Another problem

I'm sorry for bothering y'all again... I have a little problem:

mov al,byte[MemoryMap_start.mapSize]

The line above points to the wrong address when ran with bochs... if I
change it to:

mov al,byte[MemoryMap_start.mapSize - Code_start + 0x500]     ;0x500
is the address where my code is loaded

then it does point to the right address...

Both versions assemble without a problem. Is there someting I'm doing
wrong? [ORG 0x0500] doesn't work.