#pragma SMALL db CODE
#include <reg51.h>

sfr Dout = 0x90;      /* Port 1: Dout */
/* Syntax:  sfr Portname = PortAddr;  */
/* P0, P1, P2, P3 are predefined Portnames */

sbit reset = P3^1;    /* bit 1: reset */
sbit ud = P3^2;       /* bit 2: up/down */
/* Syntax:
  (1) sbit BitName = PortName^BitNumber;
  (2) sbit BitName = PortAddr^BitNumber;
  (3) sbit BitName = BitAddr;
      (Note that SFRs are bit addressable)
*/

void main()
{
  /*
  reset = 1;
  ud = 1;
  */

  while (1)
  {
    if (reset==1)
      Dout = 0;
      else if (ud == 1)
        Dout=(Dout+1) % 16;
        else
          Dout=(Dout-1) % 16;
  }
}