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The first PC IBM designed and marketed in 1981 was called the IBM PC - XT; XT standing for eXtended Technology. This computer was designed around the Intel 8088 processor which itself was able to "digest" 8 bits per clock tick. We call the way an XT computer is wired XT Architecture and the expansion slots one can find on an XT are called XT slots. By virtue of the 8088 processor being an 8-bit processor, the XT slots are also 8-bit slots -- and any peripheral inserted correctly into an XT slot can also be called an 8-bit peripheral.

When IBM introduced their next PC in 1983, they called it the AT, meaning "Advanced Technology." This computer was designed around the Intel 080286 chip -- which was a 16-bit processor. This chip was able to "digest" 16 bits per clock tick, or two characters, more or less. (see note below) In order for a new peripheral to send or receive 16 bit per clock into the motherboard on these new '286 computers, designers added a small extension to the XT slot and called the whole black slot an ISA slot -- Industry Standard Architecture.

Note: Bits per clock ticks have been simplified for ease of explanation. For those who demand the utmost accuracy, here's the real poop: For example, suppose you're typing the following on your keyboard -- "The quick brown fox jumped over the lazy dog's tail."

On an 8088 CPU with 8 - bit architecture, the processor requires 52 clock ticks x 12 clock cycles/instruction or a total of 624 total clock ticks. On a 16 bit 80286 CPU, this would require 27 clock ticks x 12 clock ticks/instruction or 324 clock ticks. A 32 bit 80386 CPU needs 13 clock ticks X 4.5 ticks/instruction or 58.5 total clock ticks. A 80486 CPU which digests 32 bits per clock tick needs 13 clock ticks X 2 ticks/instruction = 26 total clock ticks. Finally, a Pentium or Pentium type 586 CPU running at 64 bits per clock tick requires only 7 clock ticks X .5 ticks/instruction or 3.5 total clock ticks to digest the whole sentence.

The reason Pentium chips need only a half clock tick to digest all 64 bits flowing around a Pentium motherboard is because Pentium chips have two instruction pipelines and thus can execute 2 instructions per clock cycle. Pentium chips have 2 - 32 bit data registers - called Superscalar Architecture.