FCB method

Contents

5.6.1 Introduction

5.6.2 Open files

5.6.3 Create a new file

5.6.4 Sequential writing

5.6.5 Sequential reading

5.6.6Random reading and writing

5.6.7 Close a file

5.6.1 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 group 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.

5.6.2 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.

5.6.3 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.

5.6.4 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.

5.6.5 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.

5.6.6 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.

5.6.7 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.

5.7 Channels of communication

Contents

5.7.1 Working with handles

5.7.2 Functions to use handles

5.7.1 Working with handles

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.

5.7.2 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.

Date: 2014-12-22; view: 920