Accroding to AMD and element of throttling can occur on Palominos, Tbreds and Bartons through the activation of STPCLK#.
Courtesy of the AMD technical documents:
"There are two mechanisms for asserting STPCLK#—hardware
and software.
The Southbridge can force STPCLK# assertion for throttling to
protect the processor from exceeding its maximum case
temperature. This is accomplished by asserting the THERM#
input to the Southbridge. Throttling asserts STPCLK# for a
percentage of a predefined throttling period: STPCLK# is
repetitively asserted and deasserted until THERM# is
deasserted.
Software can force the processor into the Stop Grant state by
accessing ACPI-defined registers typically located in the
Southbridge.
The operating system places the processor into the C2 Stop
Grant state by reading the P_LVL2 register in the Southbridge.
If an ACPI Thermal Zone is defined for the processor, the
operating system can initiate throttling with STPCLK# using
the ACPI defined P_CNT register in the Southbridge. The
Northbridge connects the AMD Athlon system bus, and the
processor enters the Probe state to service cache snoops during
Stop Grant for C2 or throttling."