Revision as of 20:27, 7 April 2016

Bonnell was a microarchitecture for Intel's 45 nm ultra-low power microprocessors first introduced in 2008 for their then-new Atom family. Bonnell, which was named after the highest point in Austin - Mount Bonnell, was Intel's first x86-compatible microarchitecture designed to target the ultra-low power market.

Architecture

Bonnell's primary goals were:

1. Reduce power consumption,
2. while staying fully x86-compatible,
3. at acceptable performance

Performance/Power new rule: +1% performance for at most +1% power consumption.

Memory Hierarchy

• Cache
  • Hardware prefetchers
  • L1 Cache:
    • 32 KB 8-way set associative instruction
      • 1 read and 1 write port
    • 24 KB 6-way set associative data
      • 1 read and 1 write port
    • 8 transistors (instead of 6) to reduce voltage
    • Per core
  • L2 Cache:
    • 512 KB 8-way set associative
    • ECC
    • Shrinkable from 512 KB to 128 KB (2-way)
    • Per core
  • L3 Cache:
    • No level 3 cache
  • RAM
    • Maximum of 2 GB, 4 GB, and 8 GB

Note that the L1 cache for data and instructions were originally both 32 KB (8-way), however due to power restrictions, the L1d$ was later reduced to 24 KB.

Functional Units

The number of functional units were kept to minimum to cut on power consumption.

• 2 Integer ALUs (1 for jumps, 1 for shifts)
• 2 FP ALUs (1 adder, 1 for others)
• No Integer multiplier & divider

Pipeline

Much like other x86 microarchitectures, Bonnell converts the complex instructions into finer micro-ops when needed. However, most instructions in Bonnell do not break down into simpler micro-ops (since Bonnell is not OoOE, there is no real advantage in doing so anyway). Intel estimates that only 5% of common software require instructions to be split up. Bonnell has a 16-stage pipeline with a 13-stage miss penalty. Bonnell is a dual-issue superscalar but with in-order execution (in fact, first microarchitecture since Pentium Pro to not feature a OoOE). The elimination of reordering logic allowed for lower power consumption and small die area. This does imply the overall MPU is less efficient in managing its own resources; memory accesses and FP operations also stall the whole pipeline. Bonnell employed Safe Instruction Recognition (SIR) and Simultaneous multithreading (SMT) to bring performance to acceptable level. Intel claimed sub-20% power consumption penalty while improving performance between 30% and 50%.

The longer pipeline allows a more evenly spreading of heat across the chip with more units. This also allows a higher clock rate.

• Instruction Fetch
  • 3 stages
  • 8 Bytes/Cycle (lower if SMT)
• Instruction Decode
  • 3 stages
  • Instructions with up to 3 prefixes/Cycle
• Instruction Dispatch
  • 2 stages
• Source Operand Read
  • 1 stage
    • reading register operand
• Data Cache Access
  • 3 stages
    • 1 stage for calculating
    • 2 stages for reading cache
• Execution
  • 2 clusters
    • integers
      • quick cache access due to direct connection
    • floating point & SIMD
• Exception & MT Handling
  • 2 stages
• Commit
  • 1 stage

Multithreading

Bonnell has support for multithreading - up to two threads per core. However each thread compete for the same resources which does inherently means they run slower than they would if they were to run alone.

Branch Prediction

• Two-level adaptive predictor
• 12-bit branch history register
• Pattern history table has 4096 entries (shared between threads)
• Branch buffer target has 128 entries (4-way, 32 sets)
• Unconditional jumps are ignored
• Always-taken and never-taken are marked in the table
• Penalties:
  • 13 stages for miss prediction
  • 7 stages for correct prediction but missing branch target buffer (BTB)

Die

	BIC - Bus Interface Cluster MEC - Memory Cluster Execution FPC - FP/SIMD execution Cluster IEC - Integer Execution Cluster FEC - Front-End FSB - Front Side Bus

• The die is 3.1mm x 7.8mm (24.2mm²)

Cores

First generation of Bonnel-based microprocessors introduced 2 cores: Silverthorne for ultra-mobile PCs and mobile Internet devices (MIDs) and Diamondville for ultra cheap notebooks and desktops.

Silverthorne

Main article: Silverthorne

Silverthorne was the codename for a series of ultra-mobile PCs introduced in 2008.

Key features:

• 32-bit
• 1 Core / 2 Threads
• FSB 400 MHz - 533 MHz
• Cache
  • 32 KB L1i$ / 23 KB L1d$
  • 512 KB L2$
  • No L3$

Diamondville

Main article: Diamondville

Diamondville was the codename for the series of ultra cheap notebooks and desktops introduced in 2008. Diamondville is very much a derivative of Silverthorne with faster FSB.

Key features:

• 32-bit
• 1 Core / 2 Threads
• FSB 533 MHz - 667 MHz
• Cache
  • 32 KB L1i$ / 23 KB L1d$ (per core)
  • 512 KB L2$ (per core)
  • No L3$