Wipe all contents from a file then delete it

;  WipeIt   is a file deletion routine that first replaces all bytes in the
;           file with a predefined byte.  This makes it harder for someone
;           to see what was in that file.
;           It would not be to difficult to make this routine loop 2 or 3
;           times and replace each byte with a different byte on each loop.
;           This makes for higher security.
;            * Use at your own risk *
;
; WipeIt.com  v1.01
; Forever Young Software
; Copyright 1984-2015
; Benjamin David Lunt
; All Rights Reserved
; 08 Mar 2001
;
; assemble with NBASM

.model tiny
.code
.386
           org  100h
           
start:     mov  ax,cs                   ; free unused part of Mem Block
           mov  es,ax                   ;   for .COM file format
           mov  bx,4096                 ;
           mov  ah,4Ah                  ;
           int  21h                     ;
           mov  ah,48h                  ; allocate a full segment
           mov  bx,4096                 ;
           int  21h                     ;
           jc   NoMemory                ;
           mov  ZeroSeg,ax              ; save seg address
           mov  es,ax                   ;
           mov  cx,16384                ;
           mov  eax,59465946h           ; and put 'FY' in segment
           xor  di,di                   ;
           rep                          ;
           stosd                        ;

           push ds                      ; make sure es=ds
           pop  es                      ;
           push ds                      ; save ds
           mov  ah,62h                  ; get segment address of PSP
           int  21h                     ;
           push bx                      ;
           pop  ds                      ; and put in ds
           mov  cl,[0080h]              ; get len of command line
           xor  ch,ch                   ;
           or   cx,cx                   ; if zero then no filename specified
           jz   NoFileN                 ;
           mov  di,offset CmmdLine      ; put it in our filename buffer
           mov  si,0082h                ;
CmmdL:     lodsb                        ; I don't rely on the amount at 0080h
           cmp  al,13                   ; this is more reliable
           je   short CmmdDone          ;
           stosb                        ;
           loop CmmdL                   ;
CmmdDone:  pop  ds                      ; restore ds
           mov  ax,2400h                ; make asciiz string and eos at end
           stosw                        ;  of asciiz for string output
           mov  dx,offset Msg1          ; string output
           mov  ah,09h                  ;    Msg1 and CmmdLine
           int  21h                     ;
           xor  ah,ah                   ;
           int  16h                     ;
           cmp  ax,1559h                ; if not 'Y' then abort
           jne  Aborted                 ;
           mov  ax,3D01h                ; open file for write access
           mov  dx,offset CmmdLine      ;
           int  21h                     ;
           jc   short FileErr           ; if carry then file/path not found?
           mov  Handle,ax               ; else save handle
           mov  bx,ax                   ; and put it in bx
           mov  ax,4202h                ; place file pointer at end of file
           xor  cx,cx                   ;
           xor  dx,dx                   ;
           int  21h                     ; returns file len in dx:ax
           shl  edx,16                  ; put dx in hi of edx, clr lo word
           and  eax,0FFFFh              ; clr hi word of eax
           or   eax,edx                 ; put file size in eax
           push eax                     ; save file size
           mov  ax,4200h                ; replace file pointer at beginning
           xor  cx,cx                   ;  of file
           xor  dx,dx                   ;
           int  21h                     ; returns file len in dx:ax
           pop  eax                     ; restore file size
           xor  dx,dx                   ; offset in buffer
           mov  bx,Handle               ; handle to use
           push ds
           mov  ds,ZeroSeg
DoItL:     or   eax,eax                 ; if bytes left to clr = 0 then done
           jz   short DoItDn            ;
           cmp  eax,0FFFFh              ; if size  <=  0FFFFh then
           jbe  short LastTime          ;   just put in ecx
           mov  ecx,0FFFFh              ; else put 0FFFFh in ecx
           jmp  short WrtIt             ;
LastTime:  mov  ecx,eax                 ;

; Please Note:  If your disk system may have 512 byte clusters and that cluster can
;  only have a single file occupying it.  So if you have a 513 byte file, this
;  file will occupy two clusters.

; So here is where you could "wipe out" the rest of the cluster as well, instead
; of only the file.

;  add the following code here:
;
;      sub  eax,ecx
;      or   ecx,01FFh
;      inc  ecx
;      jmp  short WrtIt1
;
;  remember, this only works with 512 byte clusters.....

WrtIt:     sub  eax,ecx                 ; sub len by ecx
WrtIt1:    push eax                     ; save len
           mov  ah,40h                  ; write the 'string' (FYFYFYFY....)
           int  21h                     ;  .
           pop  eax                     ; restore len
           jmp  short DoItL             ; go to top of loop
DoItDn:    pop  ds                      ; restore the data segment
           mov  bx,Handle               ; close the file
           mov  ah,3Eh                  ;
           int  21h                     ;
           mov  dx,offset CmmdLine      ; the filename of file to delete
           mov  ah,41h                  ; delete it
           int  21h                     ;
           jmp  short Done              ; finished (exit normaly)
NoMemory:  mov  dx,offset NoMemS        ; string output
           jmp  short PrntIt            ;
FileErr:   mov  dx,offset FireErrS      ; string output
           jmp  short PrntIt            ;
Aborted:   mov  dx,offset AbortedS      ; string output
           jmp  short PrntIt            ;
NoFileN:   pop  ds                      ; restore ds from above
           mov  dx,offset NoFlErr       ; string output
PrntIt:    mov  ah,09h                  ;
           int  21h                     ;
Done:      int  20h                     ; exit to DOS
                                        ;  (uses less code then service 4Ch)
Handle     dw   00h
ZeroSeg    dw   00h
NoFlErr    db   13,10,'No file specified.',36
AbortedS   db   13,10,' Aborted by user.  File not wiped out...',36
FireErrS   db   13,10,'  *Aborted*  File Error...',36
NoMemS     db   13,10,'  *Aborted*  Out of Memory...',36
Msg1       db   13,10,'   Wipe out the file (Y/N):  '      ; no 36 needed
CmmdLine   db   00h   ; we don't need to assign this any thing longer than
                      ; just one byte.  In the startup code, we allowed
                      ; for the whole segment to be in use for our code
.end