dear Zeeshan,

After reading your post it seems you have 2 worries
1. stack initialisation in idata.
2. bios variables in data should not be corrupted due to C routines.

as far as data variables are concern in many compilers/assemblers you can specify memory location(if critical)

I didn't used keil much but in sdcc there is a switch that tells sdcc where to start stack from during compiling. Search if similar feature is there in keil.

in sdcc I can do above things as

1. stack initialisation
during compiling add switch --stack-at 0x80
(note: sdcc is command line compiler, keil is not)

2. variable at specific addres
in code you write
data unsigned char my_var at 0x20

the method I mentioned is specifically for sdcc
but keil also must be having some facility for doing this kind of stuff, for which you better read help

Regards,
Mahesh