Assembler 9

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.