IDEAL
MODEL small
STACK 100h
DATASEG
x      dw  40
y      dw  40 
xcoord equ [bp+4]
ycoord equ [bp+6]
sqsz   equ 15   ;square size in pixels. 
color  db  0Dh  ;0Ch-red, 9- light blue, 0Ah - light green, 0Dh - light magenta, 0Eh - yellow

CODESEG

proc  drawpixel
      push bp
      mov bp,sp
      push ax
      push bx
      push cx
      push dx
      mov cx,xcoord
      mov dx,ycoord
      mov al,[color]
      mov bh,0h
      mov ah,0Ch
      int 10h
      pop dx
      pop cx
      pop bx
      pop ax
      pop bp
      ret 4  
endp  drawpixel

start:
      mov ax, @data
      mov ds, ax 
; Graphic mode
      mov  ax,13h
      int  10h
      push 30          ;y
      push 80          ;x
      call drawpixel
; Wait for key press
      mov  ah,00h  
      int  16h
      inc  bl     

; Return to text mode
      mov  ah,0
      mov  al,2
      int  10h 
exit:         
      mov  ax, 4c00h
      int  21h
END start