bits 16
org 100h
 
jmp init.tsr
 
int1c:
push ds
push ax
push cs
pop ds
call checkdos2
jc int1c_e
cmp byte [hotkey_found], 1
jnz int1c_e
call tsrapp
int1c_e:
pop ax
pop ds
iret
 
int28:
push ds
push ax
push cs
pop ds
call checkdos
jc int28_e
cmp byte [hotkey_found], 1
jnz int28_e
call tsrapp
int28_e:
pop ax
pop ds
iret
 
checkdos2:
push bx
push es
les bx, [InDOS]
mov al, 0
cmp al, byte [es:bx]
jb checkdos2_e
cmp al, byte [es:bx-1]
checkdos2_e:
pop es
pop bx
ret
 
checkdos:
push bx
push es
les bx, [InDOS]
mov al, 1
cmp al, byte [es:bx]
jb checkdos_e
xor al, al
cmp al, byte [es:bx-1]
checkdos_e:
pop es
pop bx
ret
 
InDOS      dd 0
Orig_isr9  dd 0
in_isr9    db 0
hotkey_found db 0
hotkey_idx dw 0
hotseq     db 0x1b
 
isr9:
push ds
push bx
push ax
push cs
pop ds
in al, 0x60
pushf
cli
call word far [Orig_isr9]
mov ah, [in_isr9]
or ah, [hotkey_found]
jnz isr9_e
inc byte [in_isr9]
cmp al, byte [hotseq]
jz isr9_foundkey
jmp isr9_exit
isr9_foundkey:
mov byte [hotkey_found], 1
isr9_exit:
dec byte [in_isr9]
isr9_e:
pop ax
pop bx
pop ds
iret
 
tsrapp:
push dx
mov ah, 0x2
mov dl, 0x41
int 0x21
pop dx
ret
 
pre equ $
 
init.tsr:
mov ax, 0x3509
int 0x21
mov [Orig_isr9], bx
mov [Orig_isr9+2], es
mov dx, isr9
mov ax, 0x2509
int 0x21
mov dx, int28
mov ax, 0x2528
int 0x21
mov dx, int1c
mov ax, 0x251c
int 0x21
mov ax, 0x3400
int 0x21
mov [InDOS], bx
mov [InDOS+2], es
mov dx, pre
add dx, 0xf
shr dx, 4
mov ax, 0x3100
int 0x21