was changed or
there is error of disk access.
Channels of communication
The use of handles to manage files greatly facilitates the creation of
files and programmer can concentrate on other aspects of the programming
without worrying on details which can be handled by the operative system.
The easy use of the handles consists in that to operate o a file, it is
only necessary to define the name of the same and the number of the handle
to use, all the rest of the information is internally handled by the DOS.
When we use this method to work with files, there is no distinction between
sequential or random accesses, the file is simply taken as a chain of
bytes.
Functions to use handles
The functions used for the handling of files through handles are described
in unit 6: Interruptions, in the section dedicated to the 21H interruption.
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