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Computer Hardware and Logic Circuits: Question 6

Syllabus 3.1

Multiple choice AS 1 mark

Which of the following correctly describes a genuine difference between static RAM (SRAM) and dynamic RAM (DRAM)?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Checking each statement against how SRAM and DRAM actually work

SRAM (static RAM) stores each bit in a small circuit called a flip-flop, usually built from around six transistors. A flip-flop holds its state (0 or 1) continuously for as long as power is supplied, with no need to periodically rewrite it, which is why SRAM is described as “static”. This flip-flop design switches state very quickly, making SRAM fast, but the extra transistors per bit make it physically larger and more expensive to manufacture than DRAM for the same number of bits. Because of this, SRAM is normally used only in small amounts, such as CPU cache, where speed matters most.

DRAM (dynamic RAM) stores each bit as a tiny electric charge held in a capacitor, alongside a single transistor that controls access to it. A capacitor’s charge leaks away over time, so DRAM must be refreshed, read and rewritten, thousands of times per second, or the stored data would be lost. This refreshing is why DRAM is described as “dynamic”. Because each DRAM cell needs far fewer components than an SRAM cell, DRAM can be packed much more densely and cheaply, which is why it is used for a computer’s main memory, where large capacity is needed.

Why the other options are wrong

  • A: this reverses the refresh requirement. It is DRAM, not SRAM, that must be refreshed periodically, because its capacitor-based cells lose their charge over time; SRAM’s flip-flop cells do not need refreshing while powered.
  • C: this reverses the storage mechanisms. It is SRAM that uses a flip-flop (several transistors), and DRAM that uses a capacitor (with one transistor) to store each bit.
  • D: both SRAM and DRAM are volatile. Like all RAM, both lose their stored contents as soon as the power supply is removed. Only ROM is non-volatile.

Final answer

B. SRAM is faster but more expensive and less dense than DRAM, so it is used for cache, while DRAM’s cheaper, denser (but slower, refresh-dependent) cells are used for main memory.