Writing and Reading Files

1. Writing and Reading Files std@kmitnb.ac.th

2. Methods for processing disk files • File Control Blocks (FCBs) • Supported by DOS • Can address drives and filenames • Cannot address directories • File Handles • Accessing of the file • Return codes to identity errors

3. Operations using File Handles • Use INT 21H services • 3CH : Create file • 3DH : Open file • 3EH : Close file • 3FH : Read record • 40H : Write record • 42H : Move file pointer

4. Operations using File Handles • Use other services • INT 25H : Absolute read • INT 26H : Absolute write

5. Operations using FCBs • Use INT 21H services • 0FH : Open file • 10H : Close file • 14H : Read record • 15H : Write record • 16H : Create file • 21H : Read record randomly • 22H : Write record randomly • 27H : Read block randomly • 28H : Write block randomly

6. ASCIIZ strings • Containing the filespec : • the location of the disk drive • directory path • filename • ‘All optional within apostrophes’ • a bytes of hex zeros • maximum length string is 128 bytes

7. ASCIIZ strings • Example1 : defines a drive and filename • PATHNAM1 DB ‘D:\Filetest.asm’, 00H • Example2 : defines a drive, subdirectory and filename • PATHNAM2 DB ‘E:\Utility\Filetest.exe’, 00H

8. File Handles • Standard devices • 00 = input • 01 = output • 02 = error output • 03 = auxiliary device • 04 = printer

9. Error Return Codes • File handle operations for disk deliver a “completion status” via • Carry flag • Register AX • Successful operation • clear carry flag & perform other functions • Unsuccessful operation • set carry flag & return an error code in AX

10. Lists of error codes : 01-36 • 01 : Invalid function number • 02 : File not found • 03 : Path not found • 04 : Too many file open • 05 : access denied • 06 : Invalid handle • 07 : Memory control block destroyed • 08 : Insufficient memory • 09 : Invalid memory block address • 10 : Invalid environment

11. Lists of error codes : 01-36 • 11 : Invalid format • 12 : Invalid access code • 13 : Invalid data • 15 : Invalid drive specified • 16 : Attempt to remove directory • 17 : Not same device • 18 : No more files • 19 : Write-protected disk • 20 : Unknown unit

12. Lists of error codes : 01-36 • 21 : Invalid function number • 22 : Unknown command • 23 : CRC data error • 24 : Bad request structure length • 25 : Seek error • 26 : Unknown media type • 27 : Sector not found • 28 : Printer out of paper • 29 : Write fault • 30 : Read fault

13. Lists of error codes : 01-36 • 31 : General failure • 32 : Sharing violation • 33 : Lock violation • 34 : Invalid disk change • 35 : FCB unavailable • 36 : Sharing buffer overflow

14. File Pointers • ตัวชี้ที่ทำหน้าที่ชี้ตำแหน่งข้อมูลของไฟล์ • เมื่อเปิดไฟล์ Pointer จะชี้ที่ไบท์แรกเสมอ • การเคลื่อนย้ายข้อมูลในไฟล์ต้องย้าย file pointer ไปยังตำแหน่งที่ต้องการ โดยใช้ INT 21H function 42H

15. Using File handles to create disk files • Procedure for writing a disk • Use an ASCIIZ string to get a file handle from the system • Use INT 21H function 3CH to create the file • Use INT 21H function 40H to write records in the file • Use INT 21H function 3EH to close the file

16. INT 21H Function 3CH : Create File • AH = 3CH • CX = File attribute • DX = Address of the ASCII string

17. INT 21H Function 3CH : Create File • File attribute • 00H : Normal file • 01H : Read only • 02H : Hidden file • 04H : System file • 08H : Volume label • 10H : Subdirectory • 20H : Archive file

18. INT 21H Function 3CH : Create File • Example • PATHNAM1 DB ‘E:\ACCOUNTS.FIL’, 00H • FILHAND1 DW ? • . . . • MOV AH,3CH • MOV CX,00 • LEA DX,PATHNAM1 • INT 21H • JC error • MOV FILHAND1,AX

19. INT 21H Function 40H : Write Record • AH = 40H • BX = Stored file handle • CX = Number of bytes to write • DS:DX = Address of the output area

20. INT 21H Function 40H : Write Record • Example • FILHAND1 DW ? • OUTAREA DB 256 DUP(‘ ‘) • . . . • MOV AH,40H • MOV BX,FILHAND1 • MOV CX,256 • LEA DX,OUTAREA • INT 21H • JC error2 • CMP AX,256 • JNE error3

21. INT 21H Function 3EH : Close File • AH = 3EH • BX = File handle

22. INT 21H Function 3EH : Close File • Example • MOV AH,3EH • MOV BX,FILHAND1 • INT 21H

23. Using File handles to read disk files • Procedure for reading a disk file • Use an ASCIIZ string to get a file handle from the system • Use INT 21H function 3DH to open the file • Use INT 21H function 3FH to read records from the file • Use INT 21H function 3EH to close the file

24. INT 21H Function 3DH : Open File • AH = 3DH • AL = Access code • 00 : Read only • 01 : Write only • 02 : Read/Write • DX = Address of the ASCII string

25. INT 21H Function 3DH : Open File • Example • FILHAND2 DW ? • . . . • MOV AH,3DH • MOV AL,00 • LEA DX,PATHNAM1 • INT 21H • JC error4 • MOV FILHAND2,AX

26. INT 21H Function 3FH : Read Record • AH = 3FH • BX = File handle • CX = Number of bytes to read • DS:DX = Address of the input area

27. INT 21H Function 3FH : Read Record • Example • FILHAND2 DW ? • INAREA DB 512 DUP(‘ ‘) • . . . • MOV AH,3FH • MOV BX,FILHAND2 • MOV CX,512 • LEA DX,INAREA • INT 21H • JC error5 • CMP AX,00 • JE endfile

28. INT 21H Function 42H : Move File Pointer • AH = 42H • BX = File handle • CX : DX = Offset address required • AL = Method code • 00 : Take the offset from the start of the file • 01 : Take the offset from the current location of the file pointer • 02 : Take the offset from the end-of-file

29. INT 21H Function 42H : Move File Pointer • Example • MOV AH,42H • MOV AL,00 • MOV BX,HANDLE1 • MOV CX,00 • MOV DX,1024 • INT 21H • JC error