KarbosGuide.com. Module 3b3.

The CPU developments and improvements

The contents:

  • Areas of development
  • The CPU speed measurement
  • CPU changes - historical review
  • 80486DX4
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  • Previous page

  • Areas of development

    In the following table, you see some of the technologies, which can be improved in the CPU design. Note that internal means inside the CPU. External speed, etc. refers to features immediately outside the CPU on the motherboard.

    Development area Significance Example
    Internal clock frequency speed of data processing inside the CPU. 800 MHz
    External clock frequency Speed of data transfer to and from the CPU via the system bus (or Front Side Bus). 133 MHz
    Clock doubling That the CPU works x times faster internally than externally. 6.0 times (like above)
    Internal data width How many data bits can the CPU process simultaneously. 32 bits
    External data width How many data bits can the CPU receive simultaneously for processing 64 bits
    Internal cache (Level 1 cache) Large and better L1 cache, which is a small fast RAM. It works as a buffer between CPU and regular RAM. 64 KB
    External cache (Level 2 cache) Larger and better implemented L2 cache, place on-die in same chip as CPU. 256 or 512 KB
    Instruction set Can the instruction set be simplified, to speed up program processing? Or can it be improved? RISC code
    More pipelines
    MMX instructions
    3DNow! or SSE

    The CPU speed measurement

    When we look at a CPU, its speed is the most significant feature. All newer CPUs can do the same. You can run Office 2000 in Windows 98 on a 486 CPU. It would be quite slow, but it is possible.

    Speed is the primary difference between newer CPUs. Speed improvement is a product of the above mentioned technologies (such as clock frequency and bus width).

    The old Speed Index

    There are many, many ways to measure CPU speed. The subject is boundless. For years, Norton's Speed Index was used. That is a test, which can be run on any PC with the Norton Utilities Sysinfo program.

    In the table below, you see a number of the most common older CPUs. You can see how they are designed regarding clock speed and bus width. The last column shows their Norton Speed Index (SI). That is a relative number, which can be used to compare different CPUs. It is not used for modern CPUs.

    CPU CPU speed Clock
    System bus
    Data width SI
    8086 4.77 MHz 1 4.77 MHz 16 bit 1
    80286 12 MHz 1 12 MHz 16 bit 8
    80386DX 25 MHz 1 25 MHz 32 bit 40
    486 DX2-66 66 MHz 2 33 MHz 32 bit 142
    5x86-133 133 MHz 4 33 MHz 32 bit 288
    Pentium 75 75 MHz 1.5 50 MHz 64 bit 235
    Pentium 90 90 MHz 1.5 60 MHz 64 bit 278
    Pentium 100 100 MHz 1.5 66 MHz 64 bit 305
    Pentium 133 133 MHz 2 66 MHz 64 bit 420
    Pentium 166 166 MHz 2.5 66 MHz 64 bit 527
    Pentium 200 200 MHz 3 66 MHz 64 bit 629

    Newer CPUs are compared by their clock frequency or by more more sophisticated ratings.

    CPU changes - historical review

    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.


    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 back to real mode without rebooting the PC, and the only operating system to use this was OS/2.


    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.


    The 486 was released April the 10th 1989. Generally speaking, the 486 runs twice as fast as its predecessor - 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.


    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 substantial 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.

    Contrary to what you might think, the DX4 were not named for a quadrupling. They were named this way because of the registry of Intel's 80486 and 80586 names. The DX4 name is separated from that context, so it could be patented. If DX3 referred to a tripling, this would not work. The same type of problem caused the next generation chip to be named Pentium, rather than 80586.

    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: 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.

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    Click for Module 3d about the clock frequencies

    Click for Module 3e about 6th generations CPUs (Pentium IIs etc.)

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