How to revalue a symbol in C51?

Have you looked into #undef?

Jon

Have you looked into #undef?

Jon

This is what I want C51 to do as A51:
;-----------A51 MACRO-------------
refStart SET 0 ;define start addr
;-----my A51 macro "Dim"-------------------
Dim Macro ref,num
ref EQU refStart
refStart Set ref+num
ENDM
;====================================
;---now I can use this macro to define my
;address
Dim MyAddr0,1 ;define 1 byte
Dim MyAddr1,2 ;define 2 bytes after
;MyAddr0
Dim MyAddr2,1
......
;that will define:
; MyAddr0 EQU 0
; MyAddr1 EQU 1
; MyAddr2 EQU 3
; ......
;I can not use #define and #undef to do this
;How can I do?

You may use instead of EQU the SET directive. Symbols defined with SET may change the value.

"This is what I want C51 to do as A51"

Not being familiar with A51 macros, I can't really see what you're trying to do.

Perhaps if you could explain what you're actually trying to achieve we could come up with some suggestions on how to do that - rather than just trying to emulate A51 macros in C51 (which is quite possibly futile, as A51 macros are very much more powerful than the 'C' preprocessor!)

It sounds to me as though David wants to declare a number of symbols, with associated sizes, and keep a running counter of the sum of the sizes to generate the symbol values, so that he doesn't have to do it manually. In other words, he wants to be able to write something like:

#define Dim(symbol,size)  <magic goes here>

Dim(MyAddr0, 1)
Dim(MyAddr1, 2)
Dim(MyAddr2, 2)

#define MyAddr0  0
#define MyAddr1  1
#define MyAddr2  3

#define Counter 0
#define Dim(sym,size)     #define sym Counter    #define Counter (Counter + size)

U8  MyVar0;
U16 MyVar1;
U16 MyVar2;

typedef struct
    {
    U8  MyAddr0;
    U16 MyAddr1;
    U16 MyAddr2;
    } MyStruct;

...
   o = offsetof(MyStruct, MyAddr0); // 0
   o = offsetof(MyStruct, MyAddr1); // 1
   o = offsetof(MyStruct, MyAddr2); // 3

"One question is why does this need to be done in the preprocessor?"

Yes - that's the point I was trying to make earlier.
We need David to explain what it is that he's actually trying to achieve

Thanks for your help!
Now I get it, Davis' STRUCT and OFFSETOF is what I want.
I am trying to define serial addresses in my EEPRom so I can
visit these addresses in my routine. And each of address contains
a number of bytes, such as MyAddr0 contains 1 byte,MyAddr1 contains
2 bytes, MyAddr2 contains 3 bytes...

I can define these addresses using "#define"
/*********start define******************************/
#define MyAddr0 0x00 //MyAddr0 start form 0x00 and use 1 byte
#define MyAddr1 0X01 //MyAddr1 start form 0x01 and use 2 bytes(0x01 and 0x02)
#define MyAddr2 0X03 //MyAddr2 start form 0x03 and use 3 bytes(0x03 to 0x05)
/* ....... */ //now MyAddr3 must start form 0x06
/*********end define*******************************/

It will bother me when there are many addresses, but if I write A51
program, I can use "Dim" MACRO list up to "EQU" these address and "SET"
the start address to the first no-use byte in EEPRom. "Dim" MACRO is easy
use :)
Now I can use Davis' STRUCT and OFFSETOF to do this easily, thank you!