ARM compiler (armcc) and ARM archiver (armar)

I usually create libraries with an extension .a or .lib. As I glanced at mbed, their own library has extension .ar. KEIL comes with ARM Compilation Tools [www.keil.com]), which is used also for embed library, as I believe.

To compile files from which we want to create a library (target is Cortex-M0+).

armcc --cpu=Cortex-M0plus --no_debug -c file1 file2 fileXY

To create static library from object files

armar --rv archive_name object_file1 object_file2 object_fileXY

I successfully imported small static library to mbed and run it on my mbed board. There are many options to select. To read more here are the links:

Creating static software libraries with armar (pdf) [infocenter.arm.com/help]
Getting Started with the Compiler [keil.com]