[ixp1200] where is the initilization of the buffer descriptors (SRAM Push) ?

I am studying the buffer management of IXP 1200. But I can not find the location of initialization of buffer descriptors (SRAM Buffer descriptor). Obviously I confirmed that : when packet is arriving at Ingress micro ace, Ingress Micro ACE gets the buffer descriptor for SDRAM from SRAM using the instruction "sram[pop,....]". I am not sure, buffer descriptor initilazation is executed "on Micro block" OR "on ARM core". I would like to know the exact initialization code (uc or cpp). Thanks.