Assembly code fragment explanation

Hi, 

I am trying to understand the x86 boot code where I encountered this
assembly code fragment - 

      asm volatile("lcallw *%0"
                 : : "m" (boot_params.hdr.realmode_swtch)
                 : "eax", "ebx", "ecx", "edx");


"lcallw *%0" - what does this mean ??

Regards,
Arun