[HELP] How to write a simple Loader in ASM on MSDOS

[stoney81]

Hi guys!

I´m having trouble protecting a DOS Exe (with Overlay) and none of the tools in the otherwise excellent DOSEXE Collection seems to work, so I thought why not try to learn something along the way.

I would need a simple Loader (best would be ASM Code I could compile with nasm) that would load a XORed or otherwise encrypted game.bin File. The idea is to encrypt the Overlay EXE and use the Loader to load the file. This way no one can just snoop around in the EXE.

Protection doesn´t have to be bullet proof, it should just fend off newbies who try to hexedit or check for changes.

Any ideas or resources I could study?

Thank You!

- stoney

[ZeNiX]

Are you sure you are going to write a protector for 8-bit/16-bit MS-DOS Exe files?

[sendersu]

Even in 202x there are COBOL/Fortran/etc engineers that earn 1K per day

[chants]

Just to clarify DOS was a 16 bit OS, based on 8 bit CPM. As for overlays there is a nice explanation of some of the details here: https://retrocomputing.stackexchange.com/a/25742 so you will need to know more than just the relevant MZ header fields as the interrupts and segmentation and all these things must be done correctly. In general overlays which are generally for limited memory systems are not needed in a modern context if you can recompile the original DOS program without them. It was designed in days RAM was scarce. That's why they don't exist in PE applications

[BlackWhite]

Originally Posted by chants

Just to clarify DOS was a 16 bit OS, based on 8 bit CPM. As for overlays there is a nice explanation of some of the details here: https://retrocomputing.stackexchange.com/a/25742 so you will need to know more than just the relevant MZ header fields as the interrupts and segmentation and all these things must be done correctly. In general overlays which are generally for limited memory systems are not needed in a modern context if you can recompile the original DOS program without them. It was designed in days RAM was scarce. That's why they don't exist in PE applications

PE may also have an overlay which for example is actually a DLL

[BlackWhite]

Originally Posted by stoney81

Hi guys!

I´m having trouble protecting a DOS Exe (with Overlay) and none of the tools in the otherwise excellent DOSEXE Collection seems to work, so I thought why not try to learn something along the way.

I would need a simple Loader (best would be ASM Code I could compile with nasm) that would load a XORed or otherwise encrypted game.bin File. The idea is to encrypt the Overlay EXE and use the Loader to load the file. This way no one can just snoop around in the EXE.

Protection doesn´t have to be bullet proof, it should just fend off newbies who try to hexedit or check for changes.

Any ideas or resources I could study?

Thank You!

- stoney

I write the following code ad lib which should meet your needs.
Pls note, after compiling the code and get an exe say ovl.exe,
you should use a hex editor to append at least one byte say 0xCB i.e. retf at the end of ovl.exe

comment #
Demo for Loading an Overlay in 16-bit Assembly Language
                           Written by [email protected]
                           Dec 20, 2024
#
.386
code segment use16
assume cs:code, ds:code
;input:
;   [bp+4] = PSP's segment address
;   [bp+6] = self_name's offset address
;   [bp+8] = self_name's segment address
;output:
;   [bp+8]:[bp+6] -> self_name
get_self_name proc
   push bp
   mov bp, sp
   push ds
   push es
   push si
   push di
   mov es, [bp+4]   ; ES = PSP's segment address
   mov es, es:[2Ch] ; The environment block's segment address resides at PSP:[2Ch]
	                 ; ES:0->environment block which contains the running program's name
	mov  cx, 7FFFh
	mov  di, 0
	mov  al, 0
	cld
scan_it:
	repnz scasb
	cmp byte ptr es:[di], 0
	je  catch_it
	loop scan_it
catch_it:
	push es
	pop  ds
	mov si, di
	add si, 3
   les di, [bp+6]; ES:DI->self_name
copy_name:	
   lodsb
	or al, al
	jz  catch_end
	stosb
	jmp copy_name
catch_end:
	stosb
   pop di
   pop si
	pop es
	pop ds
   pop bp
   ret
get_self_name endp


;input:
;  [bp+4]->file_name
;  [bp+6]->handle
;output:
;  EAX = file_len
;  *[bp+6] = handle
;  CF = 1 for error
;  CF = 0 for success
get_self_size proc
   push bp
   mov bp, sp
   push bx
   mov ax, 3D00h
   mov dx, [bp+4]
   int 21h
   mov bx, [bp+6]
   mov [bx], ax
   jc get_self_size_done
   mov bx, ax
   mov ax, 4202h
   xor cx, cx
   xor dx, dx
   int 21h
   shl edx, 10h
   mov dx, ax
   mov eax, edx
   push eax
   mov ax, 4200h
   xor cx, cx
   xor dx, dx
   int 21h
   pop eax
   clc
get_self_size_done:
   pop bx
   pop bp
   ret
get_self_size endp

;input:
;   [bp+4] = handle
;output:
;   EAX = head_and_image_size
get_head_and_image_size proc
   push bp
   mov bp, sp
   sub sp, 6
   push bx
   push ds
   mov ah, 3Fh
   mov bx, [bp+4]
   mov cx, 6
   lea dx, [bp-6]
   push ss
   pop ds
   int 21h
   movzx eax, word ptr [bp-2]
   shl eax, 9
   mov dx, word ptr [bp-4]
   sub dx, 200h
   neg dx
   and dx, 1FFh
   movzx edx, dx
   sub eax, edx
   pop ds
   pop bx
   mov sp, bp
   pop bp
   ret
get_head_and_image_size endp

psp        dw 0
handle     dw 0
self_name  db 40h dup(0)
self_size  dd 0
head_and_image_size dd 0
overlay_size dd 0
overlay_entry dw 0
overlay_seg  dw 0
main:
   mov cs:[psp], ds
   push cs
   pop ds
   ;
   push cs
   mov ax, offset self_name
   push ax
   push [psp]
   call get_self_name
   add sp, 6
   ;
   ;
   mov ax, offset handle
   push ax
   mov ax, offset self_name
   push ax
   call get_self_size
   pop cx
   pop cx
   mov [self_size], eax
   jnc continue
   jmp done
   ;
continue:
   push [handle]
   call get_head_and_image_size
   add sp, 2
   mov [head_and_image_size], eax
   ;
   mov eax, [self_size]
   sub eax, [head_and_image_size]
   mov [overlay_size], eax
   ;
   mov bx, offset end_flag
   add bx, 100h+0Fh
   shr bx, 4
   push es
   mov ah, 4Ah
   mov es, [psp]
   int 21h; reallocate memory
   pop es
   ;
   mov ah, 48h
   mov bx, word ptr [overlay_size]
   add bx, 0Fh
   shr bx, 4
   int 21h
   mov [overlay_seg], ax
   ;
   mov ax, 4200h
   mov bx, [handle]
   mov ecx, [head_and_image_size]
   mov dx, cx
   shr ecx, 10h
   int 21h; fseek 
   ;
   push ds
   mov ah, 3Fh
   mov bx, [handle]
   mov cx, word ptr [overlay_size]
   xor dx, dx
   mov ds, [overlay_seg]
   int 21h; read overlay
   mov ah, 3Eh
   int 21h; close file
   pop ds
   ;
   push ds
   push es
   mov cx, word ptr [overlay_size]
   mov es, [overlay_seg]
   mov ds, [overlay_seg]
   xor si, si
   xor di, di
decode_overlay:
   lodsb
   ; xor al, 42h; put your decoding algorithm here
   stosb
   loop decode_overlay
   pop es
   pop ds
   ;
   call dword ptr [overlay_entry]
   ;
   push es
   mov ah, 49h
   mov es, [overlay_seg]
   int 21h; free memory
   pop es
   ;
done:
   mov ah, 4Ch
   int 21h
end_flag label byte
code ends
end main