Direct memory access

An instruction to transfer input or output data is executed only after the processor determines the I/O devices is ready. To do this, the processor determines whether the in-put or output device is ready. To do this, the processor either polls a status flag in the device interface or waits for the device to send an interrupt request. In either case, considerable overhead is incurred, brcause several program instructions must be executed for each data word transferred. In addition to polling the status register of the device, instructions are neede for incrementing the memory address and keeping track of he word count. When interrupts are used there is additional overhead associated with saving and restoring the program counter and other state information. To tranfer large blocks of data at high speed, an alternative approach is used. A special control unit may be provided to allow transfer a block of data directly.