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AMD's Athlon: How It Works (Page 2/5)

Posted: April 12, 2000
Written by: Tuan "Solace" Nguyen

Cache Flow

The Athlon has 512KB cache on external modules that are on the PCB and run at core speed. However, AMD has lowered cache speeds to 2/5ths on Athlons above 750MHz. Reaching 900 and above, AMD has lowered its cache speed again to 1/3. This is to keep the SRAM modules operating within its manufacturing tolerance. The interesting thing about the Athlon is that its cache can be incremented all the way up to a whopping 8MB's. This would give the Athlon a lot of headroom and information would be processed much faster because a lot of the data is close at hand. The 8MB-cached Athlon will be showcased in the Athlon Ultra series.

Running at core speed, the cache constricts the Athlon in terms of overclockability and higher-speed stability. The success rate in overclocking a processor is determined much by its cache speed. And unfortunately, cache frequency cannot be changed with simple jumpers or software.

On the Level 1 side (L1), the Athlon is outfitted with 128KB of full core speed cache and is right on processor die. The Athlon contains 4 times more L1 cache than Intel's Pentium III.

Let's take a ride on the EV6.

Digital Architecture

AMD licensed architectural technology from Digital to incorporate it into their Athlon processor. Called the EV6 Bus, this technology will take the Athlon well above what Intel is currently capable of. Originally, the technology was designed for Digital's Alpha 21264 RISC (Reduced Instruction Set Computing) processor.

The EV6 bus architecture gives the Athlon more I/O bandwidth than normal desktop PCs currently stuffed with Intel Inside. As a side effect though, the use of the same architecture will theoretically let AMD Slot A motherboards use the Alpha 21264. Unfortunately, this will mean that it is totally incompatible with your Slot 1 processors. Intel and AMD will not sleep together this time around.

The Athlon's incredible bus bandwidth doesn't stop with 200MHz default speed. It also has a separate independent backside bus for the external L2 cache, just like a Pentium 2/III/Celeron. Unfortunately, like we said before, this hampers the chances of your overclocking success. Boo. :) But take it from us, the tradeoff is well worth it. At 200MHz, the bus's I/O throughput brings in 1.6GB/sec, which is double the capacity of Intel's GTL+ (Gunning Transceiver Logic) architecture - the one inside the Pentium 2/III/Celeron. Future Athlons (the Ultras) could double that bandwidth again by driving the frequency up to 400MHz. In comparison, Intel's will be running at 133MHz. Too shabby for our liking. :)

In the higher end systems that will feature the Athlon Ultra series, faster RAM types will be used. The Athlon can use late-write SRAMs that scorch at full core frequency - 500MHz or faster! Using this bus addressing method, the Athlon can address up to 512KB of L2 cache without any external tag RAM. With an external tag RAM however, the Athlon can be outfitted with cache heaven - 8MBs. This is overkill for any desktop user and is only used in large data servers or high end graphics workstations.

So exactly how does the Athlon push data faster than Intel's counterpart? Using a hybrid architecture, that's how.

Editor's Note: A tag RAM is a memory chip that acts like a map for the L2 cache - showing address locations for cache use. If you don't have enough tag RAM, your cache won't be able to "see" all of your system memory.

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