sniper模拟器中，SimRoiStart()这个接口的功能是什么啊

Question

#define SimMagic0(cmd) ({                    \
   unsigned long _cmd = (cmd), _res;         \
   __asm__ __volatile__ (                    \
   "mov %1, %%" MAGIC_REG_A "\n"             \
   "\txchg %%bx, %%bx\n"                     \
   : "=a" (_res)           /* output    */   \
   : "g"(_cmd)             /* input     */   \
      );                   /* clobbered */   \
   _res;                                     \
})

#define SimRoiStart()             SimMagic0(SIM_CMD_ROI_START)
#define SimRoiEnd()               SimMagic0(SIM_CMD_ROI_END)

SimRoiStart()起到了什么作用啊，调用它会有什么效果