Multiwave Direct Digital Alpha

DEC Alpha 21164PC Features

• 24KB of on-chip L1 Cache (16-KB Instruction + 8KB Data)

• 512KB - 4MB of off-chip L2 Cache

• 128-bit Memory Data Path

• 138mm² Die Size

• 3.4 Million Transistors

• 413-pin Ceramic PGA Packaging (for use with 413-Socket Motherboards)

• 50mm x 50mm Package Size

Technical Features

• 2-Integer and 2-Floating Point Instruction Issue Rate

• 7-stage Integer Pipeline

• 9-stage Floating Point Pipeline

• 128-bit Bus Width

• 4-Way Superscalar Instruction Issue

RISC vs CISC

First of all, the DEC Alpha, unlike x86 processors, is a RISC Processor or a processor that accepts and processes Reduced Instruction Sets hence the name Reduced Instruction Set Computing (RISC).   More familiar to all of you would be the x86 processor, like the 486, Pentium, Pentium MMX, Pentium II, K6, 6x86MX etc..., all of those processors in contrast are CISC Processors, or processors that accept and process Complex Instruction Sets hence the name Complex Instruction Set Computing (CISC). 

Bridging the Gap

Bridging the gap between RISC and CISC processors is quite difficult, in some cases CISC processors prevail and in others RISC processors rule the kingdom.  Is comparing a 533MHz DEC Alpha to a 333MHz Pentium II fair?  Actually, yes.  The Alpha processor achieves its immense speed through the use of extremely high clock speeds, as with all Alpha processors, underclocking them to the levels of their Intel competitors is quite unfair to the Alpha processor since the processor itself was never designed to run at clock speeds that low.  One of the benefits of RISC architecture is the ability to achieve those high clock speeds, ever wonder why the industry is moving towards a non-CISC standard?  Although CISC chips are extremely fast at lower clock speeds, pushing them up to clock speeds 1.5 and 2x those we are at now is slowly becoming much more difficult due to limitations of the architecture.  That's why some of the current x86 processors out today do feature some RISC-like qualities, such as the Intel Pentium II and the AMD K6, the AMD K6 for example features a RISC86 decoder which converts the Complex Instructions it receives into easy to manage Reduced Instructions.

The 21164 and its PC Brother

Now lets talk about the Alpha you'll be seeing much more of here, the 21164PC.  You may be wondering why the Alpha is mentioned very infrequently as being a competitor to the Pentium II, AMD K6, etc..., that is because until now, Alpha systems have simply been too expensive for most "High End - Home Users" to purchase.  What made the Alpha so much more expensive than x86 systems?

Remember the Pentium Pro?  It was essentially a more advanced Pentium processor, with 256KB - 512KB (now even up to 1MB) of L2 cache on the chip itself running, and here's the keyword, *at* clock speed.   Meaning a 200MHz Pentium Pro had 16KB of L1 cache running at 200MHz in addition to 512KB of L2 cache running at 200MHz.  For those of you that aren't too familiar with the functions of L1 and L2 cache, Cache is essentially a small amount of high speed RAM that serves as a mid-way point between your CPU and Memory.  It stores frequently accessed data for future use, since in the case of the Pentium Pro, the L1 and L2 cache is located on chip there are no Bus Lines to traverse to access the Cache and therefore data extracted from the Cache can be performed at a much faster rate than data from the System Memory.  In the same case of that 200MHz Pentium Pro, the System Memory would be running at a 66MHz bus speed, while the Cache operates at 200MHz...about 3x as fast.   The one downside to placing both the L1 and L2 cache on chip was that it significantly raised the price of the processor, and as you might be able to guess Digital's fastest Alpha chips were also plagued by this hefty price-tag. 

Digital's solution?  Kick the L2 cache off the chip, pump up the clock rate, and provide a low-cost alternative Alpha chip for the High End Home/Office Market that could compete directly with the Pentium II.  This they did, and the result was the original 21164 Alpha, with the letters 'PC' tacked on to the end of it...the 21164PC.

* Unofficially Achieved Bus Speed - this processor is capable of running at the 100MHz Bus Speed