The US Department of Justice (DoJ) has extended its anti-trust investigation into the SRAM memory market by serving both Toshiba and Samsung with sub-poenas.

Just hours after asking competitors Mitsubishi and Cypress for information about their sales practices in the SRAM (static RAM) chip market, the two memory giants were also served.

Samsung said it would comply with the request for information about sales and marketing practices throughout the industry, a spokeswoman said. "Samsung will cooperate fully with the Department of Justice on this matter," she said. "Samsung is committed to fair competition and ethical practices and forbids anticompetitive behavior."

Samsung has already been charged with anti-trust violations in a DoJ investigation of the DRAM (dynamic RAM) market. Justice Department lawyers found evidence that several DRAM chip makers manipulated prices while bidding for business with computer makers Dell, Apple, HP and IBM. Several DRAM makers have been ordered to pay fines as high as $300 million and three Samsung executives were sentenced to time in prison.

Toshiba gave a less welcoming response, saying in a statement: "Toshiba's American subsidiary, Toshiba America Electronics Components, has received a subpoena from the Department of Justice and is co-operating in what appears to be an industry-wide investigation. We refrain from commenting further."

SRAM chips can retain data as long as they have power, and are typically used as a memory buffer in devices such as computers and hard-disk drives, or to handle data in low-power devices like mobile phones.