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A Tour of the Pentium Pro Processor Microarchitecture

A Tour of the Pentium Pro Processor Microarchitecture

Introduction

One of the Pentium Pro processor's primary goals was to significantly exceed the
performance of the 100MHz Pentium processor while being manufactured on the same
semiconductor process. Using the same process as a volume production processor
practically assured that the Pentium Pro processor would be manufacturable, but
it meant that Intel had to focus on an improved microarchitecture for ALL of the
performance gains. This guided tour describes how multiple architectural
techniques - some proven in mainframe computers, some proposed in academia and
some we innovated ourselves - were carefully interwoven, modified, enhanced,
tuned and implemented to produce the Pentium Pro microprocessor. This unique
combination of architectural features, which Intel describes as Dynamic
Execution, enabled the first Pentium Pro processor silicon to exceed the
original performance goal.

Building from an already high platform

The Pentium processor set an impressive performance standard with its pipelined,
superscalar microarchitecture. The Pentium processor's pipelined implementation
uses five stages to extract high throughput from the silicon - the Pentium Pro
processor moves to a decoupled, 12-stage, superpipelined implementation, trading
less work per pipestage for more stages. The Pentium Pro processor reduced its
pipestage time by 33 percent, compared with a Pentium processor, which means the
Pentium Pro processor can have a 33% higher clock speed than a Pentium processor
and still be equally easy to produce from a semiconductor manufacturing process
(i.e., transistor speed) perspective.

The Pentium processor's superscalar microarchitecture, with its ability to
execute two instructions per clock, would be difficult to exceed without a new
approach. The new approach used by the Pentium Pro processor removes the
constraint of linear instruction sequencing between the traditional "fetch" and
"execute" phases, and opens up a wide instruction window using an instruction
pool. This approach allows the "execute" phase of the Pentium Pro processor to
have much more visibility into the program's instruction stream so that better
scheduling may take place. It requires the instruction "fetch/decode" phase of
the Pentium Pro processor to be much more intelligent in terms of predicting
program flow. Optimized scheduling requires the fundamental "execute" phase to
be replaced by decoupled "dispatch/execute" and "retire" phases. This allows
instructions to be started in any order but always be completed in the original
program order. The Pentium Pro processor is implemented as three independent
engines coupled with an instruction pool as shown in Figure 1 below.

What is the fundamental problem to solve?

Before starting our tour on how the Pentium Pro processor achieves its high
performance it is important to note why this three- independent-engine approach
was taken. A fundamental fact of today's microprocessor implementations must be
appreciated: most CPU cores are not fully utilized. Consider the code fragment
in Figure 2 below:

The first instruction in this example is a load of r1 that, at run time, causes
a cache miss. A traditional CPU core must wait for its bus interface unit to
read this data from main memory and return it before moving on to instruction 2.
This CPU stalls while waiting for this data and is thus being under-utilized.

While CPU speeds have increased 10-fold over the past 10 years, the speed of
main memory devices has only increased by 60 percent. This increasing memory
latency, relative to the CPU core speed, is a fundamental problem that the
Pentium Pro processor set out to solve. One approach would be to place the
burden of this problem onto the chipset but a high-performance CPU that needs
very high speed, specialized, support components is not a good solution for a
volume production system.

A brute-force approach to this problem is, of course, increasing the size of the
L2 cache to reduce the miss ratio. While effective, this is another expensive
solution, especially considering the speed requirements of today's L2 cache SRAM
components. Instead, the Pentium Pro processor is designed from an overall
system implementation perspective which will allow higher performance systems to
be designed with cheaper memory subsystem designs.

Pentium Pro processor takes an innovative approach

To avoid this memory latency problem the Pentium Pro processor "looks-ahead"
into its instruction pool at subsequent instructions and will do useful work
rather than be stalled. In the example in Figure 2, instruction 2 is not
executable since it depends upon the result of instruction 1; however both
instructions 3 and 4 are executable. The Pentium Pro processor speculatively
executes instructions 3 and 4. We cannot commit the results of this speculative
execution to permanent machine state (i.e., the programmer-visible registers)
since we must maintain the original program order, so the results are instead
stored back in the instruction pool awaiting in-order retirement. The core
executes instructions depending upon their readiness to execute and not on their
original program order (it is a true dataflow engine). This approach has the
side effect that instructions are typically executed out-of-order.

The cache miss on instruction 1 will take many internal clocks, so the Pentium
Pro processor core continues to look ahead for other instructions that could be
speculatively executed and is typically looking 20 to 30 instructions in front
of the program counter. Within this 20- to 30- instruction window there will be,
on average, five branches that the fetch/decode unit must correctly predict if
the dispatch/execute unit is to do useful work. The sparse register set of an
Intel Architecture (IA) processor will create many false dependencies on
registers so the dispatch/execute unit will rename the IA registers to enable
additional forward progress. The retire unit owns the physical IA register set
and results are only committed to permanent machine state when it removes
completed instructions from the pool in original program order.

Dynamic Execution technology can be summarized as optimally adjusting
instruction execution by predicting program flow, analysing the program's
dataflow graph to choose the best order to execute the instructions, then having
the ability to speculatively execute instructions in the preferred order. The
Pentium Pro processor dynamically adjusts its work, as defined by the incoming
instruction stream, to minimize overall execution time.

Overview of the stops on the tour

We have previewed how the Pentium Pro processor takes an innovative approach to
overcome a key system constraint. Now let's take a closer look inside the
Pentium Pro processor to understand how it implements Dynamic Execution. Figure
3 below extends the basic block diagram to include the cache and memory
interfaces - these will also be stops on our tour. We shall travel down the
Pentium Pro processor pipeline to understand the role of each unit:

•The FETCH/DECODE unit: An in-order unit that takes as input the user program
instruction stream from the instruction cache, and decodes them into a series of
micro-operations (uops) that represent the dataflow of that instruction stream.
The program pre-fetch is itself speculative.

•The DISPATCH/EXECUTE unit: An out-of-order unit that accepts the dataflow
stream, schedules execution of the uops subject to data dependencies and
resource availability and temporarily stores the results of these speculative
executions.

•The RETIRE unit: An in-order unit that knows how and when to commit ("retire")
the temporary, speculative results to permanent architectural state.

•The BUS INTERFACE unit: A partially ordered unit responsible for connecting the
three internal units to the real world. The bus interface unit communicates
directly with the L2 cache supporting up to four concurrent cache accesses. The
bus interface unit also controls a transaction bus, with MESI snooping protocol,
to system memory.

Tour stop #1: The FETCH/DECODE unit.

Figure 4 shows a more detailed view of the fetch/decode unit:

Let's start the tour at the Instruction Cache (ICache), a nearby place for
instructions to reside so that they can be looked up quickly when the CPU needs
them. The Next_IP unit provides the ICache index, based on inputs from the
Branch Target Buffer (BTB), trap/interrupt status, and branch-misprediction
indications from the integer execution section. The 512 entry BTB uses an
extension of Yeh's algorithm to provide greater than 90 percent prediction
accuracy. For now, let's assume that nothing exceptional is happening, and that
the BTB is correct in its predictions. (The Pentium Pro processor integrates
features that allow for the rapid recovery from a mis-prediction, but more of
that later.)

The ICache fetches the cache line corresponding to the index from the Next_IP,
and the next line, and presents 16 aligned bytes to the decoder. Two lines are
read because the IA instruction stream is byte-aligned, and code often branches
to the middle or end of a cache line. This part of the pipeline takes three
clocks, including the time to rotate the prefetched bytes so that they are
justified for the instruction decoders (ID). The beginning and end of the IA
instructions are marked.

Three parallel decoders accept this stream of marked bytes, and proceed to find
and decode the IA instructions contained therein. The decoder converts the IA
instructions into triadic uops (two logical sources, one logical destination per
uop). Most IA instructions are converted directly into single uops, some
instructions are decoded into one-to-four uops and the complex instructions
require microcode (the box labeled MIS in Figure 4, this microcode is just a set
of preprogrammed sequences of normal uops). Some instructions, called prefix
bytes, modify the following instruction giving the decoder a lot of work to do.
The uops are enqueued, and sent to the Register Alias Table (RAT) unit, where
the logical IA-based register references are converted into Pentium Pro
processor physical register references, and to the Allocator stage, which adds
status information to the uops and enters them into the instruction pool. The
instruction pool is implemented as an array of Content Addressable Memory called
the ReOrder Buffer (ROB).

We have now reached the end of the in-order pipe.

Tour stop #2: The DISPATCH/EXECUTE unit

The dispatch unit selects uops from the instruction pool depending upon their
status. If the status indicates that a uop has all of its operands then the
dispatch unit checks to see if the execution resource needed by that uop is also
available. If both are true, it removes that uop and sends it to the resource
where it is executed. The results of the uop are later returned to the pool.
There are five ports on the Reservation Station and the multiple resources are
accessed as shown in Figure 5 below:

The Pentium Pro processor can schedule at a peak rate of 5 uops per clock, one
to each resource port, but a sustained rate of 3 uops per clock is typical. The
activity of this scheduling process is the quintessential out-of-order process;
uops are dispatched to the execution resources strictly according to dataflow
constraints and resource availability, without regard to the original ordering
of the program.

Note that the actual algorithm employed by this execution-scheduling process is
vitally important to performance. If only one uop per resource becomes data-
ready per clock cycle, then there is no choice. But if several are available,
which should it choose? It could choose randomly, or first-come-first-served.
Ideally it would choose whichever uop would shorten the overall dataflow graph
of the program being run. Since there is no way to really know that at run-time,
it approximates by using a pseudo FIFO scheduling algorithm favoring back-to-
back uops.

Note that many of the uops are branches, because many IA instructions are
branches. The Branch Target Buffer will correctly predict most of these branches
but it can't correctly predict them all. Consider a BTB that's correctly
predicting the backward branch at the bottom of a loop: eventually that loop is
going to terminate, and when it does, that branch will be mispredicted. Branch
uops are tagged (in the in-order pipeline) with their fallthrough address and
the destination that was predicted for them. When the branch executes, what the
branch actually did is compared against what the prediction hardware said it
would do. If those coincide, then the branch eventually retires, and most of the
speculatively executed work behind it in the instruction pool is good.

But if they do not coincide (a branch was predicted as taken but fell through,
or was predicted as not taken and it actually did take the branch) then the Jump
Execution Unit (JEU) changes the status of all of the uops behind the branch to
remove them from the instruction pool. In that case the proper branch
destination is provided to the BTB which restarts the whole pipeline from the
new target address.

Tour stop #3: The RETIRE unit

Figure 6 shows a more detailed view of the retire unit:

The retire unit is also checking the status of uops in the instruction pool - it
is looking for uops that have executed and can be removed from the pool. Once
removed, the uops' original architectural target is written as per the original
IA instruction. The retirement unit must not only notice which uops are complete,
it must also re-impose the original program order on them. It must also do this
in the face of interrupts, traps, faults, breakpoints and mis- predictions.

There are two clock cycles devoted to the retirement process. The retirement
unit must first read the instruction pool to find the potential candidates for
retirement and determine which of these candidates are next in the original
program order. Then it writes the results of this cycle's retirements to both
the Instruction Pool and the RRF. The retirement unit is capable of retiring 3
uops per clock.

Tour stop #4: BUS INTERFACE unit

Figure 7 shows a more detailed view of the bus interface unit:

There are two types of memory access: loads and stores. Loads only need to
specify the memory address to be accessed, the width of the data being retrieved,
and the destination register. Loads are encoded into a single uop. Stores need
to provide a memory address, a data width, and the data to be written. Stores
therefore require two uops, one to generate the address, one to generate the
data. These uops are scheduled independently to maximize their concurrency, but
must re-combine in the store buffer for the store to complete.

Stores are never performed speculatively, there being no transparent way to undo
them. Stores are also never re- ordered among themselves. The Store Buffer
dispatches a store only when the store has both its address and its data, and
there are no older stores awaiting dispatch.

What impact will a speculative core have on the real world? Early in the Pentium
Pro processor project, we studied the importance of memory access reordering.
The basic conclusions were as follows:

•Stores must be constrained from passing other stores, for only a small
impact on performance.
•Stores can be constrained from passing loads, for an inconsequential
performance loss.
•Constraining loads from passing other loads or from passing stores creates
a significant impact on performance.

So what we need is a memory subsystem architecture that allows loads to pass
stores. And we need to make it possible for loads to pass loads. The Memory
Order Buffer (MOB) accomplishes this task by acting like a reservation station
and Re-Order Buffer, in that it holds suspended loads and stores, redispatching
them when the blocking condition (dependency or resource) disappears.

Tour Summary

It is the unique combination of improved branch prediction (to offer the core
many instructions), data flow analysis (choosing the best order), and
speculative execution (executing instructions in the preferred order) that
enables the Pentium Pro processor to deliver its performance boost over the
Pentium processor. This unique combination is called Dynamic Execution and it is
similar in impact as "Superscalar" was to previous generation Intel Architecture
processors. While all your PC applications run on the Pentium Pro processor,
today's powerful 32-bit applications take best advantage of Pentium Pro
processor performance.

And while our architects were honing the Pentium Pro processor microarchitecture,
our silicon technologists were working on an advanced manufacturing process -
the 0.35 micron process. The result is that the initial Pentium Pro Processor
CPU core speeds range up to 200MHz.

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