Cracker Handbook 1.0 part 62 ppsx

5H 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.