Multiple choiceQuestion 1 · Recall
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By the end you can explain the differences between static RAM (SRAM) and dynamic RAM (DRAM), and say which one a device uses for a job, and why.
Your laptop has two kinds of RAM, and you've probably only heard of one. The "16 GB of memory" on the box is DRAM; the few megabytes of cache inside the processor are SRAM.
Both are RAM: read-write, directly accessed by the processor, and volatile. The difference is how each one physically holds a bit.
Where the picture breaks: a real light switch stays put in a power cut. SRAM doesn't: like DRAM, it loses everything when the power goes off.
Dynamic RAM (DRAM)
RAM in which each bit is stored as charge on a capacitor, controlled by a transistor. The charge leaks away, so every cell must be refreshed (read and recharged) many times a second.
Static RAM (SRAM)
RAM in which each bit is held by a flip-flop (a small circuit of several transistors). It keeps its value for as long as power is supplied, so it does not need refreshing.
Worked example: Easy first: is that still a 1?
A DRAM cell was charged to full to store a 1. A cell reads as 1 if its charge is above 50% of full.
Since its last refresh, its charge has leaked to 70%. What is read, and what does the next refresh do?
A typical DRAM chip refreshes every row once every 64 ms, working through the rows in turn.
While a row is being refreshed it can't be read. That waiting is one more reason DRAM is slower than SRAM.