IBM POWERPC 750GL - Manual (Page 335)

User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_10.fm.(1.2)
March 27, 2006 Power and Thermal Management
Page 335 of 377
10. Power and Thermal Management
The 750GX microprocessor is specifically designed for low-power operation. It provides both automatic and
program-controlled power reduction modes for progressive reduction of power consumption. It also provides
a thermal assist unit (TAU) to allow on-chip thermal measurement, allowing sophisticated thermal manage-
ment for high-performance portable systems. This chapter describes the hardware support provided by the
750GX for power and thermal management.
10.1 Dynamic Power Management
Dynamic power management (DPM) automatically powers up and down the individual execution units of the
750GX, based upon the contents of the instruction stream. For example, if no floating-point instructions are
being executed, the floating-point unit is automatically powered down. Power is not actually removed from the
execution unit; instead, each execution unit has an independent clock input, which is automatically controlled
on a clock-by-clock basis. Since complementary metal-oxide semiconductor (CMOS) circuits consume negli-
gible power when they are not switching, stopping the clock to an execution unit effectively eliminates its
power consumption. The operation of DPM is completely transparent to software or any external hardware.
Dynamic power management is enabled by setting the DPM bit in Hardware-Implementation-Dependent
Register 0 (HID0[DPM] = 1).
10.2 Programmable Power Modes
The 750GX provides four programmable power modes—full on, doze, nap, and sleep. Software selects these
modes by setting one (and only one) of the three power saving mode bits in the HID0 Register.
Hardware can enable a power management state through external asynchronous interrupts. Such a hard-
ware interrupt causes the transfer of program flow to interrupt handler code, which then invokes the appro-
priate power saving mode. The 750GX also contains a decrementer, which allows it to enter the nap or doze
mode for a predetermined amount of time and then return to full power operation through a decrementer
interrupt.
Note: The 750GX cannot switch from one power management mode to another without first returning to full-
on mode.
The sleep mode disables bus snooping. Therefore, a hardware handshake is provided to ensure coherency
before the 750GX enters this power management mode.
These power states and power saving modes are shown Figure 10-1, 750GX Power States and Table 10-1
on page 336 summarizes the four power modes.