South Korea's benchmark KOSPI fell nearly 5% to around 6,100 on Tuesday morning, extending a 3.7% drop from Monday and leaving the index 10.1% lower over five sessions. At 9:49 a.m. in Seoul, it was still down 4.06%. The trigger was a weekend disclosure from ChangXin Memory Technologies, China's largest memory maker, that it had made progress on a denser 4F² chip design. Shares of Samsung Electronics and SK Hynix, the two companies that together account for more than half the KOSPI's market value, each fell roughly 5% by 9:13 a.m.
What CXMT announced over the weekend
The Chinese company said it had advanced work on a 4F² architecture — a transistor layout that packs more memory cells into the same area. TrendForce estimates the design could increase memory density by 30% to 50%. CXMT intends to ship server memory built on that design by the end of the year, aiming at a higher-value market segment where Samsung and SK Hynix currently compete.
The disclosure landed hard because investors had already been jittery about South Korea's heavy concentration in memory chips. Samsung and SK Hynix had pushed the KOSPI past 9,000 in June. The index now trades more than 30% below that mark. Even after the selloff, the KOSPI remains up 44.89% for 2026, a reminder of how much of this year's rally rode on the two chipmakers.
Foreign selling meets share buyback fatigue
Tuesday's selling wasn't only about CXMT. Foreign investors sold a net 341.4 billion won of shares, while individuals and institutions bought. Monday's slide also reflected foreign selling and the near-completion of chipmaker share buybacks, according to SBS News. Those buybacks had been a steady source of demand; with that support fading, the market had less cushion for bad news.
The pain spread beyond Seoul. Micron fell 3.9% and Sandisk 6% in US trading overnight, according to Bloomberg data. The reaction reflects a single worry: if a Chinese newcomer can match density and move into server memory, the pricing power that Samsung, SK Hynix, and Micron have enjoyed could loosen.
CXMT's actual position
It helps to keep scale in view. CXMT held about 8% of global DRAM revenue in the first quarter, per Counterpoint Research. Samsung led with 38%, SK Hynix followed with 29%, and Micron held 22%.
CXMT also has a technical hurdle. The company still struggles to stabilize yields on high-bandwidth memory, according to a Korea Times report. Meanwhile, Samsung and SK Hynix already sample the newer HBM4E generation. So the 4F² news is about a future production plan, not an immediate shipment that displaces incumbent suppliers.
That distinction didn't stop the selling. Trader memory of CXMT's July debut in Shanghai is still fresh: the KOSPI fell more than 8% that day and triggered a circuit breaker. Tuesday's move was smaller but followed the same reflex.
A rebound attempt that hasn't stuck
Kiwoom Securities researcher Han Ji-young expected dip buying to spark a rebound during the session. By late morning, that view was tested but not confirmed. The KOSPI was still down 4.06% at 9:49 a.m. in Seoul.
The next concrete marker is CXMT's yield progress on high-bandwidth memory and whether its 4F² server chips actually ship by year-end. Until then, the KOSPI's direction leans on two stocks that hold more than half its value — and on whether investors treat this as a buying opportunity or the start of a margin reset.
