Книга: Introduction to Microprocessors and Microcontrollers

12. The Pentium family

12. The Pentium family

The Pentium is a 32-bit microprocessor just like the previous Intel 80386 and 80486 but has been considerably enhanced to improve its speed of operation. Even the 132 pins of the 80386 have increased to 296 on the Pentium.

Other full RISC chips were being well-received at the time the CISC Pentium was launched in 1993 and Intel took these new designs into account but it was boxed into a corner by its own success. It had to maintain absolute compatibility with the previous 8086, 80286, 80386 and the 80486 together with their numerical co-processors. The compromise was to use all the RISC while maintaining the CISC codes. It has over 400 instruction codes. Some are performed by hardware and some by microcode. Its two million plus transistors have been incorporated into a superscalar structure. This means that it has duplicated arithmetic and logic units that can allow it to carry out two instructions at the same time under favourable conditions.

It was launched at 66 MHz and in its first year became famous as the microprocessor that couldn’t count. There was a flurry of letters in the computer magazines and a host of ‘How many Pentiums does it take to change a light bulb?’ type jokes. At first, Intel denied there was a problem even though they must have known about it. ‘And, no, you can’t have your money back.’ More letters. ‘Alright, there is a very, very small matter of a few division sums.’ The error actually produced inaccuracies in the sixth or ninth decimal place in some particular division sums. This was insufficient error to affect more that a small minority of users but it started to undermine confidence in the Pentium. The real problem was that two errors occurred during its design at the same time. Either one, on its own, would have been spotted but the two mistakes served to hide each other. Anyway, it’s been fixed. It only affected the early versions and is no longer significant.

Over time the speed has increased to 200 MHz with the inevitable rumours of the Pentium II running at 400 MHz that will support a 100 MHz system clock.

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