what is the deffrent between pa-xt and pc-at

A "PC/XT" would be either the IBM Personal Computer XT, or one of it's many clones. The XT was very similar to the original PC, but included a hard drive and updated floppy controller. It also removed the often unused data casette port on the back.

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personal computer and extended technology

The differences are relatively minor. The XT included a hard drive, while the original PC needed a card and drive provided for it. The XT also lacked a jack to connect a cassette drive as storage. No version of the IBM PC had a processor faster than 4.77 MHz, but the XT had a version with a 6 MHz processor.