gistfile1.asm

org 100h

jmp init

	resident_begin:

	handler_09:

		pusha
		push ds
		mov dx, cs
		mov ds, dx

		push 40h
		pop ds
		mov ax, [17h]
		and ax, 0h
		cmp ax, 0h
		jne .no_ast 

		mov ah, 02h
		mov dl, '*'
		int 21h

		.no_ast:

		pop ds
		popa

		jmp dword [cs:original_handler_09]

		.reboot:
		mov     ax, 0040h 
	    mov     ds, ax 
	    mov     WORD [0072h], 1234h ; Sets warm boot 
	    jmp     0F000h:0FFF0h       ; Jump to reset code 

	original_handler_09 dw 0
	original_handler_09_seg dw 0

	handler_2F:

		cmp ah, 0D3h
		jne .next

		cmp al, 01h
		je .selfremove

		mov al, 0FFh
		iret

		.selfremove:
			pusha
			push es
			push ds

			mov ax, 3509h
			int 21h

			cmp bx, handler_09
			jne .cant_uninstall

			push cs
			pop dx
			push es
			pop ax
			cmp ax, dx
			jne .cant_uninstall

			mov ax, 352Fh
			int 21h

			cmp bx, handler_2F
			jne .cant_uninstall

			push cs
			pop dx
			push es
			pop ax
			cmp ax, dx
			jne .cant_uninstall


			mov ax, 2509h
			mov dx, [cs:original_handler_09_seg]
			mov ds, dx
			mov dx, [cs:original_handler_09]
			int 21h

			mov ax, 252Fh
			push word [cs:original_handler_2F_seg]
			pop ds
			mov dx, [cs:original_handler_2F]
			int 21h

			mov ah, 49h
			push cs
			pop es
			int 21h

			pop ds
			pop es
			popa

			mov al, 11h
			iret

		.cant_uninstall:
			mov al, 10h
			iret

		.next:
			jmp dword [cs:original_handler_2F]

	original_handler_2F dw 0
	original_handler_2F_seg dw 0

	resident_end:



	init:
		cmp byte [82h], 'u'
		je uninstal_request

		mov ax, 0D300h
		int 2Fh

		cmp al, 0
		je install

		mov ah, 09h
		mov dx, .already_installed_str
		int 21h

		ret

	.already_installed_str db "Already installed.$"

	install:

		cli

		mov ax, 3509h
		int 21h
		mov [original_handler_09_seg], es
		mov [original_handler_09], bx

		mov ax, 352Fh
		int 21h
		mov [original_handler_2F_seg], es
		mov [original_handler_2F], bx

		mov ax, 2509h
		mov dx, handler_09
		int 21h

		mov ax, 252Fh
		mov dx, handler_2F
		int 21h

		sti

		mov ah, 09h
		mov dx, .installed_str
		int 21h

		mov cx, cs
		mov dx, resident_end
		int 27h

		ret
	
	.installed_str db "Installed.$"

	uninstal_request:
		mov ax, 0D300h
		int 2Fh

		cmp al, 0
		jne .not_installed_over
			mov ah, 09h
			mov dx, .not_installed_str
			int 21h
			ret
		.not_installed_str db "Not installed.$"
		.not_installed_over:

		mov ax, 0D301h
		int 2Fh

		cmp al, 11h
		jne .ok_over
			mov ah, 09h
			mov dx, .uninstalled_str
			int 21h
			ret
		.uninstalled_str db "Uninstalled.$"
		.ok_over:
			mov ah, 09h
			mov dx, .cant_uninstall_str
			int 21h
			ret
		.cant_uninstall_str db "Cannot uninstall.$"