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» Computer Architecture: Form factors «

Motherboard Form Factors


PC
The original IBM Personal Computer was released in August 1981. It cost a little under $3000 and came with 64Kb of Random Access Memory (RAM), 40Kb of Read Only Memory (ROM) which included a BASIC interpreter, a 160Kb floppy disk, version 1.0 of MS-DOS, and a cassette interface.

XT
In March 1983, the IBM Personal Computer XT (eXtended Technology) was unveiled with its built-in 10 MB fixed disk. IBM worked with another company (Xebec, I believe) to take the controller components normally located in the external cabinet and build them right onto a bus interface card, creating what we commonly call the "hard disk controller". Program code was provided by a ROM chip on the controller card which supplemented subroutines in the BIOS, and the internal power supply was upgraded to 135 watts to provide power for the internally mounted drive.

It also included more memory, more expansion slots, and a newer version of MS-DOS (version 2.0).

The architecture of the IBM PC/XT is quite simple by today's standards. It consists of a Central Processing Unit (CPU) which is an Intel 8088 running at 4.77 Mhz, some RAM, some ROM containing the machine's Basic Input Output System (BIOS), and a number of support chips to handle various peripheral devices. In order to execute PC-compatible programs a computer's CPU, BIOS and support chips need to provide similar functions to those provided with the IBM PC/XT.

The XT design utilised :


Several companies started manufacturing and selling similar drive/controller packages with various improvements including higher capacities, superior performance and built-in floppy controller components on the same card (to save an expansion slot). These third party subsystems could even be added to an original PC, so long as the internal power supply was also upgraded. All PC-compatible computers (even a 733Mhz PIII) are equipped with hardware to provide this backward compatibility and are therefore able to execute the large no. of applications which have been developed for PC-compatible computers in the last 18+ years.

AT
In 1984, the IBM AT (Advanced Technology) brought a complete overhaul to hard disk systems. Program code was included in the motherboard ROM BIOS, eliminating the need for a ROM chip on the new 16 bit controller card, and a higher quality drive improved access times. The system included use of newly added higher IRQ's, eliminated the use of DMA for hard drives, and changed the range of I/O addresses.

The AT design utilized :


Hardware configuration details for the AT, including hard disk parameters, were stored in a low power CMOS chip connected to a small battery, eliminating many of the motherboard configuration dip switches and jumpers. The battery allowed information to be maintained while the computer was turned off and information could be changed by running a setup program from disk.

The original AT supported 14 different drive types, recognising specific hard disks ranging from 10 to 112 MB. Any drive with physical parameters that did not match one of these types needed a ROM extension on the controller card or a device driver loaded during boot-up. Many people opted instead to divide their drives into 32 MB partitions creating a C: D: E: etc. up to the physical capacity. Prior to DOS 3.3, even this was not possible, because DOS did not recognise extended partitions!

Using an Intel 80286 processor, a 16-bit bus, a medium-speed hard disk and a 1.2 megabyte floppy disk drive, it had a larger case than the PC, which allowed it to accept "tall cards". The AT keyboard corrected the PC's non-standard placement of the return and left shift keys but shortened the backspace key, making it harder to reach.

ATX
An open PC motherboard specification by Intel.   Picture of an ATX motherboard

ATX is a development of the Baby AT specification with the motherboard rotated 90 degrees in the chassis. The CPU and SIMM sockets have been relocated away from the expansion card slots meaning that all the slots support full-length cards. More I/O functions are integrated on the motherboard. As the longer edge of the board is now at the back of the chassis, there is more space for connectors; also, the I/O opening on the back panel of the chassis has been defined as double the previous height, allowing vendors to add extra on-board I/O functions over and above the standard.

Most Pentium Pro boards use this form factor.

As well as the motherboard size, layout, and placement, the ATX specification also includes requirements for power supply and fan specification and location.

The full size ATX board measures 305mm wide by 244mm deep. There is also a Mini-ATX form factor, 284mm by 208mm.

NLX
A low-profile, low TCO motherboard design created jointly by Intel Corp., IBM, DEC and other PC vendors. In contrast to the traditional single-board design, NLX uses a riser card to carry PCI, ISA and AGP bus data (despite Intel's stated intent to rid PC motherboards of the ISA bus by 2000).

WTX
A new form factor set to be introduced next year. (2000) Primarily designed to facilitate cpu and component cooling. [ Update 2005. Note: This was never actually released as far as I'm aware, instead evolving into the BTX ]

Addenda June 2005
This page will be updated and expanded as soon as I get time