<P> A monitor and any floppy or hard disk drives are connected to the motherboard through cables connected to graphics adapter and disk controller cards, respectively, installed in expansion slots . Each expansion slot on the motherboard has a corresponding opening in the back of the computer case through which the card can expose connectors; a blank metal cover plate covers this case opening (to prevent dust and debris intrusion and control airflow) when no expansion card is installed . Memory expansion beyond the amount installable on the motherboard was also done with boards installed in expansion slots, and I / O devices such as parallel, serial, or network ports were likewise installed as individual expansion boards . For this reason, it was easy to fill the five expansion slots of the PC, or even the eight slots of the XT, even without installing any special hardware . Companies like Quadram and AST addressed this with their popular multi-I / O cards which combine several peripherals on one adapter card that uses only one slot; Quadram offered the QuadBoard and AST the SixPak . </P> <P> Intel 8086 and 8088 - based PCs require expanded memory (EMS) boards to work with more than 640 kB of memory . (Though the 8088 can address one megabyte of memory, the last 384 kB of that is used or reserved for the BIOS ROM, BASIC ROM, extension ROMs installed on adapter cards, and memory address space used by devices including display adapter RAM and even the 64 kB EMS page frame itself .) The original IBM PC AT used an Intel 80286 processor which can access up to 16 MB of memory (though standard DOS applications cannot use more than one megabyte without using additional APIs .) Intel 80286 - based computers running under OS / 2 can work with the maximum memory . </P> <P> The set of peripheral chips selected for the original IBM PC defined the functionality of an IBM compatible . These became the de facto base for later application specific integrated circuits (ASICs) used in compatible products . </P> <P> The original system chips were one Intel 8259 programmable interrupt controller (PIC) (at I / O address 0x20), one Intel 8237 direct memory access (DMA) controller (at I / O address 0x00), and an Intel 8253 programmable interval timer (PIT) (at I / O address 0x40). The PIT provides the 18.2 Hz clock ticks, dynamic memory refresh timing, and can be used for speaker output; one DMA channel is used to perform the memory refresh . </P>

When did ibm design the first personal computer and why did it fail