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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Instruction Timing
Page 214 of 377
gx_06.fm.(1.2)
March 27, 2006
The notation conventions used in the instruction timing examples are as follows:
Figure 6-3 shows the stages of the 750GX’s execution units.
Table 6-1. Notation Conventions for Instruction Timing
Symbol Description
Fetch. The fetch stage includes the time between when an instruction is requested and when it is brought into
the instruction queue. This latency can vary, depending upon whether the instruction is in the branch target
instruction cache (BTIC), the L1 instruction cache, the L2 cache, or system memory (in which case latency
can be affected by bus speed and traffic on the system bus, and address-translation issues). Therefore, in the
examples in this chapter, the fetch stage is usually idealized. That is, an instruction is usually shown to be in
the fetch stage when it is a valid instruction in the instruction queue. The instruction queue has six entries,
IQ0–IQ5.
In dispatch entry (IQ0/IQ1). Instructions can be dispatched from IQ0 and IQ1. Because dispatch is instanta-
neous, it is perhaps more useful to describe it as an event that marks the point in time between the last cycle
in the fetch stage and the first cycle in the execute stage.
Execute. The operations specified by an instruction are being performed by the appropriate execution unit.
The black stripe is a reminder that the instruction occupies an entry in the completion queue, described in
Figure 6-3.
Complete. The instruction is in the completion queue. In the final stage, the results of the executed instruction
are written back, and the instruction is retired. The completion queue has six entries, CQ0–CQ5.
In retirement entry. Completed instructions can be retired from CQ0 and CQ1. Like dispatch, retirement is an
event that, in this case, occurs at the end of the final cycle of the complete stage.
Figure 6-3. PowerPC 750GX Microprocessor Pipeline Stages
Fetch In Dispatch Execute1 Complete/Retire
Fetch In Dispatch Complete/RetireEA
Fetch In Dispatch Complete/Retire
IU1/IU2/SRU Instructions
LSU Instructions
FPU Instructions
Normalize
Multiply Add Round/
1. Several integer instructions, such as multiply and divide instructions, require multiple cycles in the execute stage.
Execute
Execute
Calculation
Cache Align
Entry
Entry
Entry
BPU Instructions
Fetch Fetch
Predict
Complete/Retire2In Dispatch
Entry
In Completion
Queue2
2. Only those branch instructions that update the LR or CTR take an entry in the completion queue.