X2: Dual-Core Part Deux

Examining the Athlon 64 X2

In the days of exclusively single-core processing, AMD’s Athlon closely resembled the server- and workstation-oriented Opteron. The two product families served their respective markets well, leveraging similar internals to deliver peak performance on the desktop and exceptional scaling characteristics in higher-end environments.

The same holds true today as the Opteron and Athlon X2 drive AMD’s dual-core offerings. Pretty much everything learned previously about the Opteron applies here and now. There are a couple of notable exceptions, but the likenesses are far more pervasive. For more information about the dual-core Opteron and how AMD designed it, check out our preview here.

For example, the X2 houses 233.3 million transistors on a 199 square millimeter die, just like the dual-core Opteron. They’re also both manufactured on a 90nm SOI lithography process per the revision E core that AMD recently introduced. Thus, all dual-core chips boast SSE3 instruction support, lower nominal operating voltage, and an improved memory controller. We can confirm that a motherboard populated with four, 1GB DDR400 memory modules will now run at the proper frequency, albeit at slightly higher timings.

Athlon 64 X2 die shot

As with the Opteron, both processing cores interface with a request queue and crossbar that’s responsible for delegating communication between cores, the memory controller, and the HyperTransport bus. Of course, a single-socket Athlon 64 X2 doesn’t have the throughput requirements you might otherwise see on an eight-way server, so there’s only one HyperTransport link running at 1 GHz. The benefit of AMD’s approach is clear--with each core communicating at full-speed, the overall gains realized by threading increase and the scaling characteristics of future processors are more likely to remain linear.

One of the ways AMD plans to differentiate various X2 models is through cache size, as it does currently. All Athlon 64 X2 processors include 64KB of L1 data cache and 64 L1 instruction cache per core, for a total of 256KB. Additionally, some will feature 512KB of L2 cache per core, while others wield 1MB per. By individually tweaking operating frequency and cache size, AMD is able to establish variability in performance and price.

Take the Athlon 64 X2 4800+, for example. Operating at 2.4 GHz and equipped with 1MB of L2 per core, the 4800+ defines AMD’s uppermost dual-core offering. A 4600+ will fall right below it, also running at 2.4 GHz with a reduced 512KB per core. The 4400+ ratchets L2 capacity back to 1MB, but drops clock speed to 2.2 GHz. Finally, the 4200+ will offer 2.2 GHz and independent 512KB caches.

Upcoming AMD Athlon 64 X2 Lineup
CPU	Clock Speed	L2 Cache Size	Price
Athlon 64 X2 4800+	2.4GHz	1MB	$1001
Athlon 64 X2 4600+	2.4GHz	512KB	$803
Athlon 64 X2 4400+	2.2GHz	1MB	$581
Athlon 64 X2 4200+	2.2GHz	512KB	$537