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jmp [cs:old_interrupt9]Is an NEAR absolute indirect jump (uses the same CS and takes the offset from the address). What you need is a FAR absolute indirect jump (takes CS and the offset from the address). The correct instruction should be:jmp far [cs:old_interrupt9]and ah,!ctrl_keyWon't do what you thing it does... ! operador is a BOOLEAN operator (as in C) and results in 0 or 1, always. What you are searching for is ~ (BINARY NOT operator). The correct instruction is:and ah,~ctrl_keyHere's your code with some corrections and comments of mine: bits 16
org 0x100
; Notice: NO segments needed!
; This program disables the usual DOS reset command
; (Ctrl-Alt-Del), by intercepting the INT 9 keyboard
; hardware interrupt. It checks the shift status bits
; in the MS-DOS keyboard flag and changes any Ctrl-Alt-Del
; to Alt-Del. The computer can only be rebooted by
; typing Ctrl+Alt+Right shift+Del.
;
; Use NASM -fbin prog.asm -o prog.com
RT_SHIFT EQU 0x01 ; Right shift key: bit 0
CTRL_KEY EQU 0x04 ; CTRL key: bit 2
ALT_KEY EQU 0x08 ; ALT key: bit 3
DEL_KEY EQU 0x53 ; scan code for DEL key
KEYB_PORT EQU 0x60 ; keyboard input port
jmp setup ; jump to TSR installation
; Memory-resident code begins here
int9_handler:
sti ; enable hardware interrupts
pushf ; save regs & flags
push ds ; Using DS instead of ES to avoid prefixes
push ax
push bx ; Using BX as base address (not DI), just to
; be compliant to 8086 effective addresses standard.
; Point DS:BX to the BIOS data area keyboard flag (0x40:0x17) byte
mov ax,0x40
mov ds,ax
mov bx,0x17
mov ah,[bx] ; copy keyboard flag into AH
; Test for the CTRL and ALT keys:
test ah,CTRL_KEY ; CTRL key held down?
jz .exit ; no: exit
test ah,ALT_KEY ; ALT key held down?
jz .exit ; no: exit
; Test for the DEL and Right-shift keys:
in al,KEYB_PORT ; read keyboard port
cmp al,DEL_KEY ; DEL key pressed?
jne .exit ; no: exit
test ah,RT_SHIFT ; right shift key pressed?
jnz .exit ; yes: allow system reset
; FIXED: !CTRL_KEY will result in 0, not 0xFB. ! is a BOOLEAN NOT operator. ~ is a BINARY NOT.
and ah,~CTRL_KEY ; no: turn off bit for CTRL
mov [bx],ah ; store modified keyboard_flag
.exit:
pop bx ; restore regs & flags
pop ax
pop ds
popf
; FIXED: Must be a far jump!
jmp far [cs:old_interrupt9] ; jump to INT 9 routine
old_interrupt9:
align 2
dd 0
service_end:
; --------------- (end of TSR program) ------------------
; Save a copy of the original INT 9 vector, and set up
; the address of our program as the new vector. Terminate
; this program and leave the int9_handler procedure in memory.
setup:
cli ; Disable interrupts to NOT allow
; IVT changes when we are tring to
; change it.
mov ax,0x3509 ; get old INT 9 vector
int 0x21
mov [old_interrupt9],bx
mov [old_interrupt9+2],es
; In tiny model DS=CS.
mov ax,0x2509 ; set new int 9 vector
mov dx,int9_handler
int 0x21
; Calculate # of paragraphs...
mov dx,service_end ; point to end of resident code
; this works only because the initial offset is
; always zero.
mov ah,dl
mov cl,4
shr dx,cl ; OBS: shr dx,4 ISN'T an 8086 instruction!
test ah,0x0f ; Any bytes remaining in the last paragraph?
jz .skip ; No, ok...
inc dx ; Yes. Add an extra paragraph
.skip:
mov ax,0x3100 ; terminate and stay resident
int 0x21call [cs: old_interrupt9]
cs" on the previous line might also work.
TITLE Reset-Disabling program (No_Reset.asm)
; This program disables the usual DOS reset command
; (Ctrl-Alt-Del), by intercepting the INT 9 keyboard
; hardware interrupt. It checks the shift status bits
; in the MS-DOS keyboard flag and changes any Ctrl-Alt-Del
; to Alt-Del. The computer can only be rebooted by
; typing Ctrl+Alt+Right shift+Del. Assemble, link,
; and convert to a COM program by including the /T
; command on the Microsoft LINK command line.
; Last update: 12/12/2004
.model tiny
.386
.code
rt_shift EQU 01h ; Right shift key: bit 0
ctrl_key EQU 04h ; CTRL key: bit 2
alt_key EQU 08h ; ALT key: bit 3
del_key EQU 53h ; scan code for DEL key
kybd_port EQU 60h ; keyboard input port
ORG 100h ; this is a COM program
start:
jmp setup ; jump to TSR installation
; Memory-resident code begins here
int9_handler PROC FAR
sti ; enable hardware interrupts
pushf ; save regs & flags
push es
push ax
push di
; Point ES:DI to the DOS keyboard flag byte:
L1: mov ax,40h ; DOS data segment is at 40h
mov es,ax
mov di,17h ; location of keyboard flag
mov ah,es:[di] ; copy keyboard flag into AH
; Test for the CTRL and ALT keys:
L2: test ah,ctrl_key ; CTRL key held down?
jz L5 ; no: exit
test ah,alt_key ; ALT key held down?
jz L5 ; no: exit
; Test for the DEL and Right-shift keys:
L3: in al,kybd_port ; read keyboard port
cmp al,del_key ; DEL key pressed?
jne L5 ; no: exit
test ah,rt_shift ; right shift key pressed?
jnz L5 ; yes: allow system reset
L4: and ah,NOT ctrl_key ; no: turn off bit for CTRL
mov es:[di],ah ; store keyboard_flag
L5: pop di ; restore regs & flags
pop ax
pop es
popf
; here is the code I am not sure about
jmp cs:[old_interrupt9] ; jump to INT 9 routine
old_interrupt9 DWORD ?
int9_handler ENDP
end_ISR label BYTE
; --------------- (end of TSR program) ------------------
; Save a copy of the original INT 9 vector, and set up
; the address of our program as the new vector. Terminate
; this program and leave the int9_handler procedure in memory.
setup:
mov ax,3509h ; get INT 9 vector
int 21h
mov word ptr old_interrupt9,bx ; save INT 9 vector
mov word ptr old_interrupt9+2,es
mov ax,2509h ; set interrupt vector, INT 9
mov dx,offset int9_handler
int 21h
mov ax,3100h ; terminate and stay resident
mov dx,OFFSET end_ISR ; point to end of resident code
shr dx,4 ; divide by 16
inc dx ; round upward to next paragraph
int 21h ; execute MS-DOS function
END start
; TITLE Reset-Disabling Program
section code
%define syscall int 0x21
rt_shift EQU 0x01
ctrl_key EQU 0x04
alt_key EQU 0x08
del_key EQU 0x53
keybd_port EQU 0x60
org 0x100
start:
jmp setup
; memory resident code starts here
int9_handler:
sti ; enable hardwrae interrupts
; save registers and CPU flags
pushf
push es
push ax
push di
; Point ES_DI to the DOS keyboard flag byte
L1:
mov ax, 0x40 ; DOS data segment is at 0x40
mov es, ax ; set es to the data segment
mov di, 0x17 ; location of keyboard flag
mov ah, [es:di] ; copy the keyboard flag to ah
; Test for the CTRL and ALT keys
L2:
test ah, ctrl_key ; is CTRL key down?
jz L5 ; no: exit
test ah, alt_key ; is ALT key down
jz L5 ; no: exit
; Test for DEL and Right Shift keys
L3:
in al, keybd_port
cmp al, del_key ; is DEL key down?
jz L5 ; no: exit
test ah, rt_shift ; is Right shft down
jnz L5 ; no: exit
L4:
and ah,!ctrl_key ; no turn off bit for ctrl
mov [es:di], ah ; store keyboard flag
L5:
pop di
pop ax
pop es
popf
-- here is my version
jmp [cs:old_interrupt9]
old_interrupt9: dd 0x00000000
end_ISR: db 0
; ------ end of TSR program -------
; Save a copy of the original interrupt vector and set up
; the address of our program as the new vector, Terminate
; this program and save the int9_handler procedure in memory.
setup:
mov ax, 0x3509 ; get int9 vector
syscall
mov word [old_interrupt9], bx ; Save old interrupt vector
mov word [old_interrupt9+2], es ; Save old interrupt segment
mov ax, 0x2509 ; set interrupt vector
mov dx, int9_handler
syscall
mov ax, 0x3100 ; terminate and stay resident
mov dx, end_ISR ; set end of ISR
shr dx, 4 ; divide by 16
inc dx ; move upword to next paragraph
syscall ; excute DOS function
Thanks Frederico, .COM files not being used anymore isn't an issue as I'm learning 8086 Assembly mostly for my interest in retrocomputing.As I said before: COM files don't have sections.
I see an interesting differences between your hello world code and the similar demo in Wikipedia's NASM entry: your code doesn't declare sectons.
bits 16
org 0x100 ; skip the PSP
; Your program start here
_start:
...Here a simple 'Hello, World' in COM format: bits 16
org 0x100
; start here
lea dx,[msg]
mov ah,9 ; PrintStr (DOS service).
int 0x21
mov ax,0x4c00 ; Exit (DOS service)
int 0x21
msg:
db `Hello, world\r\n$`