SINGAPORE/BEIJING (Reuters) -Two Chinese chipmakers are in the early stages of producing high bandwidth memory (HBM) semiconductors used in artificial intelligence chipsets, according to sources and documents. The progress in HBM - even if only in older versions of HBM - represents a major step forward in China's efforts to reduce its reliance on foreign suppliers amid tensions with Washington that have led to restrictions on U.S. exports of advanced chipsets to Chinese firms. CXMT, China's top manufacturer of DRAM chips, has developed sample HBM chips in partnership with chip packaging and testing company Tongfu Microelectronics, according to three people briefed on the matter.