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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
PowerPC 750GX Overview
Page 54 of 377
gx_01.fm.(1.2)
March 27,2006
• The execution units process instructions from their reservation stations using the operands provided from
dispatch, and notifies the completion stage when the instruction has finished execution. With the excep-
tion of multiply and divide, integer instructions complete execution in a single cycle.
The FPU has three stages (multiply, add, and normalize) for processing floating-point arithmetic. All sin-
gle-precision arithmetic (add, subtract, multiply, and multiply/add) instructions are processed without
stalls at each stage. They have a 1-cycle throughput and a 3-cycle latency. Three different arithmetic
instructions can be in the execution unit at one time, with one instruction completing execution each
cycle. Double-precision arithmetic multiply requires two cycles in the multiply stage, one cycle in the add
stage, and one cycle in the normalize stage, which yields a 2-cycle throughput and a 4-cycle latency. All
divide instructions require multiple cycles in the first stage for processing.
The load/store unit has two reservation registers and two pipeline stages. The first stage is for effective
address calculation and the second stage is for MMU translation and accessing the L1 data cache. Load
instructions have a 1-cycle throughput and a 2-cycle latency.
In the case of an internal exception, the execution unit reports the exception to the completion pipeline
stage and (except for the FPU) discontinues instruction execution until the exception is handled. The
exception is not signaled until it is determined that all previous instructions have completed to a point
where they will not signal an exception.
• The completion unit retires instructions from the bottom two positions of the completion queue in program
order. This maintains the correct architectural machine state and transfers execution results from the
rename buffers to the GPRs and FPRs (and CTR and LR, for some instructions) as instructions are
retired. If the completion logic detects an instruction causing an exception, all subsequent instructions are
cancelled, their execution results in rename buffers are discarded, and instructions are fetched from the
appropriate exception vector.
Because the PowerPC Architecture can be applied to such a wide variety of implementations, instruction
timing varies among PowerPC processors. For a detailed discussion of instruction timing with examples and
a table of latencies for each execution unit, see Chapter 6, Instruction Timing, on page 209.
1.10 Power Management
The 750GX provides the following four power modes, selectable by setting the appropriate control bits in the
MSR and HID0 registers:
Full-power This is the default power state of the 750GX. The 750GX is fully powered, and the
internal functional units are operating at the full processor clock speed. If the
dynamic power management mode is enabled, functional units that are idle will
automatically enter a low-power state without affecting performance, software
execution, or external hardware.
Doze All the functional units of the 750GX are disabled except for the Time Base/Decre-
menter Registers and the bus snooping logic. When the processor is in doze mode,
an external asynchronous interrupt, a system management interrupt, a decre-
menter exception, a hard or soft reset, or a machine check brings the 750GX into
the full-power state. The 750GX in doze mode maintains the PLL in a fully powered
state and locked to the system external clock input (SYSCLK) so a transition to the
full-power state takes only a few processor clock cycles.