; quick debugger
DOSSEG
.MODEL SMALL
.STACK 4096

.DATA

.CODE

VideoSegment EQU 0B800h

Old9  label  FAR

int9 PROC               ; start of ISR for 9h

 push ax
 push bx
 push cx
 push dx
 push si
 push di
 push ds
 push es
 push bp                ; preserve stack
 mov  bp,sp

 mov  ax,0040h
 mov  ds,ax
 mov  bx,ds:[001Ah]
 mov  cx,ds:[001Ch]

 pushf                  ; call old int9 routine
StartOf9: 
 call Old9

 cmp  bx,ds:[001Ch]
 je   endint

 mov  ax,ds:[bx]
 cmp  ax,3b00h
 jne  endint

 mov  ax,0b800h
 mov  es,ax
 mov  byte ptr es:[0],'*'

 mov  ds:[001Ah],bx
 mov  ds:[001Ch],cx

 mov  ax,[bp+16]
 mov  bx,152
 call outputnum

 mov  ax,[bp+14]
 mov  bx,152+160
 call outputnum

endint:
 mov  sp,bp             ; restore stack
 pop  bp
 pop  es
 pop  ds
 pop  di
 pop  si
 pop  dx
 pop  cx
 pop  bx
 pop  ax
 iret                   ; return from interrupt

int9 ENDP

outputnum PROC
 mov  cx,VideoSegment
 mov  ds,cx
 mov  byte ptr ds:[bx+1],7
 mov  byte ptr ds:[bx+3],7
 mov  byte ptr ds:[bx+5],7
 mov  byte ptr ds:[bx+7],7

 push ax
 and  ax,0f000h
 mov  cl,12
 shr  ax,cl
 cmp  al,10
 jae  alpha1
 add  al,48
 jmp  endout1
alpha1:
 add  al,55
endout1:
 mov  ds:[bx],al
 pop  ax

 push ax
 and  ax,00f00h
 mov  cl,8
 shr  ax,cl
 cmp  al,10
 jae  alpha2
 add  al,48
 jmp  endout2
alpha2:
 add  al,55
endout2:
 mov  ds:[bx+2],al
 pop  ax

 push ax
 and  ax,0000f0h
 mov  cl,4
 shr  ax,cl
 cmp  al,10
 jae  alpha3
 add  al,48
 jmp  endout3
alpha3:
 add  al,55
endout3:
 mov  ds:[bx+4],al
 pop  ax

 push ax
 and  ax,00000fh
 cmp  al,10
 jae  alpha4
 add  al,48
 jmp  endout4
alpha4:
 add  al,55
endout4:
 mov  ds:[bx+6],al
 pop  ax
 
 ret
outputnum ENDP

EndofCode:              ; end of resident part of code

ProgramStart:
 mov  ax,cs             ; set data segment = code segment
 mov  ds,ax
 
 mov  ax,3509h          ; get original interrupt vector 9h
 int  21h
 mov  word ptr Startof9+1,bx   ; patch old procedure address into code
 mov  word ptr Startof9+3,es

 lea  dx,int9           ; set interrupt vector 9h
 mov  ax,2509h
 int  21h

 lea  dx,EndOfCode      ; get length of code to remain resident
 mov  cl,4              ; divide by 4 to calculate paragraphs
 shr  dx,cl
 add  dx,11h            ; add 1 for remainder and 10 for PSP
 mov  ah,31h            ; terminate and stay resident
 int  21h

END ProgramStart