Understanding DRAM Characteristics: Assertion and Reason Analysis

The question presents two statements about Dynamic Random-Access Memory (DRAM): an Assertion (A) and a Reason (R).

Analyzing Assertion A: DRAM Charge Loss and Refresh

Assertion A states: "Major advantage of DRAM is that the capacitor cell never loses its charge and hence there is no need to refresh periodically."

Let's break down this statement based on how DRAM works:

• DRAM stores data using a capacitor and a transistor for each bit.
• The charge stored in the capacitor represents either a '0' or a '1'.
• Due to leakage currents, the charge stored in the capacitor dissipates over time.
• This dissipation means the charge is gradually lost, and the data held by the capacitor fades.

Therefore, DRAM cells do lose their charge and require periodic refreshing to restore the charge and maintain the data. The need for periodic refresh is a characteristic, and often seen as a disadvantage compared to Static RAM (SRAM) which doesn't need refreshing. The statement that the capacitor cell never loses its charge and requires no refresh is fundamentally incorrect for DRAM.

Thus, Assertion A is false.

Analyzing Reason R: Data Access During DRAM Refresh

Reason R states: "When DRAM is being refreshed, the data cannot be accessed."

Let's consider the DRAM refresh process:

• Refreshing involves reading the data from a row of memory cells and rewriting it back into the same cells, restoring the original charge levels.
• This process requires using the memory's internal circuitry and buses to access the specific rows being refreshed.
• During the time a specific row is being refreshed, that row (or sometimes the entire memory bank) is busy with the refresh operation.

While refresh operations are typically interleaved with normal memory access cycles, a refresh cycle does take precedence or occupy the memory resources needed for normal read/write operations on the affected rows. So, yes, when a portion of the DRAM is actively being refreshed, it cannot simultaneously be accessed for read or write operations by the CPU or other devices.

Thus, Reason R is true.

Evaluating the Statements Together

Based on our analysis:

• Assertion A is false.
• Reason R is true.

We are looking for an option that reflects "A is false but R is true".

Conclusion

Assertion A is false because DRAM capacitors lose charge and need periodic refresh. Reason R is true because DRAM cannot be accessed for normal operations during the refresh cycle.

The correct answer is the option stating that A is false but R is true.

Revision Table: Key DRAM Facts

Additional Information: DRAM vs. SRAM

It's helpful to compare DRAM with Static RAM (SRAM) to better understand their characteristics.

• SRAM uses latches (usually 6 transistors) to store data. Once data is written, it stays as long as power is supplied, without needing refresh.
• DRAM uses a capacitor and a transistor per bit. The capacitor's charge leaks, requiring periodic refresh.

Here's a quick comparison:

Understanding the fundamental difference (capacitor vs. latch) explains why DRAM needs refresh and SRAM does not, and subsequently impacts their speed, density, and cost.