- XT(Extended Technology)
It is use 8086 and 8088 Micro processors developed bathe Intel Com pony. The speed of the processors is 4.77MHZ. Everything was based on the text based system. It can not support Graphics Software. It's I/O devices are not flexible and faster. Its word length is only 16 bit (8 bit is equal to 1 byte, two nibbles).
- AT computer ( Advance Technology)
It is the new technology computer. They are faster in processing then the XT computer and can run any types of software weather that is text based or GUI based. Its processing speed is more then two GHZ. It can support Graphic software as well. It uses 80386,80486,Pentium I,II,III,IV(80586) processors. Any type of complex and long processing can be done depending on the capacity of the computer. I/O devices are interactive, flexible and faster. Word length exceeds 16 bit. Co-processors are used to help the main processors for complex mathematics. The main processor of this computer is Micro processors.
- PS/2 computer(Personal System/2)
Actually , these are not totally different model of the computer but are refinement of AT computer. It is Advance from AT computer. It was build after 1990's. It is mostly used in the Laptop computers. Rechargeable and battery operated system with faster and flexible I/O devices are some important characteristics of these computer. Os/2 operating system was used at beginning, but now a days Windows operating system is used.