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Alpha Microprocessor

The Alpha processor had a cruel fate. The legend of the computer world, the first 64-bit platform, this processor was far ahead of its time in many respects. Sometimes revolutionary solutions don’t get what they deserve. The Alpha platform is no exception. Digital Equipment Corporation, its developer, went bankrupt, mostly because of marketing slips. Having the best microprocessor of that time, the developers thought little of promoting it into the market. They must have thought that a good processor would sell by itself. Alas, the outcome is quite logical - however good your processor is, you should first tell about it to the potential customer.

Compaq was the second owner of the processor. It was more complex here: when Compaq bought Digital, the then-existing Alpha platform was already somewhat obsolete and the development of the new generation was far behind the schedule (partially because of personnel problems as many developers of the first processor generations had quit the company). Besides that, Compaq didn’t use the international dealer net it had inherited from Digital. That platform was somewhat alien to Compaq and no one managed it, no one promoted it. The result was again predictable: the platform lost all its performance advantage and had no development perspectives (the proposed minor innovations couldn’t change the overall situation).

Thus, when Compaq and Hewlett-Packard went for a merger, the Alpha platform found itself in a position of a stepdaughter: HP has no use for a third 64-bit platform and the corporation says this explicitly. So now we can have a look at the past grandeur - there’s nothing permanent in this world of change and microprocessor architectures grow and die, too.

So, the Alpha is a microprocessor capable of performing four operations per clock cycle. The processor has a two-port dual-channel L1 cache of 64KB size and a 64KB dual-channel instruction cache. The processor supports from 1MB to 16MB of an external L2 cache (I mean CPUs of the EV68CB/EV68DC series or Alpha 21264C) and tags of the L2 cache are stored in the processor core for faster processing. The L2 cache is joined with the core by means of a 128-bit-wide bus (plus error correction). The 21364 series Alpha microprocessor has 1.75MB of L2 cache on-die.

The CPU core contains two ALUs (each with an address computation unit) and two FPUs (one is responsible for multiplication, another with addition, division, and root extraction). The processor has a branch-prediction unit. Speculative execution is possible and the processor has 80 integer and 72 FPU registers (32 and 32 architectural) for that. All operations are pipelined, save for division and root extraction, but it’s possible to launch other operations with root extraction and division going on in the background (with some restrictions).

The Alpha has the shortest pipeline - 7 stages only, both for integer and floating-point calculations. Thus, the processor feels great in branching algorithms where longer-pipelined CPUs would stumble. This reason also makes impossible an easy growth of the clock rate: the processor units perform too much work per clock cycle to be easily scalable in frequency.

Well, this processor evidently loses to its competitors in the amount of execution units as well as in their functionality. We should make allowances, though, that the Alpha micro-architecture appeared first of all, being a kind of example for all developers of future CPUs. Thanks to the well-designed and polished-off architecture, this processor can show good results in benchmarks still, although cannot match modern CPUs. Its performance numbers are rather weird, by the way. It seems like an older generation of CPUs should be slower than the younger, but that’s not all that simple. The AlphaServer GS1280 7/1150 based on the Alpha 21364 processor (1.75MB on-chip L2 cache) scores 795 in SPEC_int base 2000, while the Alpha Server ES45 68/1250 with the previous-generation Alpha 21264C processor with a higher frequency and 16MB of off-chip L2 cache scores 845 points. The difference in results may probably be caused by the gross difference in the cache size. The AlphaServer ES45 68/1250 with the Alpha 21264C (16MB off-chip L2 cache) is the leader in SPEC_fp base 2000 with 1019 points.

In fact, nothing changed principally in the Alpha architecture since then – there have been only minor innovations like addition of 1.75MB L2 cache into the core in the 21364 model or integration of an RDRAM controller into the processor in the 21464 model.

Strangely enough, in spite of the outdated architecture, systems with Alpha CPUs show good performance; they are faster than the PA8700, for example. This is a great achievement of Digital developers who created a well-balanced processor, a good platform for it and an efficient compiler. Particularly, the Alpha processor bus, also known as EV6, was licensed by AMD and employed in the Athlon MP CPU. Besides that, the Alpha platform uses the "point-to-point" topology. It used it first - then AMD took this idea over.

Overall, we can put down a full stop in the history of this processor. There’s no hope left that this platform will continue its evolution. It will be serving its owners for some time yet and then will go into museums of computer equipment.

 
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