I'm now developing my own bootloader on the P89C668.

Unfortunately there is no way back after changing the bootvector.

Only the Atmels allow reprograming the boootvector by forcing PSEN low.

Thus I insert this code on my developing. Since my application use memory mapped IOs, /RD and /WR acting as outputs normally.

So if something goes wrong during developing, I can /RD or /WR force low during reset to have a second chance to reprogram it by the Philips bootloader.

There is also denoising done, to avoid false decision.

#define CALL(addr)      (((void(*)(void))(char code *)addr)())


void emergency_exit(void)
{
  unsigned char i;

  for( i = 0x55; i; ){          // 8 consecutive zeros = run Philips bootloader
    i <<= 1;
    if( P3_7 && P3_6 )          // check /RD or /WR if pulled low
      i++;
    if( i == 0xFF )             // 8 consecutive ones = run user bootloader
      return;
  }

  EA = 0;
  AUXR1 |= 0x20;                // enable boot
  CALL( 0xFC00 );               // invoke Philips bootloader
}

Peter