Optimizing problems (volatile hardware register)

I have had a rather ugly problem caused due to the optimization.

I am reading data from a fifo. The read port is at address 0x8200 (read only access). It also has a read reset port. Its address is also 0x8200 (both xdata) (write onle access).

When I declare the varables for accessing the fifo like this:

xdata uchar fifo_data     _at_ 0x8200; xdata uchar fifo_rres     _at_ 0x8200;

It will result in a linker warning memory overlap. Thus I have decided to do it like this:

xdata uchar fifo_data     _at_ 0x8200;
#define fifo_rres fifo_data

In a routine I have programmed this code:

  fifo_rres = 0xff;                // reset fifo read
  jpg_adr = 1;                     // set jpg file address to the next address to be read
  fbuf_start = 0;                  // set buffer start pointer to zero
  fbuf_in = 1;                     // set buffer input pointer to next free byte in buffer
  fbuf_pos = 0;                    // set buffer read pointer to zero
  fbuf_startadr = 0;               // set buffer startadress to zero
  fbuf[0] = fifo_data;             // read first byte from jpg file

The result was, that fbuf[0] was always 0xff. Then I have read the disassembly and found that when I want to set fbuf[0], I don't access the fifo read register, but it is written with 0xff.

I think, this is matter of the compiler optimization. The compiler thinks, that when I write 0xff into some address and read it some lines later, it will still be 0xff.

How can I tell the compiler, that it is not the same? I didn't find anything in my C51 manual about that topic. Maybe I just didn't find it.