This is part of the SYS:0499 Open Function but the main purpose is to move the file pointer to the beginning of the filem if the file is new/empty/to be overwritten, or to the end of the file if it is to be appended.
SYS0526: XOR DX,DX
SYS0528: XOR CX,CX
SYS052A: MOV BX,[DI:Handle]
SYS052C: MOV AX,4202
SYS052F: INT 21
Move file pointer to end of the file with a call to DOS INT 21h AH = 42h service with parameters:
Upon return: DX:AX contains the new file pointer location, with CF clear on success, an error code in AX and CF is set. This purpose of this first call to DOS INT 21h AH = 42h is to get the file size.
SYS0531: SUB AX,SizeOf(TextBuf)
SYS0534: SBB DX,+00
SYS0537: JNB 053D
Check if the file is at least 128 (80h) bytes in size then set DX:AX to 128 (80h) bytes from the end.
SYS0539: XOR AX,AX
SYS053B: XOR DX,DX
If the file is less than 128 (80h) bytes, then set the file pointer DX:AX to the beginning of the file.
SYS053D: MOV CX,DX
SYS053F: MOV DX,AX
Copy file pointer in DX:AX to CX:DX.
SYS0541: MOV BX,[DI:Handle]
SYS0543: MOV AX,4200
SYS0546: INT 21
Move the file pointer to new location using DOS INT 21h AH = 42h service.
SYS0548: LEA DX,[DI:TextBuf]
SYS054C: MOV CX,SizeOf(TextBuf)
SYS054F: MOV BX,[DI:Handle]
SYS0551: MOV AH,3F
SYS0553: INT 21
Read 128 (80h) bytes from the File and store it in DS:DX which points to its TextBuf. After the call, AX contains the number of bytes read and CF flag is clear on success and set on error.
SYS0555: JNB 0559
Check for errors.
SYS0557: XOR AX,AX
If there were errors, clear AX, i.e. 0 bytes read.
SYS0559: XOR BX,BX
Set BX to the beginning of the buffer.
SYS055B: CMP BX,AX
SYS055D: JZ 057F
Check all bytes up to (the count in) AX have been verified then exit.
SYS055F: CMP BYTE PTR [BX+DI+TextBuf],1A
SYS0564: JZ 0569
Search for the CTRL-Z/26/1Ah byte, which is usually the last byte (or signature mark) of a text file.
SYS0566: INC BX
SYS0567: JMP 055B
Loop back (to SYS:055B) until BX = AX.
SYS0569: MOV DX,BX
SYS056B: SUB DX,AX
SYS056D: MOV CX,FFFF
Setup a signed 32-bit number as file pointer (or offset) in CX:DX.
SYS0570: MOV BX,[DI:Handle]
Get file handle indicated by [DI].
SYS0572: MOV AX,4202
SYS0575: INT 21
Move file pointer to offset CX:DX from end of the file using INT 21h AH = 42h DOS Move File Pointer service.
SYS0577: XOR CX,CX
SYS0579: MOV BX,[DI:Handle]
SYS057B: MOV AH,40
SYS057D: INT 21
Write 0 bytes to File/Device using DOS Write to a File or Device INT 21h AH = 40h with parameters:
SYS057F: RET
Because it returns with a NEAR RET, it can only be called from within the system library.
See also: SYS:0499 Open Function or go back