Floppy disk controller

A floppy disk controller (FDC) is a special-purpose chip and associated circuitry that directs and controls reading from and writing to a computer's floppy disk drive (FDD). This article contains concepts common to FDCs based on the NEC µPD765 and Intel 8072A or 82072A and their descendants, as used in the IBM PC and compatibles from the 1980s and 1990s. The concepts may or may not be applicable to, or illustrative of, other controllers or architectures.

Overview

A single floppy disc controller (FDC) board can support up to four floppy disk drives. The controller is linked to the system bus of the computer and appears as a set of I/O ports to the CPU. It is often also connected to a channel of the DMA controller. On the x86 PC the floppy controller uses the CPU interrupt number 6, on other systems other interrupt schemes may be used. The floppy disc controller usually performs data transmission in direct memory access (DMA) mode.

The diagram below shows an floppy disc controller which communicates with the CPU via an Industry Standard Architecture (ISA) bus. An alternative arrangement which is more usual in recent designs has the FDC included in a super I/O chip which communicates via a Low Pin Count bus.

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