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Thermal management is an important design issue in high-performance, low-power portable computers. If the computer system is designed for worst-case processor power dissipation and environmental operating conditions, it carries an area and cost penalty for the system designer. The next-generation PowerPC/sup TM/ microprocessor includes a thermal assist unit (TAU) comprised of an on-chip thermal sensor and associated logic. The TAU monitors the junction temperature of the processor and dynamically adjusts processor operation to provide maximum performance under changing environmental conditions. The TAU is used in conjunction with other low-power features such as dynamic power management, instruction cache throttling and static low-power modes to provide comprehensive power and thermal management. This paper describes the implementation of the TAU and presents its characterization and operating data from first silicon.