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AMD's Carrizo brings power savings to mainstream laptops

Excavator and GCN combine at 15W
— 8:25 PM on June 2, 2015

The Carrizo processor is AMD's follow-on to Kaveri and a direct competitor to Intel's Broadwell CPUs. After a lengthy prelude, AMD is officially taking the wraps off of Carrizo today at the Computex trade show in Taipei. The firm expects laptops based on Carrizo to be available near the end of this month, and now that the chip is official, we know a number of juicy details about it that had previously been murky.

The long and short of it is that Carrizo is a quad-core processor with integrated Radeon graphics. Unlike prior "big-core" CPUs from AMD, this one is a true system on a chip (SoC), with an integrated south bridge I/O section and no need for a companion chipset. Chips like this one, which AMD calls "APUs" or accelerated processing units, pack a ton of complexity into a single die. To give you some idea, have a look at Carrizo's basics, neatly baked into a stat sheet from AMD.

Carrizo's highlights include a next-gen x86-compatible CPU core code-named Excavator and revised Radeon graphics based on the third revision of the GCN architecture.

Unlike Kaveri before it, Carrizo will not be making its way into socketed desktop form, largely because it's tuned for optimal operation in a frugal 15W power envelope. This new APU's benefits likely wouldn't translate well into full-sized desktop systems. Instead, AMD has chosen to target laptops, all-in-one desktops, and small-form-factor systems with power envelopes of 35W and lower. Smartly, the firm has also decided to consolidate the infrastructure for its high-performance and low-power APU lines into a single setup. Upcoming processors known as Carrizo-L, similar to the low-power Beema APU, will be drop-in compatible with Carrizo motherboards, so system makers can mix and match AMD processors as they wish.

You may have noticed in the specs table above that Carrizo is manufactured on a 28-nm fabrication process. That's the same basic generation of process tech as Kaveri before it, and it's a far cry from the 14-nm tech Intel is now using for Broadwell. Fortunately, AMD tells us it has managed to squeeze some formidable improvements into this new APU regardless. In fact, I've already covered Carrizo's advances in power efficiency and transistor density in some depth, so go read that article if you'd like to know more. I'll try not to repeat too many of those claims here. Instead, I'll focus on new information, including filling in some details about Carrizo's various components and the overall performance of the chip.

Excavator digs in
The newest member of AMD's Bulldozer lineup of x86-compatible CPU cores is Excavator, which debuts exclusively aboard Carrizo. Like the past "heavy equipment" cores from AMD, Excavator includes a number of targeted tweaks intended to improve its performance and power efficiency. AMD's architects estimate Excavator's performance has improved between four and 15% on a clock-for-clock basis, and the firm is quick to point out that this new core occupies no more silicon die area than Piledriver did before it.

To achieve those gains and save on die space, the Excavator team slashed the size of the L2 cache from 2MB per dual-core module on Steamroller to 1MB per module here. They then doubled the size of each core's L1 data cache to 32KB while keeping access latencies the same. That tradeoff between L1 and L2 cache capacities was apparently a worthwhile one. They also found ways to reduce the caches' power consumption, such as clock gating, that cumulatively produced a 50% power savings. Here's a look at the dynamic power use of the L1 caches in Excavator versus, presumably, Steamroller, straight from AMD's presentation:

Beyond the cache tuning, the team improved the core's branch prediction accuracy by growing the size of the branch target buffer from 512 to 768 entries. They also implemented a fast-flush capability in the floating-point unit that, I presume, allows the pipelines to more quickly recover from a branch misprediction.

Furthermore, Excavator adds support for some new x86 instructions, including the AVX2 suite and MOVBE, SMEP, and BMI1/2. Applications that employ those instructions could see some nice increases in performance and efficiency.

The final bit of goodness AMD built into Excavator is something that has been sorely missing from these big APUs: support for low-power standby modes. These deep sleep states enable features like Windows 8's Connected Standby and Windows 10's InstantGo, although those are apparently two names for the same basic thing. The idea is to bring smartphone-style sleep modes with periodic wakeups for notification checking to Windows-based systems. Intel's CPUs have supported this capability for several years at least. Interestingly enough, AMD tells us that the InstantGo capability on Carrizo systems will make use of the ARM Cortex-A5 CPU built into the security processor portion of the chip in order to control wake and sleep behavior with minimal power use.

Carrizo's combination of better per-clock performance, improved power efficiency, and higher clock speeds translates into some measurable gains in CPU performance. Here's a look at some results AMD supplied from Cinebench, an FPU-intensive image rendering benchmark.

True to its billing, Carrizo shines brightest in the 15W power envelope, where it's up to 55% faster than Kaveri. The lion's share of the gains come from higher clock frequencies, while the remaining 10-15% is attributable to increased per-clock throughput.