Tham khảo tài liệu 'cracker handbook part 65', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 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. 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. 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. Channels of communication Table of Contents Working with handles Functions to use handles 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. 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. 6 Macros and procedures table of Contents Procedures Macros .
