#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