By 1993, Windows was standard, and users expected a lot more from PCs in performance and features. Increasing software sophistication led to increasing memory usage and hard disk drive requirements. The market was ready for a major upgrade in CPUs, and Intel once again addressed that need. The new Pentium processor signaled a radical redesign of both the CPU and naming conventions.
With their CPUs identified by numbers, Intel faced a business problem: numbers cannot be trademarked. The company's strategy was to substitute a trademarkable name, "Pentium" for their upcoming chips that would otherwise have been named "586." The word is based on the Latin word for the number five, and this chip would have been the 80586. The original design has been revamped several times since 1993, and now there are Pentium IIs and IIIs. Like the older PC CPUs, the Pentium has spawned its share of clones, leading to entry-level PCs priced under $400.
The Pentium (Series I) offers the following features:
- Speeds of 60 to over 200 MHz.
- 32-bit address bus and 32-bit registers.
- 64-bit data path to improve the speed of data transfers.
- Dual pipeline, 32-bit data bus that allows the chip to process two separate lines of code simultaneously.
- At least 8-KB write-back cache for data and an 8-KB write-through cache for programs.
- "Branch prediction"-in which the program cache attempts to anticipate branching within the code. The CPU stores a few lines of code from each branch so that when the program reaches the branch, the Pentium already has the code stored within the cache.
The following table lists the first generation of Pentium and Pentium-compatible chips.
|Chip||Speed (MHz)||Register Width||External Data Bus||Address Bus||Internal Cache|
|Intel Pentium||60, 66||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|Intel Pentium||75||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|Intel Pentium||90, 100||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|Intel Pentium||120, 130||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|Intel Pentium||150, 166||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|Intel Pentium||180, 200||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|CYRIX 6x86(P-rating)||100, 120, 133, 200||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
|AMD K5 (P-rating)||75, 90||32-bit||64-bit||32-bit||8 KB W/B and 8 KB W/T|
Mass-producing reliable P66 Pentiums proved difficult, and many were rejected during quality control. The faulty chips were stable at clock speeds of 60 MHz, so Intel sold them as the P60. Some users change their P60 processor clock speed to 66 MHz by changing a jumper on the motherboard. This might work, but computer performance and longevity can be unpredictable.
Intel continued to use the 0.8-micron manufacturing process (the ability to draw lines as fine as 1/1000 of a millimeter on the die, about 16,000 lines per inch), begun with the 486, to fit 3.1 million transistors on the Pentium chip. The P66 used considerable power and consequently generated a large amount of heat. Operating a reliable heat sink and fan became critical with the advent of the Pentium.
The Pentium 75 was released in 1994. These chips were made using a 0.6-micron manufacturing process (approximately 21,000 lines and spaces per inch) and, as a result, they required considerably less power, despite an additional 200,000 transistors. Intel was able to change the power supply from 5 volts to 3.3 volts (the DX4 also had a reduced power supply), which reduced by nearly one half the amount of heat produced. The P90 and P100 processors were released at this time. These processors ran internally at 1.5 times the external speed (60 or 66 MHz, which was the fastest system board). A P75 processor was also released for use in lower specification machines and laptop computers.
The main components of a processor-registers, decoders, and ALUs (arithmetic/ logic units)-are collectively known as the instruction pipeline. To carry out a single instruction, a processor must:
- Read the instruction.
- Decode the instruction.
- Fetch operands (for math functions).
- Execute the instruction.
- Write back the results.
Early processors carried out these steps one at a time. Combining these steps into a single clock cycle, a process known as pipelining, thereby increases the speed of processing. Superscalar technology allows the Pentium to have two instruction pipelines-called U and V. The U pipeline can execute the full range of Pentium instructions, while the V pipeline can execute a limited number. When possible, the Pentium processor breaks up a program into discrete tasks that are then shared between the pipelines, thus allowing the Pentium to execute two simple instructions simultaneously. Software must be specifically written to take advantage of this innovative feature, which is known as multithreading.
Pentium On-Board Cache
The original Pentium series came with two 8-KB caches-one for data and one for program code, compared with the single 8-KB cache on the 486 (16 KB on the DX4). As described with the 486 chip, the cache uses a technique called "branch prediction" to improve its ability to guess what data or program code will be required next by the processor.