An Intel CPU history lesson -- more or less:
In the beginning, there was Intel's 4004 chip. I don't know much about that. They changed some stuff around and turned that into an 8088 (8-bit bus, 8-bit word size I think).
They turned that into the 8086 (16-bit instead of 8) a bit later. It could access up to 1MB of memory with its 20-bit address bus (4 bits of significant segment, 16 bits of offset, but DOS put a hard limit at 640KB because the BIOS was mapped in above that point).
The 80186 actually existed, but not in any PCs. IIRC it was used for mostly controller applications (kinda like a PLC but not as versatile).
The 80286 was the first Intel chip with protected mode, but its memory address bus was only 24 bits wide. So you could access 16MB of memory, but DOS was still stuck at 640KB. Various schemes (XMS, EMS) existed to access some of the higher memory, but they didn't get used much.
Then, Intel came out with the 80386. It had a slightly better protected mode (paging was possbile). It also had a full 32-bit address bus (4GB of memory), though the data bus was I think still smaller. Not that the data bus matters... The 80386 is an i386 or a 386; it was the first chip that Linux ran on.
The 80486 (i486) was an i386 with cache. I'm pretty sure the i386 didn't have any caches, but it didn't need them either (the highest its clock ever got was 33MHz). The 486DX chips had an integrated math coprocessor (to do floating-point); the 486SX and all 386's and lower had an external math coprocessor as an option. The data bus may have gotten wider at this point if it was actually not 32 bits on the 386.A
The i586 (Pentium) was the first chip where all of its revisions had a math coprocessor. It also had some better caching, I think. It did introduce a bunch of new assembly-language instructions, but I don't remember what most of them are anymore. Somewhere near the Pentium Classic, Intel started putting fans on their chips (some of the later 486s (like the DX4-100MHz ones) may have had fans too, I don't remember for sure).
i686 is the Pentium Pro and anything that's compatible with PPro's. It's the first Intel chip to have a 36-bit address bus (capable of accessing 64GB of memory), along with PAE and PSE (PAE / PSE is the way you access memory above 4GB).
Last edited by michaelk; 09-23-2004 at 07:13 PM.