STC8，keil扩展SFR读写指令被优化的问题。
在调试STC8扩展串口时，由于扩展的SCON不能位寻址，只能字节寻址。但是写出来的代码被优化掉。声明SFR的时候加volatile 直接报错。
目前只能分两步进行才不被优化掉。请问这个问题大家是怎么解决的。
分两步不会被优化
直接写会被优化
求教各位
sfr
The sfr type defines a special function register (SFR). It is used as follows:
sfr name = address;
Where
name is the name of the SFR.
address is the address of the SFR.
SFRs are declared in the same fashion as other C variables. The only difference is that the type specified is sfr rather than char or int. For example:
sfr P0 = 0x80; /* Port-0, address 80h */
sfr P1 = 0x90; /* Port-1, address 90h */
sfr P2 = 0xA0; /* Port-2, address 0A0h */
sfr P3 = 0xB0; /* Port-3, address 0B0h */
P0, P1, P2, and P3 are the SFR name declarations. Names for sfr variables are defined just like other C variable declarations. Any symbolic name may be used in an sfr declaration.
The address specification after the equal sign ('=') must be a numeric constant. Expressions with operators are not allowed. Classic 8051 devices support the SFR address range 0x80-0xFF. The NXP 80C51MX provides an additional extended SFR space with the address range 0x180-0x1FF.
Note
sfr variables may not be declared inside a function. They must be declared outside of the function body.
sfr variables are always volatile. The compiler will not optimize accesses to this type of variables.
事实却不像keil说的那样。
.H
//串行口特殊功能寄存器
sfr SCON = 0x98;
sbit SM0 = SCON^7;
sbit SM1 = SCON^6;
sbit SM2 = SCON^5;
sbit REN = SCON^4;
sbit TB8 = SCON^3;
sbit RB8 = SCON^2;
sbit TI = SCON^1;
sbit RI = SCON^0;
sfr SBUF = 0x99;
sfr S2CON = 0x9a;
#define S2SM0 0x80
#define S2ST4 0x40
#define S2SM2 0x20
#define S2REN 0x10
#define S2TB8 0x08
#define S2RB8 0x04
#define S2TI 0x02
#define S2RI 0x01
sfr S2BUF = 0x9b;
优先级问题，加括号 （xxx & xxx) == 0 寄存器是直接寻址，不会被优化掉的，仔细检查程序。 真是丢脸了，居然踩回初学者的坑里面去了。
印象中一直记得==的优先级很低的。居然就是一前一后。
看来还是要随时记得用括号括起来，毕竟括号免费{:biggrin:}
一般用 if(xxx&yyy)和if(!(xxx&yyy)) , 没遇到这个坑 加括号是一个非常非常好的习惯，我也不知道优先级别，但是我都是加括号的。 keil 选择大模式时，在函数中定义个临时变量也被放在外部存储器，效率一下子低了很多很多，如果能优化成临时变量编译器能自动定义在内部直接寻址的128个寄存器就好了。， 从来不写==0,免得与=0混淆，写!x。 对于位变量，我会写！，对于字节，多字节变量，我习惯写==0.虽然很多时候编译出来的代码一样。
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