; NORESET.ASM
;
; A short TSR that patches the int 9 interrupt and intercepts the
; ctrl-alt-del keystroke sequence.
;
; Note that this code does not patch into int 2Fh (multiplex interrupt)
; nor can you remove this code from memory except by rebooting.
; If you want to be able to do these two things (as well as check for
; a previous installation), see the chapter on resident programs. Such
; code was omitted from this program because of length constraints.
;
;
; cseg and EndResident must occur before the standard library segments!
cseg segment para public 'code'
OldInt9 dword ?
cseg ends
; Marker segment, to find the end of the resident section.
EndResident segment para public 'Resident'
EndResident ends
.xlist
include stdlib.a
includelib stdlib.lib
.list
DelScanCode equ 53h
; Bits for the various modifier keys
CtrlBit equ 4
AltBit equ 8
KbdFlags equ <byte ptr ds:[17h]>
cseg segment para public 'code'
assume ds:nothing
; SetCmd- Sends the command byte in the AL register to the 8042
; keyboard microcontroller chip (command register at
; port 64h).
SetCmd proc near
push cx
push ax ;Save command value.
cli ;Critical region, no ints now.
; Wait until the 8042 is done processing the current command.
xor cx, cx ;Allow 65,536 times thru loop.
Wait4Empty: in al, 64h ;Read keyboard status register.
test al, 10b ;Input buffer full?
loopnz Wait4Empty ;If so, wait until empty.
; Okay, send the command to the 8042:
pop ax ;Retrieve command.
out 64h, al
sti ;Okay, ints can happen again.
pop cx
ret
SetCmd endp
; MyInt9- Interrupt service routine for the keyboard hardware
; interrupt. Tests to see if the user has pressed a
; DEL key. If not, it passes control on to the original
; int 9 handler. If so, it first checks to see if the
; alt and ctrl keys are currently down; if not, it passes
; control to the original handler. Otherwise it eats the
; scan code and doesn't pass the DEL through.
MyInt9 proc far
push ds
push ax
push cx
mov ax, 40h
mov ds, ax
mov al, 0ADh ;Disable keyboard
call SetCmd
cli ;Disable interrupts.
xor cx, cx
Wait4Data: in al, 64h ;Read kbd status port.
test al, 10b ;Data in buffer?
loopz Wait4Data ;Wait until data available.
in al, 60h ;Get keyboard data.
cmp al, DelScanCode ;Is it the delete key?
jne OrigInt9
mov al, KbdFlags ;Okay, we've got DEL, is
and al, AltBit or CtrlBit ; ctrl+alt down too?
cmp al, AltBit or CtrlBit
jne OrigInt9
; If ctrl+alt+DEL is down, just eat the DEL code and don't pass it through.
mov al, 0AEh ;Reenable the keyboard
call SetCmd
mov al, 20h ;Send EOI (end of interrupt)
out 20h, al ; to the 8259A PIC.
pop cx
pop ax
pop ds
iret
; If ctrl and alt aren't both down, pass DEL on to the original INT 9
; handler routine.
OrigInt9: mov al, 0AEh ;Reenable the keyboard
call SetCmd
pop cx
pop ax
pop ds
jmp cs:OldInt9
MyInt9 endp
Main proc
assume ds:cseg
mov ax, cseg
mov ds, ax
print
byte "Ctrl-Alt-Del Filter",cr,lf
byte "Installing....",cr,lf,0
; Patch into the INT 9 interrupt vector. Note that the
; statements above have made cseg the current data segment,
; so we can store the old INT 9 value directly into
; the OldInt9 variable.
cli ;Turn off interrupts!
mov ax, 0
mov es, ax
mov ax, es:[9*4]
mov word ptr OldInt9, ax
mov ax, es:[9*4 + 2]
mov word ptr OldInt9+2, ax
mov es:[9*4], offset MyInt9
mov es:[9*4+2], cs
sti ;Okay, ints back on.
; We're hooked up, the only thing that remains is to terminate and
; stay resident.
print
byte "Installed.",cr,lf,0
mov ah, 62h ;Get this program's PSP
int 21h ; value.
mov dx, EndResident ;Compute size of program.
sub dx, bx
mov ax, 3100h ;DOS TSR command.
int 21h
Main endp
cseg ends
sseg segment para stack 'stack'
stk db 1024 dup ("stack ")
sseg ends
zzzzzzseg segment para public 'zzzzzz'
LastBytes db 16 dup (?)
zzzzzzseg ends
end Main