MPC8544COMEDEV Freescale Semiconductor, MPC8544COMEDEV Datasheet - Page 201

• Manufacturer Part Number: MPC8544COMEDEV
• Description: KIT DEV EXPRESS MPC8544COM
• Manufacturer: Freescale Semiconductor
• Type: MPUr

Specifications of MPC8544COMEDEV

• Contents: Board
• For Use With/related Products: MPC8544
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Freescale Semiconductor
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Instruction fetch timing depends on many variables, such as whether an instruction is in the on-chip
instruction cache or an L2 cache (if implemented). Those factors increase when it is necessary to
fetch instructions from system memory and include the processor-to-bus clock ratio, the amount of
bus traffic, and whether any cache coherency operations are required.
Because there are so many variables, unless otherwise specified, the instruction timing examples
in this chapter assume optimal performance and show the portion of the fetch stage in which the
instruction is in the instruction queue. The fetch1 and fetch2 stages are primarily involved in
retrieving instructions.
The decode/dispatch stage fully decodes each instruction; most instructions are dispatched to the
issue queues (however, isync, rfi, sc, nops, and some other instructions do not go to issue queues).
The two issue queues, BIQ and GIQ, can accept as many as one and two instructions, respectively,
in a cycle. The behavior of instruction dispatch is covered in significant detail in the e500 Software
Optimization Guide. The following simplification covers most cases:
— Instructions dispatch only from the two lowest IQ entries—IQ0 and IQ1.
— A total of two instructions can be dispatched to the issue queues per clock cycle.
— Space must be available in the CQ for an instruction to decode and dispatch (this includes
Dispatch is treated as an event at the end of the decode stage. The issue stage reads source operands
from rename registers and register files and determines when instructions are latched into the
execution unit reservation stations. Note that the e500 has 14 rename registers, one for each
completion queue entry, so instructions cannot stall because of a shortage of rename registers.
The general behavior of the two issue queues is described as follows:
— The GIQ accepts as many as two instructions from the dispatch unit per cycle. SU1, SU2, MU,
instructions that are assigned a space in the CQ but not in an issue queue).
and all LSU instructions (including 64-bit loads and stores) are dispatched to the GIQ, shown
in
Instructions can be issued out-of-order from the bottom two GIQ entries (GIQ1–GIQ0). GIQ0
can issue to SU1, MU, and LSU. GIQ1 can issue to SU2, MU, and LSU.
Note that SU2 executes a subset of the instructions that can be executed in SU1. The ability to
identify and dispatch instructions to SU2 increases the availability of SU1 to execute more
computational-intensive instructions.
An instruction in GIQ1 destined for SU2 or the LSU need not wait for an MU instruction in
GIQ0 that is stalled behind a long-latency divide.
Figure
MPC8544E PowerQUICC III Integrated Host Processor Family Reference Manual, Rev. 1
5-6.
From IQ0/IQ1
Figure 5-6. GPR Issue Queue (GIQ)
GIQ3
GIQ2
GIQ1
GIQ0
To SU2, MU, or LSU
To SU1, MU, or LSU
Core Complex Overview
5-15