Hi Peter,
Thanks for these informations.

Where could I find the C51-primer document which you're speaking of ?

In order to understand correctly the compiler and linker, I'm trying to build a small example project.
There is a main.c and a serial.c
I'm just trying to receive and send text on a terminal.

I've chosen the Small model. All variables are located in DATA and the serial buffers (256 bytes each) are located in XDATA.
For the moment I'm still confusing between DATA & PDATA.
As far I've understood, DATA (128 first bytes of RAM) contains the variables, IDATA are the 128 following and are containing the bit RAM & registers, and the 768 following are XDATA for general purpose. Where is PDATA ? I've seen that it's for compact model but what is the advantage ?

Then, where specify the limits of each segment ? Is it necessary ? Currently I don't how the linker knows when the internal 768 bytes XDATA will be full !!

Here under is my simple batch file I'm using with the command line controls :

echo off
c:keilbinc51 main.c CD LC SB MODP2
if errorlevel 1 goto end
c:keilbinc51 iserial.c CD LC SB MODP2
if errorlevel 1 goto end
c:keilbinbl51 main.obj, iserial.obj TO TRX RS(256)
c:keilbinoh51 trx

:end

Do you see anything to add ?

Thanks
Stephane