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.
|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
3DNow! or SSE
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).
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.
|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|
CPU changes - historical review
This describes briefly the changes throughout the early CPU generations:
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.
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.
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 .
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.
Click for Module 3d about the clock frequencies
Click for Module 3e about 6th generations CPUs (Pentium IIs etc.)
Copyright (c) 1996-2005 by Michael B. Karbo. www.karbosguide.com.