1642
	.MODEL TINY
	.386

	.CODE
	ORG 100h

start:	jmp main

;**************************** Resident data

org_int9   dd ?
refresh_toggle db 0

;**************************** NewInt9 -- New INT9 (key-press) proc.

newint9	PROC

	sti
	push 	ax
	in		al,60h                  ; Get scancode from keyboard processor
	cmp	al, 27h                 ; Check if hotkey (Alt-™)
	jnz	no_hotkey

	push	es                      ; look into BIOS data area
	sub	ax, ax                  ; (segment 0) to check shift state
	mov	es, ax
	mov	ax, es:[417h]           ; Get SHIFT-key flags
	test	ax, 0000000000001000b   ; Check if Alt-key pressed 
	pop	es
	jz		no_hotkey 

	cli                            ; Disable interrupts while resetting
	in		al, 61h                  ; Get current port 61h state
	or		al, 10000000b           ; Turn on bit 7 to signal clear keybrd
	out	61h, al                 ; Send to port
	and	al, 01111111b           ; Turn off bit 7 to signal break
	out	61h, al                 ; Send to port

	push	bx
	mov	ah, 12h                 ;Disable/enable screen refresh
	xor	cs:refresh_toggle,1
	mov	al,cs:refresh_toggle
	mov	bl,36h
	int	10h
	pop 	bx

	mov	al, 20h                 ; Reset interrupt controller
	out	20h, al
	sti                             ; Reenable interrupts

	pop 	ax
  	iret

no_hotkey:
	pop	ax                      ; Recover AX and fall through
	cli                           ; Interrupts cleared for service
	jmp	cs:org_int9             ; jump to old Int 09 routine

newint9	ENDP


;/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/ Transient Section


;**************************** Transient data

header		db 'Screen Saver installed. Press Alt-™ to activate/deactivate.',13,10,'$'
uninstalled db 'Un'
installed   db 'Installed Successfully',13,10,'$'
syntax      db 'Syntax: SCRSAV [/U]',13,10,'$'
nomemory    db 'Out of memory',13,10,'$'
alreadyin   db 'Already installed',13,10,'$'
notfound    db 'Resident copy not found',13,10,'$'
cantunload  db 'Can''t uninstall',13,10,'$'
resname     db 'ScrSav  '
resseg      dw 0

;**************************** Main -- Main procedure

main	PROC

	mov	ah,9                		;Print install message
	mov	dx, offset header
	int 21h

  	call	findres            		;Find resident copy

	mov	al,'/'              		;Search for slash
	xor	cx, cx              		;in command line
	mov	cl, byte ptr cs:[80h]
	mov	di, 81h
	repne	scasb
	jne	nounload						;No slash, don't unload
	mov	dx, offset syntax    	;DX = 'Syntax:' string
	cmp	byte ptr[di], 'U'			;/U or /u, unload
	je		unload
	cmp	byte ptr[di], 'u'
	je		unload
	cmp	byte ptr[di], '?'			;/?, print syntax
	je		error

nounload:   
	mov	dx, offset alreadyin		;Point DX to error string
	cmp	resseg, 0					;Already installed?
	jne	error

	cld									;Clear direction flag
	mov	ax, offset lastbyte		;AX = last byte + 256
	inc	ah
	cmp	ax, sp						;Check for too little memory
	jbe	memOK

	mov	dx, offset nomemory		;Print 'Out of memory' string
error: 
	mov	ah, 9
	int	21h
	mov	ax, 4C01h					;Exit with error
	int	21h

memOK:      
	mov	sp, ax						;Shrink stack
	jmp	load							;Jump to Load procedure

main	ENDP

;**************************** Load -- TSR loader procedure

load	PROC

	mov 	ah,49h						;Free environment segment
	mov	es, cs:[2Ch]
	int	21h

	mov	ax, 3509h					;Get and store old INT9 address
	int	21h
	mov	word ptr[org_int9], bx
	mov	word ptr[org_int9+2], es

	mov	ax, 2509h					;Set new INT16 handler
	mov	dx, offset newint9
	int	21h

	mov	ax, cs						;ES = MCB segment (CS - 1)
	dec	ax
	mov	es, ax

	mov	si, offset resname		;Change resident name
	mov	di, 8
	mov	cx, di
	rep	movsb
            
	mov	ah, 9							;Print 'Installed' string
	mov	dx, offset installed
	int	21h

	mov	dx, offset header-1		;Last byte of program
lastbyte:   
	int 27h								;DOS TSR service

load	ENDP

;**************************** Unload -- Uninstall procedure

unload	PROC

	mov	cx, resseg					;CX = resident segment

	mov	dx, offset notfound		;Point DX to error string
	test	cx, cx						;Make sure it's in memory
	je		error

	mov	dx, offset cantunload	;Point DX to error string

	mov	ax, 3509h					;Check to make sure that the
	int	21h							;interrupts still point to
	cmp	bx, offset newint9		;the resident copy...
	jne	error

	mov	es, resseg					;ES = resident segment

	mov	ax, 2509h					;Reset INT9 vector
	mov	dx, word ptr es:[org_int9]
	mov	ds, word ptr es:[org_int9+2]
	int	21h

	mov	ah, 49h						;Release memory segment
	int	21h

	push	cs								;DS = CS
	pop	ds

	mov	byte ptr[installed], 'i';Fix 'Uninstalled' string
	mov	ah,9							;Print 'Uninstalled' string
	mov	dx, offset uninstalled
	int	21h

	mov	ax, 4C00h					;Quit program
	int	21h

unload	ENDP


;**************************** FindRes -- Find resident copy

findres	PROC

	pusha									;Save registers
	push 	es

	mov	ax,5802h						;Save UMB link status
	int	21h
	push	ax

	mov	ax, 5803h					;Link UMBs
	mov	bx, 1
	int	21h

	mov	ah, 52h						;Get Internal Config Table
	int	21h

	sub	bx, 2							;BX = first MCB
	mov	bx, es:[bx]

fr_lp:      
	mov	es, bx						;ES = MCB segment
	cmp	word ptr es:[9], 7263h	;Check for signature
	je		fr_found
	add	bx, es:[3]              ;Step to next one
	inc	bx
	cmp	byte ptr es:[0],'Z'     ;Last one in chain?
	jne	fr_lp                   ;Loop back

	mov	bx,-1							;Return 0 (-1 + 1 = 0)

fr_found:   
	inc bx
	mov	resseg, bx					;Return resident segment

	mov	ax, 5803h					;Restore UMB link status
	pop	bx
	int	21h

	pop	es
	popa
	ret

findres	ENDP

	END start
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