Cracker Handbook 1.0 part 62 ppsx

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

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