Question

What registers are significantly incremented and decremented respectively for the transmission of each byte by Direct Memory Access (DMA)?

• A. \( \text{Address Register \& Byte Count Register} \)
• B. \( \text{Control Register \& Byte Count Register} \)
• C. \( \text{Transmitter Register \& Byte Count Register} \)
• D. \( \text{Status- Register \& Byte Count Register} \)

Hint:

DMA controllers are essential for efficient data movement in modern computer systems. Remember the three primary registers in a DMA controller: \textbullet \ Address Register Points to the memory location and increments (or decrements) for sequential access. \textbullet \ Byte/Word Count Register Tracks the amount of data remaining to be transferred and decrements with each transfer. \textbullet \ Control Register Configures the DMA operation (e.g., source/destination, transfer direction, mode).

Answer 1

The Correct Option is A

Direct Memory Access (DMA) is a feature of computer systems that allows certain hardware subsystems to access main system memory (RAM) independently of the central processing unit (CPU). This is crucial for high-speed data transfer operations, such as disk read/write, network data transfer, or graphics data transfer, as it frees up the CPU to perform other tasks. For DMA operation, a DMA controller (DMAC) typically manages the data transfer. The DMAC uses specific registers to control the transfer process: 1. Address Register (or Memory Address Register): This register holds the starting memory address for the data transfer. During a DMA transfer, as each byte (or word) is transferred, this address register is automatically incremented (for reading from memory or writing to memory sequentially) or decremented (if transferring in reverse order) to point to the next memory location. This is how the DMA controller keeps track of where data is being read from or written to in memory. 2. Byte Count Register (or Word Count Register): This register holds the total number of bytes (or words) to be transferred during a DMA operation. As each byte (or word) is transferred, this register is automatically decremented. When the byte count reaches zero, it signifies that the entire data block has been transferred, and the DMA operation is complete. Let's evaluate the options: (A) \( \text{Address Register \& Byte Count Register} \): This aligns perfectly with the roles of these registers in DMA. The Address Register is incremented to traverse memory, and the Byte Count Register is decremented to track the remaining data. (B) \( \text{Control Register \& Byte Count Register} \): A Control Register is used to configure the DMA operation (e.g., transfer direction, mode), but it is not typically incremented/decremented per byte transfer. The Byte Count Register is correct. (C) \( \text{Transmitter Register \& Byte Count Register} \): A Transmitter Register might be part of a peripheral device (like a UART) involved in data transmission, but it's not a core register within the DMA controller that is incremented/decremented per byte for memory access. The Byte Count Register is correct. (D) \( \text{Status- Register \& Byte Count Register} \): A Status Register holds information about the current state of the DMA controller (e.g., busy, error, transfer complete), but it is not incremented/decremented per byte transfer. The Byte Count Register is correct. Therefore, the Address Register and Byte Count Register are the two registers that are significantly incremented and decremented respectively during byte-by-byte DMA transmission.