Instruction Timing - Motorola MPC8260 PowerQUICC II User Manual

2.7 Instruction Timing

The processor core is a pipelined superscalar processor. A pipelined processor is one in
which the processing of an instruction is broken into discrete stages. Because the
processing of an instruction is broken into a series of stages, an instruction does not require
the entire resources of an execution unit at one time. For example, after an instruction
completes the decode stage, it can pass on to the next stage, while the subsequent
instruction can advance into the decode stage. This improves the throughput of the
instruction ßow. The instruction pipeline in the processor core has four major stages,
described as follows:
¥ The fetch pipeline stage primarily involves retrieving instructions from the memory
system and determining the location of the next instruction fetch. Additionally, the
BPU decodes branches during the fetch stage and folds out branch instructions
before the dispatch stage if possible.
¥ The dispatch pipeline stage is responsible for decoding the instructions supplied by
the instruction fetch stage, and determining which of the instructions are eligible to
be dispatched in the current cycle. In addition, the source operands of the
instructions are read from the appropriate register Þle and dispatched with the
instruction to the execute pipeline stage. At the end of the dispatch pipeline stage,
the dispatched instructions and their operands are latched by the appropriate
execution unit.
¥ During the execute pipeline stage, each execution unit that has an executable
instruction executes the selected instruction (perhaps over multiple cycles), writes
the instruction's result into the appropriate rename register, and notiÞes the
completion stage that the instruction has Þnished execution.
The execution unit reports any internal exceptions to the completion/writeback
pipeline stage and discontinues execution until the exception is handled. The
exception is not signaled until that instruction is the next to be completed. Execution
of most load/store instructions is also pipelined. The load/store unit has two pipeline
stages. The Þrst stage is for effective address calculation and MMU translation and
the second stage is for accessing the data in the cache.
¥ The complete/writeback pipeline stage maintains the correct architectural machine
state and transfers the contents of the rename registers to the GPRs and FPRs as
instructions are retired. If the completion logic detects an instruction causing an
exception, all following instructions are cancelled, their execution results in rename
registers are discarded, and instructions are fetched from the correct instruction
stream.
The processor core provides support for single-cycle store operations and it provides an
adder/comparator in the SRU that allows the dispatch and execution of multiple integer add
and compare instructions on each cycle.
Performance of integer divide operations has been improved in the processor core. A divide
instruction takes half the cycles to execute as described in the MPC603e UserÕs Manual.
MOTOROLA Chapter 2. PowerPC Processor Core
Part I. Overview
2-29