By Naga Hymavathi Ulli Assistant Professor
The DMA I/O technique provides direct access to the memory while the microprocessor is temporarily disabled.
Three ways to synchronize the processor to data rate of peripherals: 1- Polling: which provides a fast response but it the processor recourses are dedicated to one peripheral. 2- Interrupt approach: is much more efficient. the processor only services the peripheral when data is required. requires high software overhead. 3-DMA is a third solution but it increases the complexity of the hardware system.
DMA • Direct Memory Access. • In memory-memory or memory-peripherals communication, the processor is a “middleman” which is not really needed. • Used with HOLD HOLDA signals. • DMA requires another processor - The DMA Controller or DMACto generate the memory and I/O addresses. • 8237 is a DMAC. • In IBM PC, 8237 was used to speed up the read or write operation by the slow 8088 processor. • Nowadays, It is usually used by sound cards and by memory controllers to generate row address for refreshing.