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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Instruction Timing
Page 212 of 377
gx_06.fm.(1.2)
March 27, 2006
• 64-bit floating-point unit (FPU)
• Load/store unit (LSU)
• System register unit (SRU)
Figure 6-1 represents a generic pipelined execution unit.
The 750GX can retire two instructions in every clock cycle. In general, the 750GX processes instructions in
four stages—fetch, decode/dispatch, execute, and complete as shown in Figure 6-2. Note that the example of
a pipelined execution unit in Figure 6-1 is similar to the 3-stage FPU pipeline in Figure 6-2.
Figure 6-1. Pipelined Execution Unit
Figure 6-2. Superscalar/Pipeline Diagram
Clock 0
Clock 1
Clock 2
Clock 3
Instruction A — —
Instruction B
Instruction C
Instruction D
Instruction A
Instruction B
Instruction C
—
Instruction A
Instruction B
Stage 1 Stage 2 Stage 3
Fetch
Complete (Write-back)
Decode/Dispatch
Execute Stage
FPU3SRU IU2IU1
Maximum 3-instruction dispatch per
clock cycle (includes one branch
instruction)
Maximum 2-instruction completion per
clock cycle
FPU2
FPU1
LSU1
Maximum 4-instruction fetch per
clock cycle
LSU2
BPU