4.3: Direct Memory Access (DMA)
Introduction
This section introduces the learners to the DMA programmed I/O which provides access to the microprocessor between devices operating at different speeds
Activity Details
Direct Memory Access and DMA-controlled I/O
The DMA I/O technique is used in personal computer systems including those using Intel family of microprocessors. The direct memory access (DMA) I/O technique provides direct access to the memory while the microprocessor is temporarily disabled. A DMA controller temporarily borrows the address bus, data bus, and control bus from the microprocessor and transfers the data bytes directly between an I/O port and a series of memory locations. The DMA transfer is also used to do high-speed memory-to memory transfers. Two control signals are used to request and acknowledge a DMA transfer in the microprocessor-based system. The HOLD signal is a bus request signal which asks the microprocessor to release control of the buses after the current bus cycle. The HLDA signal is a bus grant signal which indicates that the microprocessor has indeed released control of its buses by placing the buses at their high- impedance states. The HOLD input has a higher priority than the INTR or NMI interrupt inputs.
Special hardware writes to / reads from memory directly (without CPU intervention) and saves the timing associated with op-code fetch and decoding, increment and test addresses of source and destination. The DMA controller may both stop the CPU and access the memory (cycle stealing DMA) or use the bus while the CPU is not using it (hidden cycle DMA). The DMA controller has some control lines (to do a handshake with the CPU negotiating to be a bus master and to emulate the CPU behaviour while accessing the memory), an address register which is auto-incremented (or auto-decremented) at each memory access, and a counter used to check for final byte (or word) count.
Conclusion
This section has introduced the learners to the DMA and its operations of synchronizing the I/O devices with the microprocessor
Assessment
Describe how DMA helps in the synchronization of different devices in accessing the microprocessor
The direct memory access (DMA) I/O technique provides direct access to the memory while the microprocessor is temporarily disabled. A DMA controller temporarily borrows the address bus, data bus, and control bus from the microprocessor and transfers the data bytes directly between an I/O port and a series of memory locations. The DMA transfer is also used to
do high-speed memory-to-memory transfers. Two control signals are used to request and acknowledge a DMA transfer in the microprocessor-based system. The HOLD signal is a bus request signal which asks the microprocessor to release control of the buses after the current bus cycle. The HLDA signal is a bus grant signal which indicates that the microprocessor has indeed released control of its buses by placing the buses at their high-impedance states. The HOLD input has a higher priority than the INTR or NMI interrupt inputs.