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Is there a way to change the DEFINE statement to get rid of the two useless mov(s)?
C51 COMPILER V6.02, COMPILATION OF MODULE MAIN OBJECT MODULE PLACED IN .\MAIN.OBJ COMPILER INVOKED BY: C:\PROGRAM FILES\KEIL\C51\BIN\C51.EXE .\MAIN.C OPTIMIZE(7,SPEED) NOINTPROMOTE MODDP2 DEBUG OBJECTEX -TEND CODE SYMBOLS NOCOND stmt level source 1 typedef unsigned char BYTE; 2 typedef unsigned int WORD; 3 4 typedef pdata struct 5 { 6 BYTE PC0 :1; 7 BYTE PC1 :1; 8 BYTE nFifoPend :1; //PC2 9 BYTE PC3 :1; 10 BYTE FpgaReset :1; //PC4 11 BYTE PC5 :1; 12 BYTE PC6 :1; 13 BYTE PC7 :1; 14 } CType; 15 16 extern volatile BYTE pdata xOUTC; 17 18 #define IoPins_FpgaReset ( ((CType pdata*)(&xOUTC))->FpgaReset ) 19 20 void main(void) 21 { 22 1 IoPins_FpgaReset = 1; 23 1 } 24 25 ; FUNCTION main (BEGIN) ; SOURCE LINE # 20 ; SOURCE LINE # 21 ; SOURCE LINE # 22 0000 7A00 E MOV R2,#HIGH xOUTC 0002 7900 E MOV R1,#LOW xOUTC 0004 7800 E MOV R0,#LOW xOUTC 0006 E2 MOVX A,@R0 0007 4410 ORL A,#010H 0009 F2 MOVX @R0,A ; SOURCE LINE # 23 000A 22 RET ; FUNCTION main (END)
However, I disagree that 'The volatile on the "extern BYTE pdata xOUTC" is not necessary'. It is not necessary because C ignores it on extern'd variable declarations. It only matters on the definition which is in some other translation unit for your project. If you don't put volatile on the definition then, yes, xOUT = xOUT will be optimized out. Remember definitions allocate storage, declarations do not have to (a definition is an implicit declaration). So I agree with your bit-fields being efficient after playing with them. What did you think of my other example using
volatile CType pdata r_portC _at_ 0x8000;
I am sorry, I am quite wrong. Obviously since the compiler cannot know that the definition might have the volatile keyword if the declaration does not then it will optimize out the xOUT = xOUT statement. You do need to make the definition and extern declaration volatile if the memory "location" is actually a memory mapped device or can be changed by an ISR or other task. Please excuse my whilst I eat this large crow. - Mark
I think the real problem here comes from the fact that the memory location that is referenced is an unsigned char type. However, the actual data is a structure. So, to access the structure you must take the address of the unsigned char, cast it to a structure pointer, and dereference an element of the structure. I think it would be much more clear if the original variable were declared as a structure of the appropriate type. Then you don't have to do all of this explicit type casting and you won't run into that sneaky volatile problem. Jon