INT 21H (0x21) Function 43H (0x43 or 67) Get or set file attributes (0x44 or 68) Input/Output Control (I/O Ctrl) sub function 00H (0x00) 01H (0x01) 02H (0x02) 03H (0x03) 04H (0x04) Read control data block-device driver 05H (0x05) 06H (0x06) 07H (0x07) 08H (0x08) 09H (0x09) 0AH (0x0A or 10) 0BH (0x0B or 11)

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INT 21H (0x21)

Function 43H (0x43 or 67) --> Get or set file attributes

Call with: AH = 43H
AL = 00H to get attributes
01H to set attributes
CX = file attribute, if AL=01H. Bits can be combined
DS: DX = segment: offset of ASCIIZ pathname

Returns: If function successful
Carry flag = clear
CX = file attribute

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This function obtains or alters the attributes of file (read-only, hidden, system, or archive) or directory. For the significance of bits for different attributes refer Bits significance table given before.

INT 21H (0x21)

Function 44H (0x44 or 68) --> Input/Output Control (I/O Ctrl)

This function provides a direct path of communication between an application program and a device driver. It allows a program to obtain hardware-dependent information and to request operations that are not supported by other MS-DOS function calls.

The sub functions of Input and output Control have been given in the following table:

Sub function

Operation Name

00H

Get Device Information                                                

01H

Set Device Information                                                

02H

Receive Control Data from Character Device Driver  

03H

Send Control Data to Character Device Driver        

04H

Receive Control Data from Block Device Driver    

05H

Send Control Data to Block Device Driver

06H

Check Input Status                                                    

07H

Check Output Status                                                 

08H

Check If Block Device Is Removable                      

09H

Check If Block Device Is Remote                           

0AH (10)

Check If Handle Is Remote                                     

0BH (11)

Change Sharing Retry Count                                    

0CH (12)

 

 

 

 

Generic I/O Control for Character Devices

Value

Description

CL = 45H

Set Iteration Count                

CL = 4AH

Select Code Page                  

CL = 4CH

Start Code Page Preparation 

CL = 4DH

End Code Page Preparation  

CL = 5FH

Set Display Information       

CL = 65H

Get Iteration Count

CL = 6AH

Query Selected Code Page   

CL = 6BH

Query Prepare List                

CL = 7FH

Get Display Information 

 

0DH (13)

 

 

 

 

 

Generic I/O Control for Block Devices

Value

Description

CL = 40H

Set Device Parameters  

CL = 41H

Write Track                  

CL = 42H

Format and Verify Track

CL = 47H

Set Access Flag

CL = 60H

Get Device Parameters 

CL = 61H

Read Track                   

CL = 62H

Verify Track                  

CL = 67H

Get Access Flag                                    

0EH (14)

Get Logical Drive Map                           

0FH (15)

Set Logical Drive Map 

 

INT 21H (0x21)

Function 44H (0x44 or 68), sub function 00H (0x00) I/O Ctrl --> get device information

Call with: AH = 44H
AL = 00H
BX = handle

Returns: If function successful
Carry flag = clear
DX = device information word

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function returns a device information word for the file or device associated with the specified handle.

 

INT 21H (0x21)

Function 44H (0x44 or 68), sub function 01H (0x01) I/O Ctrl --> set device information

Call with: AH = 44H
AL = 01H
BX = handle
DX = device information word

Returns: If function successful
Carry flag = clear

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function of function 44H of INT 21H, sets certain flags for a handle associated with a character device. This sub function may not be used for a handle that is associated with a file.

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 02H (0x02) I/O Ctrl --> read control data character device driver

Call with: AH = 44H
AL = 02H
BX = handle
CX = number of bytes to read
DS: DX = segment: offset of buffer

Returns: If function is successful
Carry flag = clear
AX = bytes read
And buffer contains control data from driver

If function is unsuccessful
Carry flag = set
AX = error code

Comments:

It reads control data from a character-device driver. The length and contents of the data are specified to each device driver and do not follow any standard format. This function does not necessarily result in any input from the physical device.

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 03H (0x03) I/O Ctrl --> write control data character-device driver

Call with: AH = 44H
AL = 03H
BX = handle
CX = number of bytes to write
DS: DX = segment: offset of data

Returns: If function successful
Carry flag = clear
AX = bytes transferred

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function transfers control data from an application to a character-device driver. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result if any output to the physical device.

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 04H (0x04) I/O Ctrl --> Read control data block-device driver

Call with: AH = 44H
AL = 04H
BL = device code (0= default, 1=A,
2=B, etc.)
CX = number of bytes to read
DS: DX = segment: offset of buffer

Returns: If function successful
Carry flag = clear
AX = bytes transferred
And buffer contains control data from device driver

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function transfers control data from a block-device driver directly into an application program’s buffer. The length and contents of the data are specific to each device driver and do not follow any standard format. This function does not necessarily result in input from the physical device.

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 05H (0x05) I/O Ctrl --> write control data block-device driver

Call with: AH = 44H
AL = 05H
BL = device code (0= default, 1=A,
2=B, etc.)
CX = number of bytes to write
DS: DX = segment: offset of data

Returns: If function successful
Carry flag = clear
AX = bytes transferred

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function transfers control data from an application program directly to a block-device driver. The length and contents of the control data are specific to each device driver and do not follow any standard format. This function does not necessarily result any output to the physical device.

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 06H (0x06) I/O Ctrl --> check input status

Call with: AH = 44H
AL = 06H
BX = handle

Returns: If function successful
Carry flag = clear
And for a device:
AL = 00H, if device not ready
FFH, if device ready
For a file:
AL = 00H, if file pointer at EOF
FFH, if file pointer not at EOF

If function unsuccessful
Carry flag = set
AX = error code

Comments:

It returns a code indicating whether the device or files associated with a handle is ready for input.

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 07H (0x07) I/O Ctrl --> check output status

Call with: AH = 44H
AL = 07H
BX = handle

Returns: If function successful
Carry flag = clear
And for a device:
AL = 00H, if device not ready
FFH, if device ready
For a file:
AL = FFH

If function unsuccessful
Carry flag = set
AX = error code

Comments:

It returns a code indicating whether the device associated with a handle is ready for output.

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 08H (0x08) I/O Ctrl --> check if block device is removable

Call with: AH = 44H
AL = 08H
BL = drive number (0 = default, 1=A,
2=B, etc.)

Returns: If function successful
Carry flag = clear
AL = 00H, if medium is removable
01H, if medium is not removable

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function checks whether the specified block device contains a removable storage medium, such as a floppy disk. If a file is not found as expected on a particular drive, a program can use this sub function to determine whether the user should be prompted to insert another disk.

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 09H (0x09) I/O Ctrl --> check if block device is remote

Call with: AH = 44H
AL = 09H
BL = drive number (0 = default, 1=A,
2=B, etc.)

Returns: If function successful
Carry flag = clear
DX = device attribute word
bit 12 = 0, if drive is local
= 1, if drive is remote

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function checks whether the specified block device is local (attached to the computer running the program) or remote (redirected to a network server).

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 0AH (0x0A or 10) I/O Ctrl --> check if handle is remote

Call with: AH = 44H
AL = 0AH
BX = handle

Returns: If function successful
Carry flag = clear

DX = attribute word for file or device
bit 15 = 0 if local
1 if remote

If function unsuccessful
Carry flag = set
AX = error code

Comments:

It checks whether the specified handle refers to a file or device that is local (located on the PC that is running program) or remote (located on a network server).

 

INT 21H (0x21)

Function 44H (0x44 or 68), Sub function 0BH (0x0B or 11) I/O Ctrl --> change sharing retry count

Call with: AH = 44H
AL = 0BH
CX = delays per retry (default = 1)
DX = number of retries (default = 3)

Returns: If function successful
Carry flag = clear

If function unsuccessful
Carry flag = set
AX = error code

Comments:

This sub function sets the number of times MS-DOS retries a disk operation after a failure caused by a file-sharing violation before it returns an error to the requesting process. This sub function is not available unless the file sharing module is loaded.

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Data Recovery with & without Programming


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