问 /*------------------------------------------------------------------------------
HELLO.C

Copyright 1995-1999 Keil Software, Inc.
------------------------------------------------------------------------------*/

#include <REG52.H>                /* special function register declarations   */
                                  /* for the intended 8051 derivative         */

#include <stdio.h>                /* prototype declarations for I/O functions */


#ifdef MONITOR51                         /* Debugging with Monitor-51 needs   */
char code reserve [3] _at_ 0x23;         /* space for serial interrupt if     */
#endif                                   /* Stop Exection with Serial Intr.   */
                                         /* is enabled                        */


/*------------------------------------------------
The main C function.  Program execution starts
here after stack initialization.
------------------------------------------------*/
void main (void) {

/*------------------------------------------------
Setup the serial port for 1200 baud at 16MHz.
------------------------------------------------*/
#ifndef MONITOR51
    SCON  = 0x50;                /* SCON: mode 1, 8-bit UART, enable rcvr      */
    TMOD |= 0x20;               /* TMOD: timer 1, mode 2, 8-bit reload        */
    TH1   = 221;                /* TH1:  reload value for 1200 baud @ 16MHz   */
    TR1   = 1;                  /* TR1:  timer 1 run                          */
    TI    = 1;                  /* TI:   set TI to send first char of UART    */
#endif

/*------------------------------------------------
Note that an embedded program never exits (because
there is no operating system to return to).  It
must loop and execute forever.
------------------------------------------------*/
  while (1) {
    P1 ^= 0x01;                 /* Toggle P1.0 each time we print */
    printf ("Hello World\n");   /* Print "Hello World" */
  }
}
-------------------------------------------------------------
char code reserve [3] _at_ 0x23;中的-at-是什么意思？为什么monitor51需要空间？ 答 1: reserve数组定位在0x23地址而0x23正好是定时器串行口中断入口地址。

不过这个code怎么没赋值呢，调试用这个是干什么用的？