IBM Apple II Intel 8088 PC XT hard drive Apple I RAM floppy disk Intel
IBM responded to the success of the Apple II with the IBM PC, released in August 1981. Like the Apple II and S-100 systems, it was based on an open, card-based architecture, which allowed third parties to develop for it. It used the Intel 8088 CPU running at 4.77 MHz, containing 29,000 transistors. The first model used an audio cassette for external storage, though there was an expensive floppy disk option. The cassette option was never popular and was removed in the PC XT of 1983. The XT added a 10 MB hard drive in place of one of the two floppy disks and increased the number of expansion slots from 5 to 8. While the original PC design could accommodate only up to 64 KB on the main board, the architecture was able to accommodate up to 640 KB of RAM, with the rest on cards. Later revisions of the design increased the limit to 256 KB on the main board.
