** Nvidia NVDA.O and Advanced Micro Devices' AMD.O shares fall

** Brokerage UBS cuts PTs on chipmakers' shares due to tighter U.S. controls on exports of advanced AI chips to China

** NVDA down 3.4% to $100.99, AMD falls 0.7% to $87.75

** Brokerage cuts Nvidia's PT to $180 from $185, AMD's PT cut to $155 from $175

** $5.5 billion in charges flagged by NVDA due to U.S. export restrictions imply a revenue impact of ~$9 billion, with $700 million of this coming in Q1 and remaining ~$8 billion spread across Q2 and Q3 - UBS

** AMD's revenue impact seen as ~$1 billion based on $800 million in charges - UBS

** Brokerage sees softer overall demand environment for AMD in the back half of the year given tariff headwinds

** YTD, Nvidia shares down 24.8%, AMD down 27.4%

(Reporting by Arsheeya Bajwa)