An illustrated Guide to CPU improvements
80286 to 80386: The change to the 386's came in the late 80's. 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 386SX's - despite the implementation of 32 bit mode. 80386SX: A 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. 80386 to 80486: 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. 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 286/386SX's. 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 in their own facilities, licensed by Intel. IBM may change the architecture, but these chips may only be used in IBM PC's and system boards. They may not be sold separately.
Further 486 developments DX4: Intel's DX4 processors represent an improvement on the 80486 series. Their speed is tripled from 25 to 75 MHZ and from 33 to 100 MHZ. Another DX4 chip is speeded up from 25 to 83 MHZ. Contrary to what you might think, the DX4 are not named for a quadrupling. They are so named 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 CPU's. Those are improved 486's, which file:///E|/education/Click & Learn/module3b.htm (6 of 7) [4/15/1999 6:20:32 AM]