Draw and illustrate the block diagram of DMA controller. Also discuss the various modes in which DMAC works.

Direct memory access (DMA) is a process in that an external device takes over the control of system bus from CPU.DMA is for high-speed data transfer to/from mass storage peripherals, for example magnetic tape, hard disk drive, CD-ROM and sometimes video controllers. E.g. a hard disk may boasts a transfer rate of 5 M bytes per second, which implies 1 byte transmission every 200 ns. To make such data transfer via CPU is both unnecessary andundesirable.

Basic idea of DMA is to transfer blocks of data directly between peripherals andmemory. The data do not go through the microprocessor but data bus is occupied. 'Normal' transfer of one data byte takes up to 29 clock cycles. DMA transfer requires only 5 clock cycles.

Modes of operation include demand mode, block mode, single mode and cascade mode. Demand mode transfers data until an external EOP is input or until DREQ input becomes inactive. Single mode releases HOLD after each byte of data transferred. Block mode automatically transfers number of bytes indicated by count register for the channel. Cascade mode is used when more than one 8237 is present in a system.