A lesson from the Computer Memory module of the bootcamp. About a 12 minute read.
Main memory is not one technology but two. SRAM holds a bit in a small loop of transistors and is fast and expensive; DRAM holds it as charge in a capacitor and is dense, cheap and slow. Why both exist, and why processors have outrun memory by a factor of hundreds, is the reason every later lesson in this module exists.
STATIC RAM stores each bit in a cell of six transistors wired as two inverters feeding each other: whatever value the loop holds, it reinforces, so the bit stays put for as long as the power is on. It is fast, reachable in about a nanosecond, and it costs a lot of silicon per bit, so it is used for the processor's caches. DYNAMIC RAM stores each bit as charge on a tiny capacitor behind a single transistor, which makes it several times denser and far cheaper, so the gigabytes in your machine are DRAM. The price is speed: roughly 100 nanoseconds to get an answer back. Fast and small, or big and slow. Every design decision in the rest of this module is a way of getting the first while paying for the second.
The rest of this lesson continues with 6 further sections. See the full curriculum.
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