TITLE ERASER: OVERWRITE AND ERASE A FILE
.MODEL SMALL
.STACK 100H
.DATA
PROMPT DB 'FILENAME:$'
FILENAME DB 30 DUP (0)
BUFFER DB 512 DUP (0)
HANDLE DW ?
OPENERR DB 0DH,0AH,'OPEN ERROR-CODE'
ERRCODE DB 30H,'$'
FILESIZE DW ?
.CODE
MAIN PROC
	MOV AX,@DATA
	MOV DS,AX			;INIT DS
	MOV ES,AX			;AND ES
	CALL GET_NAME		;READ FILENAME
	LEA DX,FILENAME	;DX HAS FILENAME OFFEST
	MOV AL,0 			;ACCESS CODE 0 FOR READING
	CALL OPEN 			;OPEN FILE
	JC OPEN_ERROR 	;EXIT IF ERROR
	MOV HANDLE, AX 	;SAVE HANDLE
;seems like here we can count how big the file is?
READ_LOOP:
	LEA DX,BUFFER 		;DX PTS TO BUFFER
	MOV BX,HANDLE 		;GET HANDLE
	CALL READ 			;READ FILE. AX=BYTES READ
	OR AX,AX 			;END FILE?
	JE EXIT 				;YES, EXIT
	MOV CX,AX 			;CX GETS NO. OF BYTES READ
	;CALL DISPLAY 		;DISPLAY FILE
	JMP READ_LOOP 	;EXIT
OPEN_ERROR:
	LEA DX,OPENERR 	;GET ERROR MESSAGE
	ADD ERRCODE, AL 	;COVERT ERROR CODE TO ASCII
	MOV AH,9
	INT 21H				;DISPLAY ERROR MESSAGE
EXIT:
	MOV BX,HANDLE 		;GET HANDLE
	CALL CLOSE 			;CLOSE FILE

	;save filesize currently in CX
	MOV FILESIZE, CX
	
	MOV CX,122
	FOR_:
	CMP CX,34
	JLE EXIT_FOR
	;first open file for writing
		LEA DX,FILENAME	;DX HAS FILENAME OFFEST
		call openre
	;write over the buffer
		call writeover	
	;we would then want to write the file back out erasing it
		call write
		call close
		dec cx
		jmp FOR_
	EXIT_FOR:

;call delete file
	call delete

	MOV AH,4CH
	INT 21H 				;DOS EXIT
MAIN ENDP

WRITEOVER PROC NEAR
;writes over buffer
;input: value of cx is used as overwrite value
			push cx
			push ax
			push si
			mov ah,cl
			mov cx, filesize
			lea si,buffer
			;mov ah,'r'
		top:
			mov [si], ah
			inc si
		loop top
			pop si
			pop ax
			pop cx
			ret
WRITEOVER ENDP

GET_NAME PROC NEAR
;READS AND STORES FILENAME
;INPUT: NONE
;OUTPUT: FILE NAMED STORED AS ASCIIZ STRING
	PUSH AX			;SAVE REGISTERS USED
	PUSH DX
	PUSH DI
	MOV AH,9 			;DISPLAY STRING FUNCTION
	LEA DX,PROMPT
	INT 21H 				;DISPLAY DATA PROMPT
	CLD
	LEA DI, FILENAME 	;DI PTS TO FILENAME
	MOV AH,1			;READ CHAR FCN
READ_NAME:
	INT 21H 				;GET A CHAR
	CMP AL,0DH			;CR?
	JE DONE			;YES EXIT
	STOSB				 ;NO,STORE IN STRING
	JMP READ_NAME	 ;KEEP READING
DONE:
	MOV AL,0
	STOSB				 ;STORE 0 BYTE (I ASSUME AS STRING ENDER LIKE IN C)
	POP DI				 ;RESTORE REGISTERS
	POP DX
	POP AX
	RET
GET_NAME ENDP

OPEN PROC NEAR
;OPEN FILE
;INPUT DS:DX FILENAME
;	AL ACCESS CODE
;OUTPUT: IF SUCCESSFUL, AX HANDLE
; 	IF UNSUCCESSFUL, CF=1, AX=ERRORCODE
	MOV AH,3DH 	;OPEN FILE FCN
	MOV AL,2		 ;INPUT ONLY  			 1 means opening for writing, 2 reading and writing
	INT 21H 			;OPEN FILE
	RET
OPEN ENDP

OPENRE PROC NEAR
;OPEN FILE
;INPUT DS:DX FILENAME
;	AL ACCESS CODE
;OUTPUT: IF SUCCESSFUL, AX HANDLE
; 	IF UNSUCCESSFUL, CF=1, AX=ERRORCODE
	push ax
	push cx
	MOV AH,3CH 	;OPEN FILE FCN
	MOV CL,0		;set to normal file type
	INT 21H 			;OPEN FILE
	pop cx
	pop ax
	RET
OPENRE ENDP

WRITE PROC NEAR
;writes buffer area out to file
;input:
;	filesize contains number of bytes to write out which must be placed in cx
;	bx contains the file handle
	push cx
	push dx
	push ax
		mov cx,filesize
		lea dx, buffer	;get buffer address
		mov ah, 40h ;write function
		int 21h ;execute the write
	pop ax
	pop dx
	pop cx
	ret
WRITE ENDP

READ PROC NEAR
;READS A FILE SECTOR
;INPUT: 	BX FILE HANDLE
; 		CX BYTES TO READ (512)
; 		DS:DX BUFFER
;OUTPUT: IF SUCCESSFUL, SECTOR IN BUFFER
; 		AX NUMBER OF BYTES READ
; IF UNSUCCESSFUL, CF=1
	PUSH CX
	MOV AH, 3FH		;READ FILE FCN
	MOV CX,512		;512 BYTES
	INT 21H			;READ FILE INTO BUFFER
	POP CX
	RET
READ ENDP

DISPLAY PROC NEAR
;DISPLAYS MEMORY ON SCREEN
;INPUT 	BX = HANDLE (1)
; 		CX = BYTES TO DISPLAY
; 		DS:DX = DATA ADDRESS
;OUTPUT: AX=BYTES DISPLAYED
	PUSH BX
	MOV AH,40H		;WRITE FILE FCN
	MOV BX,1 		;HANDLE FOR SCREEN
	INT 21H			;DISPLAY FILE
	POP BX
	RET
DISPLAY ENDP

DELETE PROC NEAR
;source for interrupt: http://webster.cs.ucr.edu/AoA/DOS/ch13/CH13-6.html
;Deletes A FILE
;INPUT	DX:DS MUST HAVE FILENAME
;OUTPUT: IF CF=1, ERROR CODE IN AX
	push dx
	push ax
		LEA dx, filename 	;DX HAS FILENAME OFFEST
		MOV AH,41H		;DELETE FILE FUNCTION
		INT 21H			;CLOSE FILE
	pop ax
	pop dx
	RET
DELETE ENDP

CLOSE PROC NEAR
;CLOSES A FILE
;INPUT: BX = HANDLE
;OUTPUT: IF CF=1, ERROR CODE IN AX
	MOV AH,3EH		;CLOSE FILE FUNCTION
	INT 21H			;CLOSE FILE
	RET
CLOSE ENDP

	END MAIN