how to use a bit variable defined in another module?

Gentlemen:
I wanna use a bit variable in file:assem.asm.And the bit variable is defined in another file:main.c;Just as follows:

in main.c
unsigned char bdata flags;
sbit red_flag = flags^1;

in assem.asm

what should I do to let the keil know I want invoke the red_flag bit variable which is defined in main.c;how to declare it? The MCU I use is atmel at89s52.Thanks.
---going to mad.