DMA signal in 8085 is/are_______.

Question

Examoogle · Accepted Answer

All

Explanation: Direct Memory Access (DMA) in the 8085 microprocessor is facilitated by two dedicated control pins: HOLD (Hold Request) and HLDA (Hold Acknowledge). These signals allow an external device to take control of the system buses by requesting access from the CPU and receiving confirmation upon bus release.

Working Principle: When an external peripheral needs to perform high-speed data transfer without CPU intervention, it sends a high signal to the HOLD pin. The 8085 CPU completes its current bus cycle, places its address, data, and control lines into a high-impedance (tri-state) condition, and then asserts a high signal on the HLDA pin to grant bus control to the external controller.

Key Formulas: $T_{bus} = T_{hold} + T_{transfer} + T_{release}$ — Cycle time components of a DMA operation

Key Points: HOLD is an active high input pin to the 8085.; HLDA is an active high output pin from the 8085 indicating bus grant status.; During bus release, the 8085 enters a tri-state mode for address and data buses.; DMA allows direct data transfer between I/O devices and memory, bypassing the CPU.

Advantages: Reduces CPU overhead for large data transfers.; Increases overall system throughput for high-speed I/O.

Disadvantages: Requires complex hardware controller logic.; CPU is completely halted during the DMA transfer window.

Applications: Disk drive controllers.; Network interface cards.; Sound cards and high-speed data acquisition systems.

Additional Info: The 8085 CPU checks the HOLD pin at the end of each machine cycle.. Option B (Hold) and Option C (HLDA) are both essential signals; thus 'All' (Option A) is the correct categorization.

Answer: A is correct — The 8085 microprocessor utilizes both the HOLD and HLDA signals to manage DMA operations.

Exam Tip: Remember that in 8085, HOLD is an input from the peripheral, while HLDA is the CPU's confirmation output. Always verify the direction of the signal for interview questions.

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