|isa=x86-64

| Sandy Bridge || SNB || Desktop performance to value, AiOs, and minis

| {{intel|Sandy Bridge E|l=core}} || SNB-E || Workstations & entry-level servers

{{further|intel/microarchitectures/westmere#Process_Technology|l1=Westmere § Process Technology}}

| rowspan="3" | [[Microsoft]] || rowspan="3" | Windows || {{tchk|yes|Windows XP}}

Sandy Bridge features an entirely new architecture with a brand new core design which is both more highly performing and power efficient. The front-end has been entirely redesigned to incorporate a new decoded pipeline using a new µOP cache. The back-end is an entirely new PRF-based renaming architecture with a considerably large parallelism window. Sandy Bridge also provides considerable higher integration versus its {{intel|microarchitectures|predecessors}} resulting a full [[system on a chip]] design.

As part of the entire system overhaul, the cache architecture has been streamlined and was made more scalable. Sandy Bridge features a high-bandwidth [[last level cache]] which is shared by all the [[physical core|cores]] as well as the [[integrated graphics]] and the [[system agent]]. The LLC is an inclusive multi-bank cache architecture that is tightly associative with the individual cores. Each core is paired with a "slice" of LLC which is 2 [[MiB]] in size (lower amount for lower-end models). This pairing of cores and cache slices scales with the number of cores which provides a significant performance boost while saving power and bandwidth. Partitioning the data also helps simplify coherency as well as reducing localized contentions and hot spots.

Sandy Bridge is Intel's first microarchitecture to integrate the graphics on-die. One of the key enablers for this feature is the new cache architecture. Conceptually, the integrated graphics is treated just like another core. The graphics itself can decide which of its buffers (e.g., display or textures) will sit in the LLC and be coherent. Because it's possible for the graphics to use large amounts of memory, the possibility of [[cache thrashing|thrashing]] had to be addressed. A Special mechanism has been implemented inside the cache in order to prevent thrashing. In order to prevent the graphics from flushing out all the core's data, that mechanism is capable of capping how much of the cache will be dedicated to the cores and how much will be dedicated to the graphics.

The entire physical address space is mapped distributively across all the slices using a [[hash function]]. On a [[cache miss|miss]], the core needs to decode the address to figure out which slice ID to request the data from. Physical addresses are hashed at the source in order to prevent hot spots. The cache box is responsible for the maintaining of coherency and ordering between requests. Because the LLC slices are fully inclusive, it can make efficienct use of an on-die snoop filter. Each slice makes use of {{intel|Core Valid Bits}} (CVB) which is used to eliminate unnecessary snoops to the cores. A single bit per cache line is needed to indicate if the line may be in the core. Snoops are therefore only needed if the line is in the LLC and a CVB is asserted on that line. This mechanism helps limits external snoops to the cache box most of the time without resorting to going to the cores.

The four rings consists of a considerable amount of wiring and routing. Because the routing runs in the upper metal layers over the LLC, the rings have no real impact on die area. As with the LLC slices, the ring is fully pipelined and operates within the core's [[clock domain]] as it scales with the frequency of the core. The bandwidth of the ring also scales in bandwidth with each additional core/$slice pair that is added onto the ring, however with more cores the ring becomes more congested and adding latency as the average hop count increases. Intel expected the ring to support a fairly large amount of cores before facing real performance issues.

The System Agent interfaces with the rest of the system via the ring in a similar manner to the cache boxes in the LLC slices. It is also in charge of handling I/O to cache coherency. The System Agent enables [[direct memory access]] (DMA), which allows devices to snoop the cache hierarchy. Address conflicts resulting from multiple concurrent requests associated with the same cache line are also handled by the System Agent.

With Sandy Bridge, Intel introduced a large number of power features to save powers depending on the workload, temperature, and what I/O is being utilized. The new power features are handled at the System Agent. Sandy Bridge introduced a fully-fledged Power Control Unit (PCU). The root concept for this unit started with {{\\|Nehalem}} which embedded a discrete microcontroller on-die which handled all the power management related tasks in firmware. The new PCU extends on this functionality with numerous state machines and firmware algorithms. The use of firmware, which Intel Fellow Opher Kahn described at IDF 2010, contains 1000s of lines of code to handle very complex power management tasks. Intel uses firmware to be able to fine-tune chips post-silicon as well as fix problems to extract the best performance.

The Sandy Bridge core focuses on extracting performance and reducing power in a great number of ways. Intel placed heavy emphasis in the cores on performance enhancing features that can provide more-than-linear performance-to-power ratio as well as features that provide more performance while reducing power. The various enhancements can be found in both the front-end and the back-end of the core.

The front-end is tasked with the challenge of fetching the complex [[x86]] instructions from memory, decoding them, and delivering them to the execution units. In other words, the front end needs to be able to consistently deliver enough [[µOPs]] from the [[instruction code stream]] to keep the back-end busy. When the back-end is not being fully utilized, the core is not reaching its full performance. A weak or under-performing front-end will directly affect the back-end, resulting in a poorly performing core. In the case of Sandy Bridge base, this challenge is further complicated by various redirections such as branches and the complex nature of the [[x86]] instructions themselves.

The entire front-end was redesigned from the ground up in Sandy Bridge. The four major changes in the front-end of Sandy Bridge is the entirely new [[µOP cache]], the overhauled branch predictor and further decoupling of the front-end, and the improved macro-op fusion capabilities. All those features not only improve performance but they also reduce power draw at the same time.  

Blocks of memory arrive at the core from either the cache slice or further down [[#Ring_Interconnect|the ring]] from one of the other cache slice. On occasion, far less desirably, from main memory. On their first pass, instructions should have already been prefetched from the [[L2 cache]] and into the [[L1 cache]]. The L1 is a 32 [[KiB]], 64B line, 8-way set associative cache. The [[instruction cache]] is identical in size to that of {{\\|Nehalem}}'s but its associativity was increased to 8-way. Sandy Bridge fetching is done on a 16-byte fetch window. A window size that has not changed in a number of generations. Up to 16 bytes of code can be fetched each cycle. Note that the fetcher is shared evenly between two threads, so that each thread gets every other cycle. At this point they are still [[macro-ops]] (i.e. variable-length [[x86]] architectural instruction). Instructions are brought into the pre-decode buffer for initial preparation.

Up to four instructions (or five in cases where one of the instructions was macro-fused) pre-decoded instructions are sent to the decoders each cycle. Like the fetchers, the decoders alternate between the two threads each cycle. Decoders read in [[macro-operations]] and emit regular, fixed length [[µOPs]]. The decoders organization in Sandy Bridge has been kept more or less the same as {{\\|Nehalem}}. As with its predecessor, Sandy Bridge features four decodes. The decoders are asymmetric; the first one, Decoder 0, is a [[complex decoder]] while the other three are [[simple decoders]]. A simple decoder is capable of translating instructions that emit a single fused-[[µOP]]. By contrast, a [[complex decoder]] can decode anywhere from one to four fused-µOPs. Overall up to 4 simple instructions can be decoded each cycle with lesser amounts if the complex decoder needs to emit additional µOPs; i.e., for each additional µOP the complex decoder needs to emit, 1 less simple decoder can operate. In other words, for each additional µOP the complex decoder emits, one less decoder is active.

Sandy Bridge brought about the first 256-bit [[SIMD]] set of instructions called {{x86|AVX}}. This extension expanded the sixteen pre-existing 128-bit {{x86|XMM}} registers to 256-bit {{x86|YMM}} registers for floating point vector operations (note that {{\\|Haswell}} expanded this further to [[Integer]] operations as well). Most of the new AVX instructions have been designed as simple instructions that can be decoded by the simple decoders.

[[x86]] has dedicated [[stack machine]] operations. Instructions such as <code>{{x86|PUSH}}</code>, <code>{{x86|POP}}</code>, as well as <code>{{x86|CALL}}</code>, and <code>{{x86|RET}}</code> all operate on the [[stack pointer]] (<code>{{x86|ESP}}</code>). Without any specialized hardware, such operations would need to be sent to the back-end for execution using the general purpose ALUs, using up some of the bandwidth and utilizing scheduler and execution units resources. Since {{\\|Pentium M}}, Intel has been making use of a [[Stack Engine]]. The Stack Engine has a set of three dedicated adders it uses to perform and eliminate the stack-updating µOPs (i.e. capable of handling three additions per cycle). Instruction such as <code>{{x86|PUSH}}</code> are translated into a store and a subtraction of 4 from <code>{{x86|ESP}}</code>. The subtraction in this case will be done by the Stack Engine. The Stack Engine sits after the [[instruction decode|decoders]] and monitors the µOPs stream as it passes by. Incoming stack-modifying operations are caught by the Stack Engine. This operation alleviates the burden of the pipeline from stack pointer-modifying µOPs. In other words, it's cheaper and faster to calculate stack pointer targets at the Stack Engine than it is to send those operations down the pipeline to be done by the execution units (i.e., general purpose ALUs).

Decoding the variable-length, inconsistent, and complex [[x86]] instructions is a nontrivial task. It's also expensive in terms of performance and power. Therefore, the best way for the pipeline to avoid those things is to simply not decode the instructions. This is exactly what Intel has done with Sandy Bridge and what's perhaps the single biggest feature that has been added to the core. With Sandy Bridge Intel introduced a new [[µOP cache]] unit or perhaps more appropriately called the Decoded Stream Buffer (DSB). The micro-op cache is unique in that it not only does substantially improve performance but it does so while significantly reducing power.

It's interesting to note that the µOPs cache only operates on full windows, that is, a full 32B window that has all the µOPs cached. Any partial hits are required to go through the legacy decode pipeline as if nothing was cached. The choice to not handle partial cache hits is rooted in this features efficiency. On partial window hits the core will end up emitting some µOPs from the micro-op cache as well as having the legacy decode pipeline decoding the remaining missed µOPs. Effectively multiple components will end up emitting µOPs. Not only would such mechanism increase complexity, but it's also unclear how much, if any, benefits would be gained by that.

On each cycle, up to 4 µOPs can be delivered here from the front-end from one of the two threads. As stated earlier, the Re-Order Buffer is now a light-weight component that tracks the in-flight µOPs. The  ROB in Sandy Bridge is 168 entries, allowing for up to 40 additional µOPs in-flight over {{\\|Nehalem}}. At this point of the pipeline the µOPs are still handled sequentially (i.e., in order) with each µOP occupying the next entry in the ROB. This entry is used to track the correct execution order and statuses. In order for the ROB to rename an integer µOP, there needs to be an available Integer PRF entry. Likewise, for FP and SIMD µOPs there needs to be an available FP PRF entry. Following renaming, all bets are off and the µOPs are free to execute as soon as their dependencies are resolved.

It is at this stage that [[architectural registers]] are mapped onto the underlying [[physical registers]]. Other additional bookkeeping tasks are also done at this point such as allocating resources for stores, loads, and determining all possible scheduler ports. Register renaming is also controlled by the [[Register Alias Table]] (RAT) which is used to mark where the data we depend on is coming from (after that value, too, came from an instruction that has previously been renamed). Sandy Bridge's move to a PRF-based renaming has a fairly substantial impact on power too. With {{x86|AVX|the new}} {{x86|extensions|instruction set extension}} which allows for 256-bit operations, a retirement would've meant large amount of 256-bit values have to be needlessly moved to the Retirement Register File each time. This is entirely eliminated in Sandy Bridge. The decoupling of the PRFs from the RAT/ROB likely means some added latency is at play here, but the overall benefits are more than worth it.

Sandy Bridge introduced a number of new optimizations it performs prior to entering the out-of-order and renaming part. Two of those optimizations are [[Zeroing Idioms]], and [[Ones Idioms]]. The first common optimization performed in Sandy Bridge is [[Zeroing Idioms]] elimination or a dependency breaking idiom. A number of common zeroing idioms are recognized and consequently eliminated. This is done prior to bookkeeping at the ROB, allowing those µOPs to save resources and eliminating them entirely and breaking any dependency. Eliminated zeroing idioms are zero latency and are entirely removed from the pipeline (i.e., retired).

The [[ones idioms]] is another dependency breaking idiom that can be optimized. In all the various {{x86|PCMPEQ|PCMPEQx}} instructions that perform packed comparison the same register with itself always set all bits to one. On those cases, while the µOP still has to be executed, the instructions may be scheduled as soon as possible because the current state of the register needs not to be known.

Following bookkeeping at the ROB, µOPs are sent to the scheduler. Everything here can be done out-of-order whenever dependencies are cleared up and the µOP can be executed. Sandy Bridge features a very large unified scheduler that is dynamically shared between the two threads. The scheduler is exactly one and half times bigger than the reservation station found in {{\\|Nehalem}} (a total of 54 entries). The various internal reordering buffers have been significantly increased as well. The use of a unified scheduler has the advantage of dipping into a more flexible mix of µOPs, resulting in a more efficient and higher throughput design.

Sandy Bridge has two distinct [[physical register files]] (PRF). 64-bit data values are stored in the 160-entry Integer PRF, while [[Floating Point]] and vector data values are stored in the Vector PRF which has been extended to 256 bits in order to accommodate the new {{x86|AVX}} {{x86|YMM}} registers. The Vector PRF is 144-entry deep which is slightly smaller than the Integer one. It's worth pointing out that prior to Sandy Bridge, code that relied on constant register reading was bottlenecked by a limitation in the register file which was limited to three reads. This restriction has been eliminated in Sandy Bridge.

Configurability was a major design goal for Sandy Bridge and is something that Intel spent considerable effort on. With a highly-configurable design, using the same [[macro cells]], Intel can meet the different market segment requirements. Sandy Bridge configurabiltiy was presented during the [[2011]] [[International Solid-State Circuits Conference]]. A copy of the paper can be [http://ieeexplore.ieee.org/abstract/document/5746311/ found here].

Sandy Bridge has two variable power planes and a single fixed power plane for the {{intel|System Agent}}. The first one covers the ring, cache, and the physical cores. Note that this is a single power plane that is shared by all those components which means they all move together up or down in frequency and voltage. Each of the individual cores is capable of being entirely power gated when needed such as when the core goes into a higher [[C state]]. When this happens, the core state is saved into one of the ways of the cache and the core is entirely shut off. As with the cores, the caches can also be power-gated per way. With each deeper idle state, additional ways are invalidated and flushed and turned off.

Optimizing for performance  means trying to deliver as much power as possible to demanding components all while meeting stringent constraints. Power algorithms take into account various constraints when considering what [[P-State]] (i.e., voltage and frequency) to operate in, which include the CPU capabilities, the platform specification (e.g. platform cooling capabilities), power delivery, graphics driver and operating system inputs as well as actual user controls (e.g. system preferences) and the type of workload (e.g. [[I/O bound]] workloads will not enjoy performance increase through increased frequency). Improvements in that area comes from throughput improvement and responsiveness (branded under "Turbo Boost 2.0").

Prior to Sandy Bridge, Intel used a static model for thermal capacitance. That is, traditionally, if a model is certified for a specific TDP wattage, under no circumstance the chip will be allowed to run any hotter than that rating. This has the implication of treating temperature changes as instant. In reality temperature changes are not instant and there is a tiny bit of time early on when the heat sink is relatively cool and can absorb heat considerably faster. With Sandy Bridge, Intel moved to dynamic model which allows the chip to take advantage of the period of time when the heat spreader is still cool and can dissipate more heat quicker. For the desktop parts this can be a period of over a minute in which Sandy Bridge can operate at considerably higher frequencies and run much hotter.

File:sandy bridge die on a wafer.png|A partial wafer shot showing a complete quad-core die along with eight other dies around it.