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Writing and Reading Files

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Writing and Reading Files std_at_kmitnb.ac.th – PowerPoint PPT presentation

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Title: 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
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