PERIPHERAL INTERFACING – III 419
However, it can be made to act to control and co-ordinate the DMA requirements of the
system. See Fig 11.19. Initially the system works as a normal computer with all the switches
in the upper position. In this state, the processor controls the address, data and control buses.
Now, say, a peripheral device wants DMA service. It sends a request to the DMA controller on
the pin DREQ. e DMA chip consults its various internal registers and issues a HRQ (Hold
Request) to the CPU. It gets a HLDA (hold acknowledge) signal from the processor. is
signals the start of DMA operations, with the processor being disconnected from the system.
Now the switches in
Fig 11.19 are toggled to the lower position, giving the ...