can anyone help me with this macro?

Anyone can explain in details what the following macro does?

#define write_XDATA(address,value) (((char *)0x010000) [address]=value)

thx!

From stackoverflow

• It assigns value to the byte at memory location 0x10000 + address. It's easier to grok if you separate it out a bit:

  char* buf = (char *)0x010000;
  buf[address]=value;
  

  (Though of course you have no choice but to mash it all together in a macro.)

  sharptooth : Not necessarily. Why could you not write a function that accepts a `char` and a `size_t` and writes that `char` at address 0x010000 + size_t?

  Marcelo Cantos : Sorry, my last statement was ambiguous. What I meant was, "...in a macro, you have no choice...".

• It maps addresses to real addresses using an offset and then writes to it. XDATA is probably a term taken over from the 8051-processor.

• You use it:

  write_XDATA( Address, Value );
  

  and it is expanded:

  ((char*)0x010000)[Address]=Value;
  

  which is equivalent to the following:

  char* baseAddress = (char*)0x010000;
  *(baseAddress + Address) = Value;
  

  so basically it writes a byte stored in Value at the address 0x010000 + Address.

• I don't know how much details you want to hear, but the macro expands itself to what you have just written -

  macro parameters address and value are put into address and value placeholders in the macro expansion definition (((char *)0x010000) [address]=value)

• This macro on address + 0x010000 saving one byte of value.

• That's most probably part of a program designed to run on an embedded platform. It's used to do memory mapped IO.

  The base address of the register-map is 0x010000. It writes value to the memory location 0x010000+address.

  The use of the square brackets [] works because of the equivalence of array-addressing and pointer arithmetic in C.