请教高手！！

blueszg

 

u8 sim900a_send_cmd(u8 *cmd,u8 *ack,u16 waittime)
{
u8 res=0; 
USART2_RX_STA=0;
if((u32)cmd<=0XFF)
{
while(DMA1_Channel7->CNDTR!=0); //?????¨??7?????ê??   
USART2->DR=(u32)cmd;
}else u2_printf("%s\r\n",cmd);//·????ü??
if(ack&&waittime) //?è??????????
{
while(--waittime) //?????????±
{
delay_ms(10);
if(USART2_RX_STA&0X8000)//?????????????????á??
{
if(sim900a_check_cmd(ack))break;//???????§???? 
USART2_RX_STA=0;
} 
}
if(waittime==0)res=1; 
}
return res;
} 
这里的if((u32)cmd<=0XFF是啥意思呢？