This describes briefly the changes throughout the early CPU generations

8088 and 8086
The 8086 from 1978 was the first 16 bit CPU from Intel using a 16 bit system bus. However 16 bit hardware such as motherboards were too expensive and even non existing at this time, where the 8 bit microcomputers were the standard.
In 1979 Intel reengineered the CPU so it fit with existing 8 bit hardware. The first PC (in 1981) had this 8088 CPU. The 8088 is a 16 bit CPU, but only internally. The external data bus width is only 8 bit giving compatibility with existing hardware.
Actually the 8088 is a 16/8 bit CPU. Logically it could have been named 8086SX. The 8086 was the first total 16 bit CPU in this family.
80286
The 286 from 1982 was also a 16 bit processor. It gave a big advance relative to the first generation chips. The clock frequency was increased, but the major improvement was in optimizing instruction handling. The 286 produced much more per clock tick than 8088/8086 did.
At the introductory speed (6 MHz) it performed four times better than the 8086 at 4.77 MHz. Later it was introduced with 8, 10 and 12 MHz clock speed being used in the IBM PC-AT from 1984.
Another innovation was the ability to run in protected mode - a new work mode with a "24 bit virtual address mode", which pointed towards the later shift from DOS to Windows and multitasking. However you could not change from protected mode back to real mode without rebooting the PC, and the only operating system to use this was OS/2.
80386
The change to the 386s came October the 17th 1985. The 80386 was the first 32 bit CPU. From the traditional DOS PC's point of view, this was not a revolution. A good 286 ran as fast as the first 386SXs - despite the implementation of 32 bit mode.
It could address up to 4 GB of memory and had a better addressing (in bigger chunks) than the 286. The 386 ran at clock speeds of 16, 20 and 33 MHz. Later Cyrix and AMD made clones working at 40 MHz.
The 386 introduced a new working mode besides the real and the protected modes of the 286. The new mode called virtual 8086 opened for multitasking since the CPU could generate several virtual 8086s running in each their own memory space.
The 80386 was the first CPU to perform well with the early versions of Windows.

This chip was a very popular discount edition of 386DX. It has only 16 bit external data bus contrary to the DX 32 bit. Also, the SX has only 24 address lines, Therefore, it can only address a maximum of 16 Mb RAM. It is not really a true 386, but the cheaper motherboard layout made it very popular.
80486
The 486 was released April the 10th 1989. Generally speaking, the 486 runs twice as fast as its pioneer - all things being equal. That is because of better implementation of the x86 instructions. They are handled faster, more in RISC mode. At the same time bus speed is increased, but both 386DX and 486DX are 32 bit chips. A novelty in the 486 is the built in math co-processor. Before, that had to be installed as a separate 387 chip. The 486 also held 8 KB of L1 cache.
80486SX
This was a new discount chip. The math co-processor was simply omitted.
Cyrix 486SLC: Cyrix and Texas Instruments have made a series of 486SLC chips. They used the same set of instructions as did the 486DX, and they run at 32 bit internally, like the DX. However, externally they run at only 16 bit (like a 386SX). Therefore, they can only handle 16 MB RAM. Furthermore, they only have 1 KB internal cache and no mathematical co-processor. Actually they are just improved 286/386SXs. They are not cloned chips. There are large differences in their architecture compared to the Intel chips.
IBM 486SLC2: IBM had their own 486 chip production. The series was named SLC2 and SLC3. The latter was also known as Blue Lightning. These chips could be compared to Intel's 486SX, since they did not have a built-in mathematical co-processor. However, they had 16 KB internal cache (compared to Intel's 8). What reduced their performance was the bus interface, which was from the 386 chip. SLC2 runs at 25/50 MHz externally and internally, while the SLC3 chip runs at 25/75 and 33/100 MHz. IBM manufactured these chips for their own PCs in their own facilities, licensing the logic from Intel. The chips were not sold separately.
DX4: Further 486 developments
Intel's DX4 processors represented an improvement on the 80486 series. The clock speed was tripled from 25 to 75 MHz and from 33 to 100 MHz. Another DX4 chip was speeded up from 25 to 83 MHz.
The DX4 has 16 KB internal cache and operates on 3.3 volt (they will tolerate 5 volt, to accommodate existing system boards). DX and DX2 have only 8 KB cache and require 5 volt with inherent heat problems.

5X86 from AMD
AMD has made a series of so called 5X86 CPUs. Those are improved 486s, which approach the 5th generation chips, hence their name. Their 120 MHz model is noteworthy. It could easily be tuned to run at 160 MHz.
With Intel's Pentium from 1993, a new era began in the continued CPU development.
In these pages, we will look at different variations and further development of 5th. generation CPUs.
Pentium Classic (P54C)
This chip was developed by Intel in Haifa, Israel and was released on March the 22th 1993.

The Pentium processor is super scalar, meaning that it can execute more than one instruction per clock tick. Typically, it handles two instructions per tick. In this respect, we can compare it to a double 486.
At the same time, there have been big changes in the system busses. The width has doubled to 64 bit, and the speed has increased to 60 or 66 MHz.
This has resulted in a substantial improvement from the 486 technology.
Two version to start with
Originally, Pentium came in two versions: a 60 MHz and a 66 MHz. Both operated on 5 Volt. This produced a lot of heat (it was said that you could fry an egg on them!).
The next Pentium (P54C) generation worked with an internal clock doubling of 1.5 times. These chips ran at 3½ Volt. This took care of the heat problem. However, heat coming from the CPU has been a problem ever since.
With these the first P5 processors, Intel carried two Pentium lines; some running at 60 MHz on the system bus (The P90, P120, P150, and P180) and others with 66 MHz system bus (the P100, P133, P166 and P200).
K5
K5 was Pentium copy. The old K5 was for example sold as PR133. This means, that the chip should perform like a Pentium P133. However, it only runs 100 MHz internally. It still has to be installed in the motherboard like a P133.

AMD's K5 also existed as PR166. As the name suggests, it was intended to compete with Intel's P166. It operated at just 116.6 MHz internally (1.75 X 66 MHz). According to the highly respected German magazine c't, issue 3.97 page 20, it actually ran at least as fast as the P166.
This was due to an optimized cache and other new developments. The only feature on which it could not match the P166 was in floating point operations. These are typically necessary in 3D calculations in AutoCAD and similar applications.
PR133 and PR166 cost far less than the similar Pentium models, and they were very popular in low budget machines.

Pentium MMX (P55C)
The P55C Pentiums were introduced January 8, 1997. MMX is a new set of instructions (57 new integer instructions, four new data types, and eight 64 bit registers), which expand the capabilities of the CPU. It is an addition to the original Pentium set of instructions.
The MMX instructions were designed for multimedia programs. The programmers can utilize these instructions in their programs. These allow the Pentium to provide improved program execution.
Both Cyrix and AMD use MMX in their 6th generation CPUs (K6 and M2). Programs, which are written with MMX instructions, can still be run on, for example, a Pentium without MMX. However, execution is slower with the traditional instructions.
More L1 cache and higher clock frequency
Compared to the Pentium Classic, the Pentium MMX were further improved with 32 KB L1 cache (the old one had 16 KB). There were also other improvements in the CPU. These improvements together meant 10-20% better performance at similar clock speeds. The clock frequency of the new processors were 166, 200 and 233 MHz.

IDT Winchip
IDT was another smaller company to produce low-priced Pentium MMX-like CPUs. Their first WinChip C6 was introduced in May 1997. The company wanted to deliver 200 MHz Pentium MMXs for $50.
• Socket 7 processors running at 266 MHz and 300 MHz
• 0.25-micron process technology
• 6 million transistors
• Superscalar MMX and 3DNow!
• Fully pipelined floating point unit
• 100 MHz bus support

Introduction to the 6th generation of CPUs
The first 6th generation CPU was Intel's Pentium Pro from 1995. However, first from 1997 with both AMD's K6 and the Pentium II the 6th generation performances have been available for us all.
Pentium Pro was an important CPU, since it became the father to the Pentium II, the Celeron, the Pentium III and made the ground other P6-like processors as K6-2.

---

Pentium Pro
Pentium Pro development started in 1991, in Oregon. It was introduced on November 1, 1995.

The Pentium Pro is a pure RISC processor, optimized for 32 bit processing in Windows NT or OS/2. The new hot feature was that the L2 cache is built-in. This is like two chips in one. The new features were:
• Built in optimized L2 cache with 256 KB or 512 KB. This is connected to the CPU itself with a 64 bit back side bus. Thus, the L2 cache runs synchronous with the CPU speed.
• 5.5 million transistors in the CPU, 15 million for the 256 KB SRAM L2 cache. (6 transistors per bit).
• 4 pipelines for simultaneous instruction execution.
• RISC instructions with concurrent x86 CISC code to MicroOps RISC instructions decoding.
A giant chip
Here you see a rectangular chip. The CPU and L2 cache are separate units inside this chip:

PPro versus Pentium II
After the introduction of Pentium II, the interest in the PPro has declined, and by the end of 1998 it was out of production. However it sold awhile after the introduction of the Pentium II.
Compared to the first generations of this one, the PPro had advantages when used in certain servers:
CPU Pentium Pro 1. generation
Pentium II
Max. RAM 4 GB 512 MB
L2 cache speed 200 MHz 150 MHz
Max. number CPU 4 2
Intel also supplied a Pentium Pro-Overdrive Kit running at 333 MHz. However, with the Intel Xeon CPU the end came to the Pentium Pro.

The Pentium II
The second 6th generation CPU was Intel's Pentium II from 1997.

Pentium II
Pentium Pro "Klamath" was the code name for Intel's top processor. It ended up as a partially reduced and partially improved Pentium Pro model.
Introduced May 7, 1997, the construction of Pentium II was a little controversial. The features include:
• A CPU mounted together with 512 KB L2 in a SECC (Single Edge Contact Cartridge) module
• MMX instructions.
• Improved 16 bit program execution (joy for Windows 3.11 users).
• Doubled and improved L1 cache (16 KB + 16 KB).
• New increased internal speed: from 233 MHz to 300 MHz (later version much higher).
• L2 cache working at half CPU speed.
L2 cache out of chip
The most interesting change was the separation of CPU and L2 cache. Intel found it too costly to combine them in one chip as in Pentium Pro. To facilitate mass production, cache RAM of a different brand (Toshiba) was used. The cache RAM is marked 7 ns allowing a clock frequency of maximum 150 MHz.

L2 cache speeds compared
With its special design, the L2 cache has its own bus. It runs at half the CPU speed, like 133 MHz or 150 MHz. This is clearly a retrogression from the Pentium Pro, which can run at 200 MHz between the CPU and L2 cache. It is countered by the improved L1 cache, which really zips along! Here you see a comparison:
CPU L1 transfer rate L2 clock speed L2 transfer rate
Pentium 200 777 MB/sec. 66 MHz 67 MB/sec.
Pentium 200 MMX 790 MB/sec. 66 MHz 74 MB/sec.
Pentium Pro 200 957 MB/sec. 200 MHz 316 MB/sec,
Pentium II 266 MHz 1,175 MB/sec. 133 MHz 221 MB/sec.
Pentium II is and has been available in 233, 266, 300, 333, 350, 400, 450, and 500 MHz editions. With the 82440BX and i810 chip sets Pentium II was an excellent performer. Read on for more information on Pentium III.

The second generation of Pentium II
The Original Celeron
Early 1998 Intel was having a hard time with the Pentium II which was pretty expensive. Many users bought the AMD K6-233, which offered very good performance at a moderate price.
So Intel created a brand new CPU called Celeron. It is a Pentium II cartridge except for the L2 cache, which has been chopped away. It uses a 'Covington' core, and we could just as well have called it the Pentium II-SX. In 1998 Intel replaced their Pentium MMX with the first Celerons. Later the design was improved a lot, and Celeron became a very successful product.

This first inexpensive Celeron cartridge fitted into Slot 1 and it ran on a 66 MHz system bus. The internal clock ran at 266 or 300 MHz and delivered good performance for floating point and MMX heavy programs such as certain games. Concerning office applications, the lack of L2 cache was a great disadvantage.
K6
Intel's Pentium II soon got competition from AMD and Cyrix. Both companies have launched several good processors, sometimes giving Intel a hard competiton.
AMD's K6 is from April 2, 1997. In 1996 AMD produced the K5 processor which was not very impressive, however very cheap. The company was put back to business by Mr. Atiq Raza, who brought in the technology from NexGen. This lead to the very successful model K6, which saved AMD from ruin.
The market soon discovered that the K6 performed a lot better than Pentium MMX, which it shared the Socket 7 motherboards with. Here are the data:
• Equipped with 32+32 KB L1 cache and MMX.
K6 is (like K5) compatible with Pentium. Thus, it can be mounted in a Socket 7 on a regular Pentium motherboard, and this soon made the K6 very popular.
This 6X86MX chip is compatible with the Pentium MMX. This gives additional possibilities to assemble PCs on ordinary Socket 7 motherboards.

AMD K6-2
The next AMD "model 8" version of the K6 had the code name "Chomper".
This processor of May 28, 1998 was marketed as K6-2, and like the model 7 version of the original K6, it is manufactured with 0.25 micron technology. These chips run on just 2.2 Voltage. They became an immense success, in many situations competing very successfully with Intel's Pentium II.
Super 7 motherboards and better MMX
The K6-2 is made for a front side bus (system bus) at the speed of 100 MHz. This is to be found with the so-called Super 7 motherboards. AMD made other vendors like VIA produce new chipsets for the traditional socket 7 motherboards, after Intel in 1997 had given up the platform.
K6-2 is also improved with an MMX performance twofold better compared to the original K6.

3DNow!
The K6-2 holds a new 3D plug-in (called 3DNow!) for better game performance. It consists of 21 new instructions that can be used by software developers giving a better 3D-performance. To benefit from it, you need a graphics driver or a game, which deals directly with the new commands.
The good thing is, that games do not have to include special programming to benefit from 3DNow!. Support is included in DirectX 6.0 (and newer) for Windows . DirectX is a so-called multimedia API (in fact a hardware abstraction layer) for Windows . It is some programs that can enhance the multimedia performance within all Windows programs.
3DNow! is not compatible with MMX, but the K6-2 holds MMX as well as the 3DNow!. Also Cyrix and IDT launch CPUs with 3DNow!. Read more on 3DNow!

Good and inexpensive power
The K6-2 gave very, very good performance. You can compare the models to the Pentium IIs. A K6-2 350 MHz performed very similar to a Pentium II-350, but was sold a lot cheaper. And you even saved more because of the cheaper motherboard.
100 MHz bus
Not all K6-2s ran with a 100 MHz bus. Here you see some of the versions, which require motherboards with crystals capable of these configurations:

K6-2 Bus Clock
266 MHz 66 MHz 4.0 X 66 MHz
266 MHz 88 MHz 3.0 X 88 MHz
300 MHz 100 MHz 3.0 X 100 MHz
333 MHz 95 MHz 3.5 X 95 MHz
350 MHz 100 MHz 3.5 X 100 MHz
380 MHz 95 MHz 4.0 X 95 MHz
400 MHz 100 MHz 4.0 X 100 MHz
Two of the CPU's in the table must be the same. AMD calls it a 350 MHz version, but in Denmark e.g it was sold as a 380 MHz version.
K6-2/400 and above
November 15, 1998. The K6-2/400 was introduced. This chip worked on a new core, which should be slightly improved. Hence the performance matched a Pentium II-400.
April 6, 1999. A 475 MHz version of the K6-2 was introduced. The latest version is 533 MHz.
AMD had 39% of the market with K6-2 in 1999!