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The fourth generation


The fourth generation of Intel’s CPU’s was called the 80486. It featured a better implementation of the x86 instructions – which executed faster, in a more RISC-like manner. The 486 was also the first CPU with built-in L1 cache. The result was that the 486 worked roughly twice as fast as its predecessor – for the same clock frequency.


With the 80486 we gained a built-in FPU. Then Intel did a marketing trick of the type we would be better off without. In order to be able to market a cheap edition of the 486, they hit on the idea of disabling the FPU function in some of the chips. These were then sold under the name, 80486SX. Itwas ridiculous – the processors had a built-in FPU; it had just been switched off in order to be able to segment the market.


Fig. 101. Two 486’s from two different manufacturers.


But the 486 was a good processor, and it had a long life under DOS, Windows 3.11 and Windows 95. New editions were released with higher clock frequencies, as they hit on the idea of doubling the internal clock frequency in relation to the external (see the discussion later in the guide). These double-clocked processors were given the name, 80486DX2.


A very popular model in this series had an external clock frequency of 33 MHz (in relation to RAM), while working at 66MHz internally. This principle (double-clocking) has been employed in one way or another in all later generations of CPU’s. AMD, IBM, Texas Instruments and Cyrix also produced a number of 80486 compatible CPU’s.




In 1993 came the big change to a new architecture. Intel’s Pentium was the first fifth-generation CPU. As with the earlier jumps to the next generation, the first versions weren’t especially fast. This was particularly true of the very first Pentium 60 MHz, which ran on 5 volts. They got burning hot – people said you could fry an egg on them. But the Pentium quickly benefited from new process technology, and by using clock doubling, the clock frequencies soon skyrocketed.

Basically, the major innovation was a superscalar architecture. This meant that the Pentium could process several instructions at the same time (using several pipelines). At the same time, the RAM bus width was increased from 32 to 64 bits.


Fig. 102. The Pentium processor could be viewed as two 80486’s built into one chip.


Throughout the 1990’s, AMD gained attention with its K5 and K6 processors, which were basically cheap (and fairly poor) copies of the Pentium. It wasn’t until the K6-2 (which included the very successful 3DNow! extensions), that AMD showed the signs of independence which have since led to excellent processors like the AthlonXP.


Fig. 103. One of the earlier AMD processors. Today you’d hesitate to trust it to run a coffee machine…


In 1997, the Pentium MMX followed (with the model name P55), introducing the MMX instructions already mentioned. At the same time, the L1 cache was doubled and the clock frequency was raised.


Fig. 104. The Pentium MMX. On the left, the die can be seen in the middle.


Date: 2016-03-03; view: 717

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