Writing and Reading Files

1
Writing and Reading Files

• std_at_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

• ?????????????????????????????????????????
• ????????????? Pointer ???????????????????
• ?????????????????????????????????? file pointer
  ?????????????????????? ?????? 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
• DSDX 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
• DSDX 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