PC Architectural Standards Page
JohnKalpus.com


XT ISA EISA MCA PCI

Slot Architecture CPU Bits Slot Color
XT XT Architecture (‘81) 8088 8 bit Black
ISA Industry Std. Arch. (‘83) 80286 16 bit Black
MCA Micro Channel Arch. ('87) IBM 80386 32 bit Blue
EISA Enhanced Ind. Std. Arch. ('88) 80386
clones
32 bit Brown
VLB VESA Local Bus ('92) 80486 32 bit Brown
PCI Peripheral Computer Interface ('93) 80586 32/64 bit

White

In order to assure that the peripherals (modems">

PC Architectural Standards Page
JohnKalpus.com


XT ISA EISA MCA PCI

Slot Architecture CPU Bits Slot Color
XT XT Architecture (‘81) 8088 8 bit Black
ISA Industry Std. Arch. (‘83) 80286 16 bit Black
MCA Micro Channel Arch. ('87) IBM 80386 32 bit Blue
EISA Enhanced Ind. Std. Arch. ('88) 80386
clones
32 bit Brown
VLB VESA Local Bus ('92) 80486 32 bit Brown
PCI Peripheral Computer Interface ('93) 80586 32/64 bit

White

In order to assure that the peripherals (modems, network interface cards, NICs, sound cards, video cards, etc.) one can purchase for a modern PC today will work with virtually any PC, motherboards manufacturers agree to adhere to several standards in the way the various boards are wired. These standards are called Architectures.

The surface of a computer motherboard is traversed by thousands of tiny embedded copper wires which connect everything on the motherboard to everything else. They're much too small for anyone to determine which architectural model they represent. Instead, the several types of expansion slots into which one can insert the above-mentioned peripherals tell us definitively which architectures are represented on the board.

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.


With the advent of the '386 processor introduced in 1987 by Intel, the various competing PC manufactures, IBM, clones, etc, now split ranks and independently designed several different architectures which competed with each other. IBM developed Micro Channel Architecture -- or the MCA slot. Not surprisingly, "Big Blue used blue MCA slots on their '386 computers also called PS2s or Personal System 2 computers. While MCA architecture is advanced, fast -- at 32 bits per clock tick, IBM asked the clone manufacturers for a licensing fee in order to use MCA architecture -- and they all refused. Instead, clone manufacturers decided to design their own architecture around the Intel '386 chip. A year later, in 1988, one could find EISA slots on these machines. EISA slots communicated in 32 bit chunks of information with the motherboard bus and started to show up on clone computers. Unfortunately, motherboard manufacturers discovered that EISA architecture and their ensuing EISA architecture peripheral cards were much more costly to produce than an ISA card was. EISA architecture fell out of favor for the general PC market and can only be found now in server PCs and higher-end PCs.
XT & ISA  slot.gif (37475 bytes)

 

 

 


In 1989, Intel announced the 80486 chip, or the venerable '486. By this time IBM and the PC clone manufacturers came up with a new architecture which satisfied them all and they called it VLB or VESA Local Bus. VESA, or Video Electronics Standards Association, is an acronym, which represents makers of video cards. They were instrumental in designing the new 32 bit VLB slot -- a simple idea which uses the standard 16-bit ISA slot with another, shorter 16-bit extension about 3 inches long. Any card which is made to make contact with all the contacts in the entire VLB slot communicates in 32-bit chunks with the motherboard.
VLB slot.gif (32594 bytes)

The Pentium chip debuted in 1993 and allowed for a 32 and 64 bit communication channel with the motherboard - this new architecture is called PCI or Peripheral Computer Interface. Designers could had added a 32-bit extension to the VLB slot but this would have made a PCI card as big as the state of Rhode Island! Instead, a new slot was created -- the off-white and very stately PCI slot.
PCI slot.gif (24511 bytes)

Nowadays, modern PC motherboards usually exhibit several ISA slots (which are also XT slots) and several PCI slots as well. ISA slots will start to disappear on motherboards in 1999 -- they've really outlived their usefulness. When purchasing new card peripherals, look for a PCI slot version to ensure usability for your subsequent motherboard upgrades. Enjoy!

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.

top of page

See also: USB & Firewire (IEEE P1394) information

Return to TechInfo Page
Return to JohnKalpus.com